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cc bn thnh phm si, tng hiu xut ca sn phm, ng thilm nh hng tt ti qu trnh nghin bt giy v cc tnh cht cagiy.

3.2. Hemixenlulo v cc polysacarit phi xenluloNh nu trn, gluxit ca g cha mt polysacarit c bn trong

g, l xenlulo. Ngoi ra, trong thnh phn cn c cc

polysacarit khc. Cc polysacarit phi xenlulo ny l thnh phn cu

thnh ca g, c gi l hemixenlulo. G cn cha mt s lng nh

cc polysacarit tan trong nc, trong s c cc poliuronit. Nhng

polysacarit ny ging vi hemixenlulo v cu to ha hc, song dotan c trong nc nn chng thuc nhm cc cht chit xut.

Khi dng khi nim hemixenlulo, ngi ta hiu n lun dng snhiu, bi l nhm cc cht c cu to ha hc v tnh cht t-ng t nhau, ch khng phi l mt cht ring bit.

Hemixenlulo, cng nh cc polysacarit tan trong nc, khng hatan trong cc dung mi hu c m ngi ta thng s dng tchcc cht chit xut, song khc vi cc polysacarit ny, cchemixenlulo khng tan trong nc. Khc vi xenlulo, hemixenlulo liha tan trong cc dung dch kim v d chu tc dng ca cc dungdch axt v c long, tc d b thy phn hn.

Trong g, hemixenlulo, cc polysacarit tan trong nc v poliuronitthc hin cc chc nng khc nhau. Hemixenlulo l cc thnh phncu trc ca thnh t bo, trong khi cc polysacarit tan trong ncng vai tr l cc cht dinh dng d tr. Tuy nhin, cn phi nhn

mnh rng, vic phn lai mt polysacarit no thuc v mttrong hai nhm chc nng ny, i khi rt kh khn, bi trong g cmt s polysacarit v cu to ha hc thuc cng mt nhm (theothnh phn monosacarit), song li rt khc bit nhau v chc nngv tnh tan.

3.3. c im cu to, phn loi v tn gi

Thnh phn cc i phn t hemixenlulo v cc polysacarit phixenlulo bao gm cc an phn t gluxit cha nm hoc su

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cc gc axit hexoronic. Cc mt ca a-xit hexoronic c th c trong

thnh phn ca cc mch nhnh polysacarit. Mt phn cc nhm

hyroxin ca mt s hemixenlulo b axetyl ha. Lin kt este khng

bn d b thy phn, c bit l trong mi trng kim. V vy,

cc nhm axetyl d b tch ra di tc dng ca kim v khi thy

phn bi nc nhit cao.

Cc polysacarit hp th c gi tn bng tn ca tt c cc

monosacarit to thnh, cui tn gi l tn ca monosacarit chnh

trong chui vi vic i ui oza bng an. Tn ca cc mt cu

thnh khc c gi theo dng tip u t ca t, sp xp theo th

t tng dn v hm lng ca chng trong hp t, v d

arabinoglucoronoxylan.

Bc trng phn ca hemixenlulo vo khong 100-200 vi chnh

lch dao ng t 30 n 300 v hn na, c ngha l chui mch

ca hemixenlulo ngn hn ca xenlulo rt nhiu. Hemixenlulo

khng ng nht v phn t lng. a tn ca chng ln hn so

vi ca xenlulo. S a dng v tnh cht ca cc polysacarit gy

nhiu kh khn trong vic phn lai chng. Hnh 3.2. biu th s

phn lai tt c cc polysacarit ca g theo tnh tan v cu to

ha hc ca chng. Polysacarit ca hemixenlulo cn c phn lai

thnh nhm axit (cha cc mch axit uronic) v trung tnh.

O

CH2OH

H

HH

H

OH

OH

HO HO

OH

OH

H

H

H

CH2

O

OH

HH

H

HOH

OH

OH

H

HOhoc

HO H

D-Xyloza -D-Xylopiranoza

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O

CH2OH

HH

H

OH

HO HO

OH

H

H

H

O hocHO H

HO H

CH2OH

H

OH

OH

OH H

H

O

HOCH2

H

L-Arabinoza -L-Arabinofuranoza

O

CH2OH

H

HH

H

OH

HO O

O

HH

H

H

OH

OHH

HOhoc

C

HO

OHH H

HO

C

HO

OH

H

H

H

H

CH2OH

HO

OH

CH2OH

D-Mannoza -D-Mannopiranoza

O

CH2OH

H

HH

H

OH

HO O

O

HH

H

HOH

OH

H

HO

hoc

C

HO

OHH H

HO

C

HO

OH

H

H

H

H

CH2OH

HO

OH

CH2OH

D-Galactoza -D-Galactopiranoza

O

H

HH

H

OH

HO O

O H

H

H

HOHOH

H

HO

hoc

C OHH H C

HO

OH

H

H

H

H

OH

OH OH

OH

COOH

COOH COOH

OH

H

OH

OH H

H

H

HO

H3CO

;

COOH

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A-xit D-Glucoronic A-xit -D-Glucoronic A-xit 4-O-Metyl

--D-glucoronic

O

H

HH

H

OH

HO O

O H

H

H

HOHOH

H

HO

hoc

C

HO

OHH H

HO

C

HO

OH

H

H

H

H

OHCOOH

OH

COOH

COOH

A-xit D-Galacturonic Ax-it -D-Galacturonic

O

H

HH

H

OH

HO

O

O

HH

H

H

OH

OH

H

HO

hoc

C

HO

OH

H

H

HO

C

OH

H

H

H

H

OHCH3

OH

CH3

CH3

HO

L-Ramnoza -L-Ramnopiranoza

O

H

HH

H

OH

HO

O

O

HH

H

HOH

OHH

HOhoc

C

HO

OH

H

H

HO

C

OH

H

H

H

H

OH

CH3

OH

CH3

CH3

HO

L-Fucoza -L-Fucopiranoza

Hnh 3.1. Cc monosacarit to thnh mch i phn t

polysacarit

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Hnh 3.2. Phn lai cc polysacarit ca g

Mc d khng cha cc mch axit uronic nhng arabinan vgalactan trong bng phn loi ny thuc nhm poliuronit tan trongnc,bi chng thuc t hp cc cht pectin. Thnh phn c bn cat hp ny l axit pectinic lin kt cht ch vi arabinan v galactan,c th, bng cc lin kt ha hc. Nha keo cng thuc nhm ccpoliuronit. Song, khi nim nha keo y, mt mc nht

nh, c tnh quy c v c cc nh nghin cu gi theo cc cchkhc nhau.

Trong phn tch g, ngi ta quy c chia hemixenlulo v ccpolysacarit phi xenlulo ra thnh pentozan, hexozan v axit poliuronic(poliuronan). Pentozan bao gm cc polysacarit c i phn t cuto ch yu t cc gc pentoza v khi b thy phn to thnh ccpentoza. Cng thc chung ca chng c k hiu l (C5H8O4)n.Hexozan gm cc polysacarit c i phn t cu to ch yu t cc

gc hexoza, cng thc chung ca chng (C6H10O5)n. Khi xc nh ccaxit uronic ngi ta quy i ra cc poliuronan. Trn c s , cc

Polysacarit(Gluxit)

Polysacarit cuthnh

(Holoxenlulo)

Polysacarit vpoliuronit tan trong n-

c

Xenlulo

Hemixenlulo

Polixacarit

Tinh bt vcc glucankhc,Arabinogalac

tan

PoliuronitCc cht pectin(axit pectinic+arabinan+galactan)Nha keo (axit

poliglucoronic,vv..)

XylanGlucorono-xylanArabinoglu-coronoxylan

MannanGlucomannanGalactoglucoman-nanFructomanan

Arabinan

Arabinan

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tnh ton v himixenlulo trong g mang tnh cht quy c. Tinh chtng polysacarit ring bit l khng th. Bng cc phng php trctip ch c th xc nh c tng s lng ca hemixenlulo bngcch tch chng t g hoc t holoxenlulo.

Hnh 3.3. Phn lai cc polysacarit phi xenlulo

3.4. c im hemixenlulo ca cy g mm v cy g cng

Hm lng hemixenlulo trong g trung bnh 20-30%, song c thdao ng trong gii hn ln t 15 n 40%. Hm lng hemixenlulo

trong g cy l rng trung bnh ln hn cy l kim 1,5 2 ln. Schnh lch v hm lng hemixenlulo trong khun kh mt loi cyhoc thm ch trong cng mt ging cy cc iu kin sinh tr-

Polysacaritphi xenlulo

Pentozan

Hexozan Poliuronit

Axit poliga-lacturonic,poliglucoronic

Xylan

Glucoro-noxylan,arabino-glucoro-noxylan

Mannan

Glucoman-nan,galac-toglucoman-nan,fructo-manan

Galactan

Galactan,arabino-galactan

Gluca

n

Tinhbt vccglucankhc

Thy phn Thy phn

D-xyloza, L-arabinoza, axit D-glucoronic, axit 4-metyl-D-glucoronic

D-manoza, D-galactoza, D-glucoza, L-arabinoza,D-fructoza

Kh

cacboxyl

CO2

(k c CO2

to thnh tglu-coronoxylan)

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ng khc nhau l rt ln. Ngoi ra, kt qu xc nh hm lnghemixenlulo bng cc phng php khc nhau cng rt khc bitnhau. V vy, cc d liu cng b trn cc ti liu i vi mt laicy cng thiu ng nht.

Thnh phn hemixenlulo ca cy l kim v cy l rng khcnhau. Cy l rng cha nhiu pentozan hn (17-25%, c trng hpln ti trn 30%) so vi cy l kim (5-13%) v ch mt lng nhhexozan (0,5-6%). Cy l kim cha nhiu hexozan (8-20%) hnpentozan. Trong thnh phn hemixenlulo ca cy l rng phn ln lxylan (ch yu l glucoronoxylan), cc polysacarit phi xenlulo khcch chim mt s lng nh. Thnh phn hemixenlulo ca cy l kimphc tp hn, bao gm arabinoglucoronoxylan, glucoronoxylan,

glucomannan, galactoglucomannan, arabinogalactan, vvS phn b hemixenlulo trong cy khng ng u. C mt

s xu hng tng hm lng hemixenlulo cnh v phn trn ca thncy, song i vi cc loi cy khc nhau quy lut ny cng khcnhau. Chng hn, trong cy l kim, hm lng pentozan tng theochiu cao ca cy, cn hm lng mannan th li gim, iu linquan ti s thay i v thnh phn ca g theo vng tui ca cy.

Theo ng knh thn cy, hm lng pentozan gim theo hng t lira v, trong khi hm lng mannan li tng. iu kin sinh trngca tng cy ring bit cng gy nh hng khng nh ti hm lngca hemixenlulo. S phn b hemixenlulo trong cy l rng tngi ng u hn so vi cy l kim.

3.5. c im cc phn ng ha hc ca cc polysacaritca g

Cc phn ng ca cc polysacarit mang mt ngha thc tinln trong cc qu trnh ch bin ha v c-ha g sn xut giy-xenlulo; thy phn g, ch bin lm sn, sn xut vn nhn to,vv... Mc ch ca sn xut giy-xenlulo l ch xut xenlulo kthut v cc bn thnh phm si khc. Trong qu trnh ch bin, ccpolysacarit phi xenlulo b kh mt mc no , nh cc qutrnh phn hy trong khi nu di tc dng ca axit, kim hoc cc

cht -xy ha. Trong sn xut thy phn, phn gluxit ca g bthy phn, t polysacarit to thnh cc ng v cc sn phm ch

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bin khc. Trong nhit phn g -mt trong nhng nghnh ch binlm sn, cc cht cao phn t ca g, k c xenlulo v ccpolysacarit phi xenlulo, b nhit phn thnh cc sn phm a dng.Cn trong sn xut vn nhn to, nhit cao, nh c hi nc,

cc polime ca g chu cc bin i thy-nhit khc nhau, qua din ra tc dng ha hc ca chng vi cc cht kt dnh tng hp.Trong phn ny, s trnh by cc bin i ha hc quan trng

nht ca cc polysacarit c trong g, cn cc phn ng trong snxut xenlulo k thut v cc dn xut ca n s c trnh by ccphn sau.

3.5.1. Phn loi cc phn ng ha hc ca cc polysacarit

trong gtrn phng din l cc polime

Cc phn ng ha hc ca cc polysacarit c chia thnh hainhm: cc bin i c trng ca polime v cc phn ng ca iphn t. Kt qu ca cc bin i c trng cho polime (cc phnng ca cc mt n phn) l s thay i v thnh phn hahc ca polysacarit, song bc trng phn v cu hnh khng gianca i phn t khng b thay i. Cc phn ng ca cc mtn phn li c chia thnh hai dng: phn ng ca cc nhm chcnng, phn ng bin i ni phn t (ni mt n phn).

Phn ng ca cc nhm chc nng ch yu l phn ng ca ccnhm ru bc nht v bc hai. Lai phn ng ny i vi xenlulo c ngha quan trng hn, Cn i vi cc polysacarit phi xenlulo thcc phn ng ny thng c thc hin khi ta nghin cu cu to ha

hc ca chng (chng hn phn ng metyl ha). Cc phn ng nycng bao gm tch cc nhm axetyl khi hemixenlulo, phn ng khcacboxyl ca cc axit uronic, phn ng -xy ha cc nhm ruPhn

ng bin i ni phn t, chng hn, phn ng kh nc trong mtn phn trong qu trnh nhit phn, khng mang ngha lni vi polysacarit.

Nhm th hai cc phn ng ca i phn t, mang nhiu ngha i vi cc polysacarit. Kt qu ca cc phn ng ny l s

thay i bc trng phn ca polysacarit (thng th gim), thayi v cu hnh khng gian ca i phn t, song thnh phn

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ha hc khng i. Cc phn ng ca i phn t c chia lm badng: phn ng phn hy, phn ng kt mch v phn ng ca ccmt u mch.

Chng ta tm dng li mt s c im ca cc phn ng

ny. Phn hy polysacarit trong mi trng kim l phn hy tnh(suy bin), cc phn ng kh trng hp mt u mt ra khi iphn t thng chim u th hn. Khi phn hy hon ton, khngph thuc vo dng phn ng, sn phm ca phn ng l cc nphn.

Nhit phn g v cc bin i thy nhit trong sn xut tmp v vn nhn to l qu trnhphn hy l hc. Trong s cc bini l hc c phn hy c ca polysacarit khi nghin trong sn

xut giy v bt giy.Phn hy ha hc ca cc polysacarit din ra di tc dng ca

cc ha cht. C ba dng phn hy ha, l -xy ha, thy ha vphn hy gy ra bi cc cht hu c hay cn gi l sonvat ha.

Qu trnh -xy ha sy ra khi ty xenlulo k thut v kim haxenlul trong sn xut si visc. -xy ha i phn t din ra ngthi vi vic -xy ha cc nhm ru chc nng. L polime d th,polysacarit d dng b phn hy hn cc polime dng chui ccbon, mc d trong g lignin li l thnh phn d b -xy ha nht.

Khi nghin cu ha hc v polysacarit, qu trnh ng ch nht l thy phn. l phn ng di tc dng ca cc cht hu ckhi c cc axit lm xc tc, trong din ra phn ng ph hy linkt glicozit, ging nh phn ng thy phn, song vi s kt hp chttham gia phn ng vo v tr ca lin kt va b ph hy.

Phn hy sinh hc polysacarit l phn hy c s tham gia ca

cc cht xc tc sinh hc cc men to ra bi cc c th sng, nhcc loi nm, khun, vv Men c tnh chn lc cao i vi ccpolime khc nhau, chng hat ng trong iu kin va phi(khng cn nhit cao).

Phn ng kt mch trong ha hc polysacarit khng c chtrng lm. Chng c s dng trong bin tnh xenlulo. Phn ngca cc mt u mch hay phn ng ca cc mt cha nhmaldehit, c s dng xc nh chiu di ca chui xenlulo k

thut.

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Cng nh cc polime, trong cc bin i ha hc ca polysacarit,cng lc c th xy ra cc phn ng a dng. Cc phn ng chnh cth km theo cc phn ng ph hoc din ra theo cc chiu hngkhc nhau.

3.5.2.c im cc bin i ha hc ca polysacarittrn phng din l cc polime

Cng nh cc polime, i vi polysacarit, trong cc phn ng camt n phn v ca cc nhm chc nng ni ring, c hin tngphn tn. Phn t phn ng nh nht trong cc phn ng khngphi c i phn t, nh trong trng hp cc cht c phn t lngthp, m l mt mt ring bit, l gc monoxacarit. V vy i

vi cc polime ngi ta nh gi kt qu theo mc bin i ha hcbnh qun. c im ny c bit c ngha khi iu ch ccdn xut ca xenlulo.

Cc phn ng ca polysacarit c th l ng th hoc d th.Trong cc phn ng ng th, chng hn, thy phn ng th capolysacarit trong dung dch axit sulfuric m c, trc tin ccpolysacarit s ha tan vo dung dch, sau mi sy ra cc phnng tip theo. V cc quy lut ng hc, cc phn ng ng thca polysacarit hu nh khng khc vi cc phn ng ng th cacc cht c phn t lng thp. i a s cc phn ng capolysacarit l d th, tc hai th, chng bt u t trng thi dth v kt thc trng thi ng th.

Cc qu trnh d th ca polysacarit khc vi cc phn ng hahc ca cc cht phn t lng thp. Cu trc siu phn t capolysacarit, siu cu trc ca thnh t bo v cu to gii phu ca

g l nhng yu t nh hng n tnh cht ca cc qu trnh dth ca polysacarit. Tt c cc c im ny xc nh kh nngchu tc ng ca cc polysacarit i vi cc cht phn ng. Vvy, kt qu ca cc qu trnh d th khng nhng ph thuc vovn tc ca chnh phn ng, m cn ph thuc v vn tc khuychtn cht tham gia phn ng vo su trong thnh t bo hoc vosu trong si xenlulo. Phn lai cc phn ng d th s c trnhby phn c im cc phn ng ca xenlulo.

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3.5.3.Phn lai cc phn ng ha hc ca polysacarittrn phng din l cc cht hu c

hiu r hn v c ch cc phn ng ca polysacarit chatrong g, ngui ta phn lai theo hai nhm cc c im sau:

- trn c s kt ni hoc kh cc thnh phn cu trc;- theo phng php phn hy hoc to thnh cc lin kt.Dng phn ng th nht c chia thnh: phn ng th (S), phn

ng lin kt(A), phn ng tch (E) v phn ng sp xp li. Trong scc phn ng ny phn ng th mang ngha ln nht. Ngoi ccphn ng lit k trn, c th k n phn ng -xy ha kh.

Theo phng php phn hy hoc to thnh cc lin kt (theo bncht ca phn t tham gia phn ng) c cc phn ng d thv phn

ng ng th,phn ng d th (phn ng ion) nh phn ng nucleofil (khi chtphn ng l cc l nucleofil) v electrofil (khi cht phn ng lelectrofil)., phn ng c s tham gia ca cc radical t do.

Theo cch phn loi ny, tt c cc phn ng phn hy theo cch ph hy lin kt glicozit c chia thnh phn hy d th vphn hy ng th. Trong phn hy l hc th cc phn hyng th (radical t do) chim u th hn. Sonvat ha l qu trnhphn hy d th. Cc qu trnh xi ha bao gm c cc phn hyd th ln ng th.

3.5.4. Xc nh cu to ha hc ca cc polysacarit

xc nh cu to ha hc ca cc polysacarit ring bit ngita s dng mt lat cc phng php ha hc da trn cc phn ngphn hy ca chng kt hp vi nghin cu cc sn phm to thnh,

nh phng php metyl ha kt hp vi thy phn, xi ha bng ccmui ca it, thy phn a-xit khng hon ton, xeton ha, thyphn bng men, kh trng hp kim, vv tch v phn bit ccsn phm phn hy ngi ta s dng cc phng php sc k (nhsc k trn giy, sc k lp mng v sc k kh lng) kt hp viphng php quang ph khi lng, vv

Ging nh phng php nghin cu cu to ha hc ca xenluloza,phng php metyl ha da trn c s metyl ha nhiu ln tt c cc

nhm hydroxin trong polysacarit bng dimetylsunfat trong mi trngkim, kt hp vi thy phn polysacarit c metyl ha tip sau

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v phn tch cc sn phm to thnh. Cc monosacarit di dngvng piranza hoc furanoza m trong phn cc nhm hydroxin cmetyl ha hon ton, c to thnh t cc u mt khng kh capolysacarit, cn cc monosacarit m trong phn cc nhm hydroxin

c metyl khng ha hon ton, th c to thnh t cc mt giaca polysacarit. Mt nhm hydroxin ru t do cho thy v tr lin ktca mt gc monosacarit khc, tc l n xc nh v tr ca linkt glicozit. Nhm hydroxin lin kt glicozit li khng b metyl ha.

Trn hnh hnh 3.4 dn v d iu ch cc monosacarit metyl hat cc mt khc nhau ca galactoglucomannan.

xi ha polysacarit bng NaIO6 bng cch iu chnh cc iukin phn ng c th xi ha cc nhm hydroxin bc hai (nhm -

glycol) trong cc mt piranoza hoc furanoza ca cc monoxacarit,trong cc lin kt C2-C3 b ph v v t mt mt gia ca chuipolixacarit to thnh mt mt diandehit-polixacarit . Khi xi hau mt kh din ra xi glixerin. S c mt ca lin kt glicozit tiv tr cacbon th hai lm thay i tnh cht ca phn ng xi ha.Ngi ta xc nh lng hao NaIO6 cho xi ha v nghin cu snphm to thnh. Ngoi ra, c th tin hnh kh polysacarit bxi ha v sau thy phn n. Ngi ta cn s dng phng phpxi ha bng Cr2O3.

xc nh dng lin kt glicozit (- hay -) ngi ta ng dng ccphng php thy phn bng cc men khc khc nhau (cc menhydrolaza), thy phn bng a-xit iu kin thng, vv Phngphp quang ph hng ngai c s dng xc nh cu hnhlin kt glicozit, xc nh cc nhm chc trong cc i phn tpolysacarit v nghin cu cc lin kt hydr. Nhng nm gn y,

phng php quang ph cng hng t ht nhn 13C ngy cng cng dng rng ri cho vic nghin cu cu to ha hc capolysacarit.

~Metyl ha

thy phn

CH2OH

OHOHHO

H

H

H

H

O O

H

H

H

CH2OCH3

H3COOCH3

H, OH

OCH3

HH

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Mt u mt 2,3,4,6- Tetra-O-metyl-ca D-manoza D-manoza

OCH3

H, OH

OCH3

CH2OCH3

H

H

H

OO

H

H

H

H

OH OH

CH2OH

thy phn

Metyl ha~H H

O~ HO

Mt gia ca D-manoza 2,3,6,-Tri-O-metyl-(lin kt 14) D-manoza

HO~O

HH~

thy phn

CH2OH

OH

OH

H

H

H

H

O O

H

H

H

CH2OCH3

OCH3

H, OH

OCH3

Metyl ha

Mt gia ca D-glucoza 2,3,6,-Tri-O-metyl-(lin kt 14) D-glucoza

HO~O

HH~

Metyl ha

thy phnOHOH

H

H

H

H

O O

H

H

H

OCH3

H, OHOCH3

O

H

H

H

H

OH

OH

CH2OH

H

CH2

O

HO

+

OCH3

H, OH

OCH3

CH2OCH3

H

HH

O

HH3CO

H2OH

Mt gia ca D-manoza 2,3,-Di-O-metyl- 2,3,4,6,-Tetra-O-(lin kt 14 vi mch nhnh D-manoza metylD-galactozal mt ca D-galactoza (lin kt 16)

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Hnh 3.4. Cc sn phm metyl ha v thy phngalactoglucomannan

3.6. Polysacarit d thy phn v kh thy phn

ca g. Thy phn bng axitS c mt ca cc lin kt glicozit trong i phn t polysacaritlm cho chng c kh nng thy phn. Khi thy phn polysacarit,cc lin kt glicozit b tch ra v din ra s kt hp cc phn t canc (hyr v nhm OH) vo ch lin kt va b ph hy . Khithy phn khng hon ton, bc trng phn ca polysacarit bgim, cn khi ton b cc lin kt glicozit b ph hy, chng binthnh cc monosacarit. Khi cc lin kt glicozit b tch ra ti to

thnh cc mt u mt kh v khng kh mi (xem hnh 3.4).Theo vn tc ca phn ng thy phn trong mi trng axit cc

polysacarit c chia thnh polysacarit d thy phn v kh thyphn. S phn lai ny c xc nh bi cc c im cu trcsiu phn t ca polysacarit. Cc polysacarit d thy phn l ccpolysacarit c th b thy phn bng cc dung dch axit long(chng hn dung dch 2-5% HCl) nhit gn 100oC. Ccpolysacarit kh thy phn bao gm cc polysacarit ch b thyphn bi tc dng ca cc axit v c m c (chng hn dungdch 70-80% H2SO4 hoc dung dch HCl siu m c (40-42%) nhit thp (khong 25oC). Thy phn cc polysacarit kh thyphn bng dung dch cc axit long ch sy ra nhit cao(160-190oC).

Trong thnh phn ca holoxenlulo th xenlulo (mt polime tinhth), l polysacarit kh thy phn, cn cc hemixenlulo (cc polime

v nh hnh), thuc dng cc polysacarit d thy phn. Cc axitlong ch c th thm nhp vo cc polysacarit v nh hnh cang m ligno-gluxit v phn v dnh hnh ca xenlulo. Ccxenlulozan (t hp cc hemixenlulo cng kt tinh vi xenlulo phnng kt tinh ca vi si xenlulo) cng thuc cc polysacarit kh thyphn. Cc xenlulozan cng chnh l cc hemixenlulo, tc l ccxylan v mannan, nhng chng ch b thy phn cng vi xenlulo vkh tch ra bng kim hn. V th, trong thnh phn ca xenlulo

k thut sn xut t g bng cc phng php nu khc nhau v

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alpha-xenlulo tinh ch t holoxenlulo, lun cha mt lng tp cht lcc hemixenlulo cn li.

Thnh phn ca holoxenlulo trn c s s khc bit v mc thy ca cc polixacarit, c biu th trn hnh 3.5.

Hnh 3.5. S thnh phn ca holoxenlulo

Nu hnh dung ton b phn gluxit ca g bao gm c cc chtchit xut tan trong nc, th thnh phn ca cc polysacarit d thyphn v kh thy phn s rt phc tp. Trong g, ngai ccpolysacarit kh thy phn ra, cc polysacarit d thy phn baogm tt c cc poliuronit v polysacarit phi xenlulo. Trn c s nyc th biu th ton b phn gluxit ca g bng s hnh 3.6.

XylanManna

nManna

nXylan

Xenlulozan(phn khthy phn

ca cchemixenlulo)

Xenlulo(phn

tinh thHemixenl

ulo

Phnv nh

hnhca

xenlulo

Holoxenlulo

Polysacaritkh thy

phn

Polysacaritd thy phn

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Hnh 3.6. Cc polysacarit d thy phn v kh thy phn cag, cc sn phm thy phn ca chng

3.6.1. Cc bin i thy phn ca polysacarit

Thy phn polysacarit bng tc dng ca nc hu nh khng thc

hin c do vn tc phn ng qu thp. Trng hp ngai l l trng

hp t thy phn ca cc polysacarit d thy phn trong nc khi

un nhit 140-180OC. iu kin ny chc nng xc tc c

to bi axit axetic to thnh nh qu trnh tch kh nhm axetyl ra

khi hemixenlulo. X l nc v hi tng t c s dng trong qu

Xenlulo(phn

tinh th)Xenluloz

an

Xylan

Glucorono

- xylan,arabino-glucoro-xylan

Mannan

Glucoman- nan,fructo-mannan

Hemixenlulo

Mannan

Glucoman

- nan,galac-togluco-mannan

Xylan

Glucorono

- xylan,arabino-glucoro-xylan

Polysaca rittan

trong nc

Tinh btv cc

glucankhc,arabi-nogalactan

Poliuronit

Cc chtpectin(axitpectinic +arabinan

+galactan),nha keo

(axitpoli-glucoronic, vv,)Thy

phnThyphnDch thy phn ca cc

poli- xacarit kh thyphn

D-glucoza, D-manoza, D-galactoza, D-fructoza, D-xyloza, L-arabinoza, axit D-glucoronic

Dch thy phn ca ccpoli- xacarit d thyphn

D-manoza, D-galactoza, D-glucoza, D-xyloza, L-arabinoza, axit D-glucoronic,axit D-galactoronic

Polysacaritca g

Polysacaritkh thy

phn

Polysacaritd thy

phn

Phn v nhhnh

ca xenlulo

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trnh gi l thy phn trc, thy phn mt phn hemixenlulo

trc khi nu sunfat trong sn xut xenlulo. Qu trnh ny ch yu

nhm ty arabinogalactan khi nu g cy l rng.

Thy phn polysacarit l phn ng ph hy d th cc lin kt

glicozit, din ra theo c ch phn ng ion. Cc phn ng ny c

xc tc bi cc axit v c mnh (thy phn bng axit) v din ra nh

mt qu trnh suy thoi. S phn hy cc lin kt glicozit din ra ti

cc im ngu nhin ca chui i phn t v km theo l bc

trng phnb gim.

Trong phn ng thy phn bng axit, cht xc tc l proton tn

ti di dng thy ha trong cc dung dch nc ca cc axit

H+ + H2O H3O+

Nng ca proton cng cao th phn ng thy phn din ra

cng nhanh. V vy, ngi ta thng s dng cc axit v c mnh lm

cht xc tc. V cng , cc axit v c v hu c c th sp xp

theo dy sau:

HCl > HBr > HI > H2SO4 > HNO3 > CF3COOH > H3PO4 > HCOOH >

CH3COOH

Trong ch bin thy phn v phn tch g ngi ta thng s

dng cc dung dch H2SO4 v HCl.

Qu trnh bin i thy phn ca polysacarit gm v s cc

phn ng thy phn cc lin kt glicozit, l cc phn ng trao

i gia glicozit v nc, din ra theo c ch phn ng thay th

nucleofil SN1. C ch phn ng ny l mt dy lin tip cc k phn

ng, ngc vi qu trnh to thnh cc axetal t andehit v ru. Trong

thy phn, lin kt glicozit ging nh mt axetal, nhm alcoxyl l

nhm kh tch ra hn, v vy phi proton ha chng trc v chuyn

thnh axit kt nhm. Khc vi thy phn cc lin kt ete, s thy

phn d dng cc lin kt glicozit bng cc axit long c to ra bi

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s n nh cacbocation trung gian bng cng hng hay cn gi l

hiu ng ng chnh (xem phn sau).

Di tc dng ca ion H3O+ thy phn polysacarit din ra theo 4

k sau (hnh 3.7a , qua th d phn ng ca xenlulo):

1. Proton ha lin kt glicozit (phn ng nhanh). Trong k ny,

lin kt glicozit ng vai tr mt baz Luis (l cht cho cp in

t cha b phn chia), v sau khi kt hp proton bin thnh axit kt

nhm, mt macro-ion.

2. Lin kt proton ha b phn hy d th, to ra

cacbocation vng chuyn tip (glicozil-cation) v mt u mt

khng kh (phn ng din ra chm v l phn ng xc nh vn

tc thy phn).

3. Kt hp nc (nucleofil) vo cacbocation (phn ng nhanh).

4. Ti sinh proton v to thnh mt kh (phn ng nhanh).

Proton ti sinh to thnh ion H3O+, ion ny li proton ha lin kt

glicozit

Khi lin kt glicozit proton ha b ph hy, vng piranoza

cu hnh gh u C1 to thnh glicozil-cation dng chuyn tip

thnh cu hnh bn gh u, trong c cc nguyn t cacbon

th nht, th hai, th 5 v nguyn t xi ca vng piran cng phn

b trn mt mt phng. Cacbocation vng di cu hnh bn gh

u c n nh bi mt cng hng hay l s chuyn dch in

tch sang mt d nguyn t (nguyn t xi) c cc cp in t

nguyn vn, kt qu l in tch dng c di ng trong cu trc

ca cation cacboxon (hnh 3.7b).

Khi thy phn, trng thi ca cc lin kt glicozit ca polisacarit

b nh hng bi mt lat cc yu t cu trc, kch thc ca vng

phn t, cc hiu ng bin dng, lp th v cm ng.Yu t gy

nh hng ln nht ti vn tc ca qu trnh thy phn xc tc bng

axit l kch thc ca vng phn t. Trng hp cc lin kt glicozit

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thuc cng mt monosacarit, vng furan thy phn nhanh hn so vi

vng piran. Nguyn do l sc cng lp th ca cc vng furan ln

hn so vi vng piran, ng thi cng c th do s khc bit v c

ch phn hy cc lin kt glicozit.

Tt c cc quy lut ca bin i thy phn trn y u

xut pht t s nh hng ca s khc bit v cu to ha hc ca

cc polysacarit i vi vn tc thy phn. Kh nng tip cn cc

cht phn ng ca polysacarit gy nh hng mang tnh quyt

nh trong nhng k u phn ng. Kh nng ny ph thuc vo

siu cu trc ca thnh t bo, ch yu l cu trc siu phn t ca

chnh cc polysacarit.

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Hnh 3.7. C ch phn ng thy phn ca polysacarit qua thd xenlulo

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3.6.2 Thy phn cc polysacarit ca g bng axit long v

axit m c

Phn ng thy phn hon ton cc polysacarit ca g c th

din t bng cc phng trnh sau (c tnh cht quy c i vi

hexozan v pentozan):

(C6H10O5)n + nH2O nC6H12O6H+

Xenlulo; D-Glucoza;Hexozan hexoza

(C5H8O4)n + nH2O nC 5H10O5H+

Pentozan Pentoza

Qu trnh thy phn cc polysacarit thnh monosacarit din ravi s to thnh mt lot cc sn phm trung gian c bc trng phngim dn. Trong axit long v axit m c, thy phn din ratheo cc s khc nhau v i hi cc iu kin phn ng khc

nhau.Thy phn bng axit long. Trong axit long, qu trnh thyphn bt u t mi trng d th, iu c c trng bi snh hng ca cu trc siu phn t ti mc thy phn. Thyphn cc polysacarit d thy phn c th thc hin nhit gn 100oC, cn i cc polysacarit kh thy phn nhit nycao hn nhiu. Chng ta hy so snh thy phn xenlulo vhemixenlulo trong axit long vi s thy phn ca tinh bt (hnh

3.8).

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Hnh 3.8. S thy phn polysacarit bng cc axit v clong

Khi thy phn xenlulo, ban u xenlulo b mt dng cu trc si

v bin thnh hydroxenlulo-mt hn hp ca xenlulo v cc snphm thy phn cc mc khc nhau. Hydroxenlulo c bctrng phn trung bnh thp, song c kt tinh cao so vi xenluloban u. Phn ng k thy phn ny din ra tng i nhanh.

Tip , thy phn phn tinh th ca xenlulo trong axit long khiun si din ra rt chm. (phn ng hu nh b dng li), v vy, tng vn tc phn ng cn tng nhit ti 160-190oC.

Khc vi xenlulo, cc polysacarit v nh hnh, nh tinh bt vhemixenlulo, k phn ng thy phn chm hu nh khng c, bichng b thy phn rt nhanh to thnh cc monosacarit tan trongnc. Tip , thy phn din ra nhanh. Cc xenlodextrin (bc trngphn 60-50 v thp hn) v cc dextrin c to thnh, tip theo lcc oligosacarit (c bc trng phn 10). Trong s trn, toligosacarit chia thnh cc disacarit (xenlobioza, maltoza, ) l ccsn phm gn nh cui cng cn gi c cc lin kt glicozit ban

u.

XenluloTinh

bt

Hemixenlu

loHydroxenlulo

Xenloextrin

Oligoxacarit dy

Xenlobioza

Dextrin

Oligoxacarit dy

Mantoza

Dextrin

Dixacarit

D-glucoza

Monoxacaritv axituronic

Oligoxacarit

Cc

oligo-v

poli

xacarittan

tr

ong

nc

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Trng thi ca cc polysacarit c tch ra trong qu trnhthy phn khc vi trng thi ca chng trong g mt cht. Khi thyphn g bng cc axit long nhit 100oC, ch c ccpolysacarit d thy phn b phn hy, l cc hemixenluo

(ngoi xenlulozan ra), tinh bt, cc poliuronit v phn v nh hnhca xenlulo. Ngoi ra, cc lin kt ca lignin vi hemixenmlulo cngb thy phn.

Thy phn din ra hai giai on. giai an th nht (hatan cc polysacarit d thy phn trong nc) thy phn din ratrong mi trng d th vi vn tc tng i nhanh, bc trng phnca polysacarit gim v to thnh cc ng dextrin (xylodextrin,mannodextrin, ). Giai an ny din ra thm ch nhanh hn so

vi giai on hai, giai an thy phn trong mi trng ng th tothnh cc monosacarit (hecxoza, pentoza, axit hecxoronic) t ccdextrin. S phn chia thy phn thnh hai giai on nh trn mangtnh cht quy c. Giai an hai hu nh bt u cng vi giai anmt. V vy khi thy phn cc polysacarit d thy phn, trongdung dch tn ti cng mt lc cc dextrin, oligosacarit vmonosacarit.

Cn nhn mnh rng, khi thy phn xenlulo v tinh bt, khngph thuc vo cu hnh ca lin kt glicozit, sau khi thy phnhon ton ta thu c cng mt hn hp cn bng cha cc dng hbin ca D-glucoza, ch yu l -D-glucopiranoza, -D-glucopiranozav mt s lng khng ln dng andehit mch h (hnh 3.9); dngfuran khng ng k. Cc monoxacarit khc cng c tng quanng nh vy. V vy , trong cc s thy phn ngi ta s dngk hiu quy c cho hn hp cc u mt kh. S bin i dng

thnh dng v ngc li ch din ra i vi dng mch h.

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Hnh 3.9. Cc dng h bin ca D-glucoza

Thy phn bng axit m c. Thy phn thc hin

nhit thng (20-30oC). Thy phn bng axit m dc c mt

s c im. Th nht, khc vi thy phn bng axit long, khi

thy phn bng axit m c, cc polysacarit ca g, trong c

xenlulo, trng ln v ha tan trong axit c, tip thy phnchng trong dung dch din ra nhanh. Axit c c hiu l lai

axit c th ha tan xenlulo. Axit sunfuric c th ha tan xenlulo khi

nng ca n khng thp hn 62% (trong thc t ngi ta thng

s dng lai 70-80%), cn axit clohydric ch c lai siu c (40-

42%). Axit F3CCOOH (100%) cng c th ha tan v thy phn

xenlulo. C ch qu trnh ha tan polysacarit qua th d xenlulo sc trnh by phn ha hc v xenlulo.

HOH

C

HOOH

HH

H

H

CH2OH

O

CH2OH

HO

OH

OHOH

O

OH

D-Glucoza(~0,02 %)

OOH

OH

OH

HO

CH2OH

O

OH

OH

OH

HOCH

HOCH2 HOCH2

HOCH

OH

OH

OHO

-D-Glucopiranoza

(64%)-D-Glucopiranoza

(36%)

-D-Glucofuranoza-D-Glucofuranoza

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Khi tc dng mt lng axit m c ln g nghin, axit s ngm

vo cc khoang bo, ri thnh t bo v lm g trng ln. Tip

cc polisacarit b ha tan. Hemixenlulo s b ha tan v thy phn

trc tin, sau mi n xenlulo v cc xenlulozan. Vn tc ca qu

trnh thy phn khng nhng b nh hng bi cu trc siu phn

t, m cn bi cu to thnh t bo v m g ni chung.

Xenlulo b ha tan trong cc axit nng cao hn so vi

hemixenlulo. Thm ch, khi xenlulo b tan trong axit m c bao

gi cng bt u bng phn ng thy phn d th, thy phn

ch yu s din ra trong mi trng ng th. Tng quan gia vn

tc tan v vn tc thy phn ca xenlulo ph thuc vo nng

axit. Vi nng H2SO4 trong dung dch nc 55-62% vn tc thy

phn ln hn so vi tc tan, cn khi nng vt qu 62% th

vn tc thy phn li nh hn. Nh vy c ngha l, nhng iu

kin thng ca thy phn g v xenlulo bng axit sunfuric c,

qu trnh ha tan lun din ra trc khi thy phn.

c im th hai ca thy phn polysacarit bng axit c,

l s khan nc trong mi trng phn ng. Thy phn din ra

trong iu kin, khi nc trong mi trng phn ng c lin kt vi

nhau di dng cc hydrat nh phn ng vi cc polysacarit v vi

chnh cc axit. S khan nc dn n cc sn phm ca thy phn

khng phi l cc monoxacarit m l cc oligosacarit. Khi khan nc,

cc oligosacarit c hnh thnh bng hai cch do thy phn tng

phn polysacarit v s hi bin (phn ng ngc vi thy phn) ca

cc monosacarit to thnh khi thy phn hon ton. Phn ng hi

bin cng c xc tc bi proton (ion H3O+). Cu to ca cc

oligosacarit to thnh nh s hi bin, khc vi cu to ca cc

oligosacarit to thnh do thy phn tng phn. Oligosacarit to

thnh t thy phn khng hon ton c kch thc vng v tnh cht

cc lin kt glicozit ging cc polysacarit ban u.

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S nguyn l thy phn xenluloza v cc polysacarit khc

bng axit m c c biu th trn hnh 3.10. S hi bin c

th din ra mc cao hn, to thnh cc dextrin c phn t l-

ng tng i thp (bc trng phn > 10). Cc dextrin v oligosacarit

(cc sn phm ca qu trnh hi bin v thy phn khng hon

ton) d thy phn bng cc axit long.V vy, iu ch cc

monosacarit, thy phn bng cc axit m c c tin hnh

hai k: x l g hoc nguyn liu thc vt khc bng axit c

nhit thng; ngc bin (thy phn ph) - ha long hn hp bng

nc (ti nng ca axit 3-4%) v un si.

Hnh 3.10. S thy phn polysacarit ca g bng axit vc m c

Hai k ca thy phn gm cc phn ng, trn th d ca

xenluloza, c th din t bng cc phng trnh sau (trong m

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(C6H10O5)n + nH2O nC6H12O6H+

thy phn, D-Glucoza20oC

mC6H12O6 - mH2O (C6H10O5)mH

+

hi bin ng hi bin

(C6H10O5)m + mH2O mC 6H12O6H

+

ngc bin, D-Glucoza100oC

Bin i ca cc monosacarit. iu kin nh nhng,

trong mi trng axit cc ng (monosacarit) rt bn vng. Song,

nhng iu kin khc nghit hn khi thy phn g bng axit

long, nhit cao, chng chu cc phn ng mt nc, phn ng

ngng kt v phn hy. Nh qu trnh ngng kt mt s sn phm hi

bin c to thnh. Song song vi qu trnh thy phn, di tc dng

ca cng mt cht xc tc (axit), din ra qu trnh ph hy cc

monosacarit to thnh cc sn phm khc nhau. Ty thuc vo nhit

v nng ca axit, cc hp cht a dng c to thnh, a s

chng l nhng cht khng bn v b phn hy ngay sau , v

vy chng tn ti trong mi trng phn ng s lng khng ng

k.

C hai kh nng phn ng mt nc ni phn t ca cc

monosacarit di xc tc ca axit. Kh nng th nht dn ti s to

thnh cc ng anhyro. Kh nng th hai to thnh cc sn phm

khng mang c tnh ca cc ng, l cc andehit mch kn

d phn t, vv T pentoza to thnh fuafurol, m sau b phn

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hy mt phn thnh axit fomic v axit levulinic. Trong mi trng

axit, cc andehit dng vng furan c th b trng hp. Mt s lng

khng ln cc cht phenol c to thnh, ch yu t cc pentoza v

axit hexoronic. iu ni ln mc phc tp ca cc bin i

ha hc ca cc monosacarit khi thy phn bng axit.

Vn tc phn hy cc monosacarit c xc nh bi chnh

nhng yu t quy nh vn tc cc phn ng to thnh chng (tc

phn ng thy phn), l nng ca axit v nhit . Ngi ta

chn iu kin thy phn nguyn liu thc vt sao cho s phn r

ca cc ng l ti thiu, cn hiu xut ca chng l i a.

Nh cc phn ng ngng kt vi s tham gia ca fuafurol,

hyroximetylfuafurol, axit fomic v cc hp cht thm, to thnh cc

cht humin (cht mn), l cc hp cht cao phn t c tnh axit,

Thnh phn v cu to ca chng n nay cn cha c xc nh r

rng, chng rt kh tan v ging cc cht mn trong th nhng (cc

cht to thnh trong t t cc xc thc vt nh qu trnh thy sinh

ha). Tham gia vo cc phn ng ngng kt cn c lignin, kt qu l

s to thnh cc cht lignohumin. R rng, chng l cc sn phm

ca qu trnh ngng kt ca cc cht to thnh t lignin, nh hp cht

thm, fuafurol hoc hyroximetylfuafurol. V vy , nn gi cc

cht ny l lignofuran thay v lignohumin.

Khi thy phn polysacarit bng cc axit m c, do thy

phn c thc hin nhit thp, nn cc monosacarit t b

phn r hn v hiu xut ca chng hu nh tng ng vi s lng

theo tnh tan. Trong , ch to thnh mt lng nh cc cht

humin.

Thy phn g v cc nguyn liu thc vt khc c s dng

trong sn xut thy phn v trong phn tch g ( xc nh

hm lng v thnh phn ca polisacarit v lignin). Khi nu g bng

axit sn xut xenluloza (chng hn nu sunphit), din ra cc

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phn ng thy phn ca hemixenlulo, lm chng b ha tan trong

dung dch, ng thi cng xy ra cc phn ng bt li, l s

phn hy mt phn xenluloza, dn n lm gim hiu xut, bc

trng phn v bn ca xenluloza.

3.6.3 Cc sn phm thy phn nguyn liu thc vt vng dng ca chng

Trong cng nghip, ngi ta thy phn g v cc nguyn liu

thc vt bng axit long, nhit cao (0,4-0,7% H2SO4; 120-

190o

C; 0,6-1,5 MPa). Qua qu trnh thy phn thu c dch thyphn (dung dch ng v cc sn phm phn hy chng), cht

thi l lignin thy phn. Cc pentoza v hexoza trong qu trnh

thy phn b kh nc to thnh fuafurol v axit levulinic tng ng.

Nhc im ca thy phn bng axit long l cc monosacarit b

mt mt tng i nhiu do b phn hy. La chn iu kin thy

phn (nng axit, nhit , thi gian phn ng) c th gim mtmt monosacarit. Trong khi , cng ngh thy phn bng axit

long n gin hn nhiu so vi s dng axit m c, iu

lm cho phng php thy phn bng axit long c ng dng rng

ri. Khi thy phn bng axit m c, mt mt ng l ti thiu,

song cng ngh ny i hi thu hi axit v buc phi s dng cc

thit b chu axit.Tip , dch thy phn c ch bin ty thuc vo chuyn

ngnh sn xut. Bng cng ngh tng t nh vy ngi ta ch bin

dch en thu c trong sn xut xenluloza bng phng php nu

sunphit.

Dich thy phn c th c ch bin bng cc phng php ha

hc hoc ha sinh. Cc sn phm ca sn xut thy phn c th lcc monoxacarit tinh th (glucoza, xyloza) v fuafurol. Cng ngh

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ha sinh cho php sn xut ra cn etylic, thc n gia sc, vvTrong

ch bin ha hc, t cc monoxacarit c th sn xut cc loi ru

nhiu ln ru, l cc pentit v hexit (xylit, sorbit, mannit) bng

cc phn ng kh cc pentoza v hexoza tng ng, xi ha ng,

vv

Trong mi ngnh sn xut, cn chn nguyn liu sao cho thnh

phn ca chng p ng c hiu xut ti a cc sn phm theo

yu cu. C mt thi gian di, mt lng cht thi v cng ln ca sn

xut thy phn, l lignin thy phn, khng c s dng. Hin

nay, ngi ta iu ch c rt nhiu sn phm c gi tr t lignin

v ngy cng c thm nhng phng php mi y trin vng v

vic s dng cht thi cng nghip ny.

3.7. Mt s polysacarit ch yu ca hemixenlulo3.7.1. Xylan

Xy lan l polixacarit thuc hemixenlulo ph bin ph binnht. Chng c trong tt c cc loi cy g, ha tho v mt s loithc vt khc. Xylan ca cc loi thc vt khc cng khc nhau bithnh phn v cu to phn t. Nhng khc bit ny c th c sdng lm du hiu phn bit cc nhm thc vt.

Xylan l cc polysacarit, m i phn t ca chng cu to chyu t cc mch ca xyloza, l cc gc -D-xylopiranoza. Tt ccc xylan l cc polysacarit hn th, ngai tr xylan ng th cacy sy alpha (esparto). y l mt polysacarit c mch nhnh,

trong mch chnh cc gc -D-xylanpiranoza lin kt vi nhau bngcc lin kt glicozit 14. Xylan ca cc thc vt khc c mch chnhng th. Cc xylan mch thng ng th ch c trong cc loirong bin.

Trong g cy l rng, xylan l polysacarit chim i a strong s cc hemixenlulo. c th ca cc cy l rng l chng chaglucoronoxylan, l mt polysacarit mch nhnh, m mch chnh ktcu t cc gc -D-xylanpiranoza kt ni vi nhau bng cc lin ktglicozit 14, cc mch nhnh l cc gc ca axit 4-O-metyl-D-glucoronic hoc D-glucoronic, kt ni vi mch chnh bng cc lin

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kt glicozit 12. Cng thc cu to v k hiu ca glucoronoxylanc biu th trn hnh 3.11. Cng thc cu to ca cc monosacaritcu thnh c trnh by phn trc (xem hnh 3.1). i khi,trong cc mch nhnh ca glucoronoxylan c cha cc gc -D-

xylanpiranoza. Trong thnh phn ca glucoronoxylan ca cy lrng c cha c nhm axetyl COCH3 vi hm lng 12-19% (chng 01nhm trn 02 mt xyloza). Cc nhm axetyl ca cc glucoronoxylanchim 3-6% khi lng g.

Trong cc glucoronoxylan cha cc mt axit 4-O-metyl-D-glucoronic, hm lng ca nhm (-OCH3) chim 1,5-3%. Trong mi tr-ng axit, nhit cao, cc nhm ny d dng b tch ra. Khi thyphn hoc nhit phn g, cc nhm axetyl to thnh axit axetic,

cn cc nhm metoxyl th to ra methanol. Bc trng phn caglucoronoxylan vo khong 100-200, song thm ch trong cng mtcy, xylan cng khng ng nht v bc trng phn (dao ngtrong khong 20-260, thm ch cao hn na).

i vi cy l kim, cu to ca xylan cn phc tp hn so vi cyl rng, song hm lng ca chng trong g li t hn, bi laipolysacarit nhiu nht trong g cy l kim l mannan. Trong g cyl kim c arabinoglucoronoxylan (arabino-4-O-metylglucoronoxylan).Chng c mch nhnh nhiu hn so vi glucoronoxylan ca cy lrng, trong cc mch nhnh, ngoi cc gc ca axit 4-O-metyl-D-glucoronic v D-glucoronic (01 mt axit trn 3-5 mt xylan), kt nivi nhau bi cc lin kt glicozit 12, cn cha cc gc -L-arabinofuranoza, kt ni vi mch chnh bng cc lin kt glicozit13.. Thng gp c cc mch nhnh c -D-xylanpiranoza (lin kt13). Xylan ca cy l kim khng cha cc nhm axetyl, ngai tr

mt s trng hp. Cu to ca arabinoglucoronoxylan trong g cy lkim c biu th hnh 3.12. Trong g cy l kim ngoiarabinoglucoronoxylan , r rng, cn cha mt lng nhglucoronxylan.

Trong xylan cacy l rng v mt s cy l kim ngi ta tmthy mt c im th v trong cu to u mt kh ca mchchnh. Trc mt kh u mt ca D-xyloza l cc mt -L-ramnozav axit -D-galactoronic. u mt kh ca mch xylan c th biudin bng cng thc k hiu sau:

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~4)--D-Xyn-(13)--L-Ramn-(12)--D-GalOn-(14)--D-Xyn

C gi thit cho rng, chnh cu trc ny to cho u mt khca xylan c kh nng bn vng trc kh trng hp.

Hm lng ca glucoronoxylan trong g cy l rng chim trungbnh 20-30%, arabinoglucoronoxylan trong g cy l kim chim 5-11%. Trong sn xut thy phn, ngoI g ra ngi ta cn s dng ccph thi nng nghip khc, tc l cc thc vt thuc h ha tho,vv. Cc loi ha tho c c im l trong thnh t bo cachng ngoi glucoronoxylan cn c arabinoglucoronoxylan chanhiu mch nhnh. Trong mt s ha tho (nh li ng) c cha ccarabinoxylan.

COOH

OH

OH

O

H3CO[HO]

~O

OH

OH

O

O

OH

OH

O

O

OH

O

O

O

OH

OH

~

O

4)--D-Xyn-(1 4)--D-Xyn-(1 4)--D-Xyn-(1 4)--D-Xyn-(12

14-O-Me--D-GluOn

[-D-GluOn]

Hnh 3.11. Mt mng i phn t glucoronoxylan (khng v ccnhm axetyl)

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COOH

OH

OH

O

H3CO[HO]

OH

OH

O

O

OH

O

O

OH

O

O

O

OH

~

O

OO

O

OH

O

OH

HO

O

OH

OH

HOH2C

4)--D-Xyn-(1 4)--D-Xyn-(1 4)--D-Xyn-(1 4)--D-Xyn-(12 3 3

1 1 1

4-O-Me--D-GluOn -L-Araf -D-Xyn

[-D-GluOn]

Hnh 4.12. Mt mng i phn t arabinoglucoronoxylan

3.7.2. Mannan

Mannan thuc nhm hecxozan v l polysacarit chnh cahemixenlulo trong g cy l kim. Hm lng mannan trong g cy lrng khng ln lm.

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Mannan l cc polysacarit m i phn t ca chng ch yuc to bi cc mt ca manoza cc gc -D-manopiranoza. Ccmannan ng th rt him thy. Ph bin rng ri l cc mannanhn th, ch yu l mch nhnh, c chui polyme chnh cu to t

D-manoza v D-glucoza, chng cn gi l glucomannan.Nhng glucomannan n gin nht l cc glucomannan ca gcy l rng. Chui phn t ca chng cu to bi cc gc -D-manopiranoza v -D-glucopiranoza lin kt vi nhau bng cc linkt glicozit. Chui phn t ca cc glucomannan ny c th l mchthng hoc mch nhnh. Mch nhnh cha cc gc -D-manopiranoza kt ni vi mch chnh bng cc lin kt 12, 13hoc 16. T l Man:Glu giao ng trong khong t 1:1 n

2,3:1, trung bnh l (1,52):1 i vi cc loi cy l rng. S phnb cc mt trong chui phn t l v trt t, trong chiu dicc an cu to bi cc mt manoza v glucoza cng thay i.Bc trng phn ca glucomannan ca cy l rng khng ln lm vvo khang t 60-70 n 100. Hm lng mannan trong g cy lrng vo khong t 1% (cy bch dng) ti 34% (trong cc gingcy khc). Do hm lng trong g nh nn cc mannan c nghin cu

rt t.Cng thc cu to v k hiu ca glucomannan c biu th trnhnh 3.13. Cc cng thc ca cc monoxacarit cu thnh ctrfinh by cc phn trc.

CH2OH

CH2OH

OH

OHOO

O

CH2OH

OH OH

CH2OH

OH OHO

O

O

O

OH OH

4--D-Mann-(14)--D-Glun-(14)--D-Mann-(14)--D-Mann-(1

Hnh 3.13. Mt mng chui glucomannan mch thng

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Cc galactomannan tan trong nc l thnh phn cha yu canha thc vt , s dng lm cc ph gia trong keo giy v tng bn ca giy, nh guaran v carubin.

G cy l kim cha ti 20% cc mannan khc nhau, l

glucomannan v galactoglucomannan. Thnh phn v cu tomannan ca cy l kim ph thuc vo ging cy. Song quan imca cc nh nghin cu v vn ny cng khc nhau. TheoSharkov, th trong cc ging cy g mm (nh cc loi thng) chaphn ln l cc glucomannan mch nhnh, galactogluccomannan chchim mt lng rt nh, cn cc ging cy g cng (nh cy tng,linh sam) th cha ch yu l galactoglucomannan. Fengel vVegener th li cho rng, trong tt c cc lai cy l kim mannan

tn ti di dng galactoglucomannan c hm lng cc mt galactozadao ng trong mt gii hn rng t 2 n 20%. Ngi ta cho rng,cc mt glucoza v manoza trong chui phn t c phn bkhng ng u, song khng lai tr trng hp kh nng cu tokhi ca chui glucomannan (mi khi glucoza cha ti 3 mt vkhi manoza cha ti 5 mt).

Galactoglucomannan l nhng polysacarit mch nhnh, cc mchchnh ca chng cu to bi cc gc -D-manopiranoza v -D-glucopiranoza kt ni vi nhau bng cc lin kt glicozit 14. Ccnhnh cha cc gc -D-galactopiranoza ni kt vi mch chnh bngcc lin kt glicozit 16. C trng hp cc nhnh l cc mt -D-manopiranoza. Cc nhnh c th ni kt vi mch chnh bng cclin kt 13. Cng thc cu to v k hiu ca galactoglucomannanbiu th hnh 3.14.

Cc i phn t ca mannan trong g cy l kim c axetyl

ha: hm lng cc nhm axetyl chim 5-8%. Chng c th phn b bt k v tr t do no (C2, C3, C6) ca cc mt D-manoza. i khik c cc mt D-glucoza cng b axetyl ha.

Trong g ca cng mt ging cy c th cha cc phngalactoglucomannan khc nhau: phn ha tan trong kim vi t lMan:Glu:Gal l (34):1:(0,10,2) v phn ha tan trong nc cha l-

ng ln cc mt galactoza. S khc bit nhau v tnh tan c to rabi phn nhnh cao hn ca cc galactoglucomannan tan trongnc, ng thi bi bc trng phn khc nhau (dao ng trongkhang t 20..30 n 100140) v kh nng tip cn ca cc

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mannan trong thnh t bo. Chc l, trong thnh phn ca ccxenlulozanc cha cc glucomannan mch thng v mch t nhnh.

CH2OH

CH2OH

OH

OHOO

OOH OH

CH2OH

OH OHO

O

O

OOH OH

OH

OH

OCH2OH

HO

CH2

O

Hnh 3.14. Mt mng i phn tgalactoglucomannan

(khng c cc nhm axetyl)

Cch xc nh mannan trong g. Cc mannan trong g thng l ccpolysacarit d thy phn, song cng tn ti cc mannan kh thy phn. Kh

thy phn l mt phn glucomannan v fructomannan, khi thy phn ngai D-manoza ra cn to thnh c D-fructoza. Ngi ta tm thy fructoza ch trong ccsn phm thy phn cc polysacarit kh thy phn. Hiu xut fructoza chimkhong 12% i vi g cy l kim. Trong mt s loi thc vt cn cha mtpolysacarit l fructan (levulan), song trong g nhng polysacarit tng tkhng c.

Xc nh hm lng mannan trong g da trn phng php thy phnchng to thnh D-manoza v xc nh monosacarit ny tip sau .

Trong phn tch g, ngi ta xc nh hm lng cc mannan d thy phn

v hm lng chung ca mannan. xc nh hm lng mannan d thy phnngi ta tin hnh thy phn g bng axt v c long, thng l dung dch 5%HCl, trong iu kin un si, cn xc nh hm lng chung th thy

38

4--D-Mann-(14)--D-Glun-(14)--D-Mann-(14)--D-Mann-(1 6

1-D-Galn

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phn bng axt m c, thng l dung dch 72% H2SO4. Manoza trong dchthy phn c xc nh bng cc phng php sc k (bng cch ny c thxc nh c D-fructoza ring bit).

Phng php phn tch trng lng c in da trn cch xc nh D-manoza di dng dn xut c trng ca n l phenylhydrazon, b kt ta di tc

dng ca phenylhydrazin theo phn ng sau:

_ O

H__ _

__ _ _ _+

H2OCH2OH (CHOH)4 C NH2 NH C6H5 CH2OH (CHOH)4 CH=N NH C 6H5

Phenylhydrazon khng tan trong nc, etanol v axeton. . kt ta honton cn c c dung dch manoza. Song, dung dch manoza khng th cc trong mi trng c cc axit v c ( trnh s bin i manoza tothnh hydroxymetylfurfurol), cng khng th trong mi trng kim hac mi tr-

ng trung tnh (trnh trng hp bin i epime D-manozaD-glucoza hocng phn ha D-manoza thnh D-fructoza v ngc li). V vy, dch thyphn sau khi trung ha bng kim ngi ta axit ha bng axit axetic ti pH 5,sau mi c c. Ngy nay, phung php phn tch trng lng gn nh c thay th honton bng cc phng php sc k.

3.8 Cc polysacarit tan trong nc

Nh trnh by trn, s phn lai cc polysacarit phi xenlulo

thnh hemixenlulo v cc polysacarit tan trong nc (cc cht chitxut cao phn t tan trong nc) ch yu mang tnh quy c. Mt s

cc polysacarit v poliuronit, nh tinh bt trong cc t bo nhu m,

nha trong cc rnh bo, vv, khng tham gia cu thnh nn thnh

t bo, chng ng vai tr bo v v d tr, v c th chit ra

khi g bng nc. Mt s khc, nh arabinogalactan, c th cha trong

thnh t bo, nhng cng c th chit ra khi g bng nc nng. Cccht pectin, l cc cht c hnh thnh trong giai an phn chia

t bo tng tng, m nhim chc nng cu trc nn thnh t bo

v v vy chng tham gia vo thnh phn ca tm gia. Mt phn

cc polysacarit v poliuronit tng t tr nn kh tip cn hn v

ha tan chng i hi cc iu kin tc ng c bit, chng

hn tc ng ca cc dung dch kim long. chit hon ton cc

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cht pectin buc phi s dng n dung dch ocxalat v nitrat

amn.

Cc polysacarit tan trong nc khc vi hemixenlulo v cc c

im cu to ha hc. Cng nh tt c cc polixacarit phi xenlul,

chng c th l cc pentozan hoc hexozan. Poliuronit l cc hp

th mang c trng ca gluxit, thnh phn c bn ca chng l cc

hexuronan, l cc chui polime to nn bi cc gc axit uronic.

Khi phn lai cc polysacarit v poliuronit tan trong nc theo

monosacarit cu thnh, ngi ta phn bit glucan (k c tinh bt),

galactan, arabinan, galacturonan. Trong s cc cht chit xut cao

phn t tan trong nc c th k n c glycoprotein, l cc hp th

cha cc chui oligo- hoc polysacarit lin kt vi protit bng cc lin

kt ha tr (k c cc lin kt glicozit). Trong cy chng m nhn

cc chc nng d tr v sinh l.

3.8.1. Tinh bt v glucan

Trong thc vt, k c thc vt g, ngai xenlulo ra cn c cc

polysacarit khc m mt phn chui phn t chnh hoc ton b

chui phn t cu to bi cc gc D-glucoza, l cc glucan.

Chng thuc lai hexozan. Mt trong nhng cht dinh dng quan

trng nht ca cy, mt glucan ng nht, l tinh bt.

Trong thc vt, k c thc vt thn g, tinh bt c tng hp t

D-glucoza cha trong cc m m trong tinh bt c hnh thnh

dn dn di dng ht. Ht tinh bt phn b trong dc g, cc t bo

sng ca libe, song khng c trong phn li cy. Tinh bt cng c

to thnh l v trong ht cy. Lng tinh bt b thay i theo ma.

i vi i a s cy, tinh bt hnh thnh vo ma h trong m

mc, vo ma thu hm lng ca n t cc i. Ma ng hm lng

tinh bt gim dn hoc bin mt hon ton do qu trnh bin i

thnh ng (glucoza). Vo u ma xun li xut hin hm lng

cc i th hai ca tinh bt. n cui ma xun chng li phn hy,

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v khi cy bt u ln chi glucoza c chuyn ra l, n

tham gia vo xc tin qu trnh quang hp. Cc ng to thnh

trong qu trnh quang hp lu chuyn theo libe v cc tia li vo

trong g, ni tinh bt li c hnh thnh.

Trong cc ti liu, hm lng tinh bt ch c ng ti i vi

cc loi cy l rng, trong cc m ca chng hm lng tinh bt vo

thi im cc i t ti 2-5%.

Tinh bt khng phi l mt cht ha hc ring bit. N l hn hp

cc polysacarit. Cc polysacarit trong tinh bt c th chia lm hai

phn l amiloza v amilopectin, khc bit nhau v bc trng phn

v cu trc khng gian ca i phn t.

Amiloza l cc polysacarit mch thng (hoc l c t mch

nhnh), cc i phn t ca chng c cu to bi cc gc -D-

glucopiranoza kt ni vi nhau bng cc lin kt glicozit 14. Bc

trng phn ca cc polysacarit ny trong gii hn t 200 n 1000.

Cng thc cu to ca amiloza hnh 3.15. Trong cc chui amiloza

ngi ta cn tm thy mt lng nh cc lin kt 16,13 v 12.

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CH2OH

O

OCH2OH

OH

OH

O

OOH

OH

O

OH

OH

CH2OH

Amiloza

OH

OH

O

OH

OHO

O

OH

OH

CH2OHO

O

CH2OH

CH2OH

O

O

OOH

OH

O

OH

OH

CH2OH

CH2CH2OH

OH

O

O

O

OH

OH

CH2OH

OO

OH

OH

CH2OH

O

O

O

O

Amilopectin

Hnh 3.15. Mng i phn t tinh bt

Amilopectin l mt polysacarit c nhiu mch nhnh. Cc chui

chnh ca n cu to bi -D-glucopiranoza kt ni vi nhau bng

cc lin kt glicozit 14. Cc nhnh di c cng cu to kt ni vi cc

mt ca mch chnh bng cc lin kt 16 v 13 s lng nh

(hnh 3.15). Cc mch nhnh cng c th c cc nhnh. Bc trngphn vo khong 600-6000.

Hm lng cc phn amiloza v amilopectin trong tinh bt ca cc

lai cy khc nhau cng khc nhau, tng ng l 15-25 v 75-85%.

Trong cc ht tinh bt chng phn b ng u hn. Tinh bt to

ra mu xanh khi tc dng vi i-t, trong amiloza c mu xanh

m, cn amilopectin th c mu tm.

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Mt glucan ng th na ca g l caloza. Polysacarit ny m

nhn chc nng bo v trong cc th bo roi ca libe v nhu bo

ca m mc, to thnh cc lp bo v trn mng khoang bo, ng

thi cha trong thnh t bo mc thim. Caloza l mt polysacarit

mch thng, i phn t ca n c cu to bi cc mt -D-

glucopiranoza kt ni vi nhau bng cc lin kt glicozit 13.

Caloza c kh nng to ra cc cu trc si.

Trong thn cy c v cc i din ca h ha tho (la m,

tre) c cha cc -glucan mch thng. Chui -glucan c xem nh

chui ca polysacarit m trong cc mng ging xenlulo (lin kt

14) c kt ni vi nhau bng cc lin kt 13. Vy th, cc

glucan ca ha tho phi thuc nhm cc polysacarit dng

hemixenlulo thc hin chc nng cu thnh v cng vi cc

hemixenlulo v xenlulo to thnh mt h cu trc thng nht trong

thnh t bo.

Mt s glucan mch nhnh hn th, nh xyloglucan, c tm thy

trong g v cc m ca cc lai thc vt khc. Xyloglucan l mt

polixacarit c nhiu mch nhnh, chui chnh c cu thnh bi cc

gc -D-glucopiranoza kt ni vi nhau bng cc lin kt glicozit

14. Cc mch nhnh l cc gc -D-glucopiranoza kt ni vi mch

chnh bng cc lin kt 16. Xyloglucan thc hin chc nng kt

ni cc si xenlulo vi cc hemixenlulo trong g, chng tham gia vo

vic hnh thnh cu trc thnh t bo, iu chnh s sinh trng

ca t bo.

3.8.2. Galactan v arabinogalactan

Galactan cng thuc lai hexozan nh mannan v glucan.

Galactan l cc polysacarit m i phn t ca chng c cu

thnh ch yu t cc mt galactoza. Galactan tng i ph bin

trong thin nhin, song trong g hm lng ca chng rt thp (ch

chim chng 0,5-3%). Trng hp ngoi l l cy haroxylon cha ti

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9% galactan. C th tnh ho nc ca galactan to cho lai cy

ny c kh nng sinh trng trong kh hu kh nng. Trong cc ging

thng khc nhau c cha mt polixacarit hn th, l

arabinogalactan. Galactan v arabinogalactan u tan trong nc.

Cu to ca cc galactan ng th rt a dng v ph thuc

vo ging cy. Ban u, ngi ta cho rng, trong g ca cc lai

cy l rng, kh nng c cy l kim na c cha galactan ng

th mch thng, m cc i phn t ca chng c cu thnh t

cc gc -D-galactopiranoza kt ni vi nhau bng cc lin kt 14.

Cng thc cu to v k hiu ca mt galactan nh vy c biu th

hnh 3.16. Cng vi nhng galactan mch thng, ngi ta cn tm

thy cc galactan mch nhnh c cc nhnh kt ni vi mch chnh

bng cc lin kt 13 v 16. C th c c lin kt 13 trong

mch chnh. Trong mt s loi cy l rng, nh cy bch dng, ngi

ta cn tm thy mt s galactan cu to t cc gc -D-

galactopiranoza.

O

OH

OH

CH2OH

O O

CH2OH

OH

OH

OO

CH2OH

OH

OH

O

4--D-Galn-(14)--D-Galn-(14)--D-Galn-(1

Hnh 3.16. Mt an chui galactan mch thng

Ngy nay, ngi ta cho rng, trong i a s cc trng hp cc

galactan tham gia vo t hp cc cht pectin. Do nhng kh khn

trong vic tch cc polysacarit tan trong nc thnh dng tinh khit v

nguyn dng, khng phi lc no cng c th phn bit c dng

galactan ng th hay hn th. Qua cc kt qu nghin cu, ngi

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ta i n kt lun rng trong g cy l kim, v c th c cy

l rng na, c cha khng phi l cc galactan ng th m l cc

galactan hp th, trong c c cc galactan mang nhm a-xit, tc

cha cc mt a-xit uronic. Trong s cc galactan hp th ph bin

nht l cc arabinogalactan mch nhnh c cu to khc nhau.

Arabinogalactan rt c trng i vi g thng. Trong g thng n l

mt polysacarit hp th c nhiu mch nhnh, mch chnh cu to

bi cc mt -D-galactopiranoza kt ni vi nhau bi cc lin kt

13, cc mch nhnh l cc gc - v -L-arabinofuranoza kt ni vi

mch chnh bng cc lin kt 16.Tng quan cc mt galactoza v

arabinoza trong i phn t vo khong 6:1, song c th dao

ng (thm ch trong cng mt ging cy) trong gii hn ln t 9,8:1

n 2,6:1. Mc phn nhnh (s lng v chiu di nhnh) cng

khng c nh.

Tn ti cc arabinogalactan c cc nhnh ngn (di dng mt

mt). Cng thc cu to v k hiu ca arabinogalactan biu th

hnh 3.17. Trong cc arabinogalactan nhiu mch nhnh ngoi cc

nhnh dng mt mt l cc mt D-galactopiranoza v L-

arabinofuranoza cn c mt i, mt ba v ln hn, ng thi c

c cc gc a-xit - v - glucoronic (glucuronoarabinogalactan).

Trong cc mch nhnh c th c cc mt L-arabinopiranoza kt ni

vi cc mt L-arabinofuranoza.

Cc arabinogalactan c a tn cao v cha cc phn c bc

trng phn t 100 n 600 v thnh phn ha hc khc nhau.

Trong cy thng arabinogalactan ch yu tp trung phn li cy,

chng phn b tng dn theo hng t tm ra ngoi v t hm l-

ng cao nht cc vch vng tui k vi dc g.

Hm lng arabinogalactan trong mt s cy l kim khc t hn,

t 1 n 3%. Cu to ca chng ph thuc vo ging cy. Ngoi

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cc mch nhnh l cc gc a-xit D-glucoronic ngi ta cn tm thy c

gc a-xit D-galacturonic.

G cy l rng cha mt lng nh cc arabinogalactan (1-1,5%),

m cc mch nhnh ca chng c cha cc gc ramnoza, l cc

ramnoarabinogalactan, i khi cng tm thy c

galactoronoramnogalactan. C l , cc polysacarit ny c trong thnh

phn ca nha cy. Do cc arabinogalactan tan c trong nc, nn

ngi ta thng xp chng vo nhm nha cy.

O

OH

OH

O

OH

O

O

O

OO

OH

OH

CH2OHO

CH2

HO

HO

HO

CH2

O

OH

OH

HOH2C O

O

OH

OH

HOH2C

CH2

O

HO

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Hnh 3.17. Mng i phn t arabinogalactan mchnhnh

Arabinogalactan gy nh hng ng k trong sn xut giy v

xenluloza t g thng. to iu kin tt cho vic ty lignin v

tng cc ch s cht lng ca xenluloza trong phng php nu

sunfat, ngi ta thng ty arabinogalactan trc bng phng php

thy phn hi. Dch thy phn ny mang ch bin nh cc lai

dch thy phn khc trong sn xut thy phn. Cng nh cc

hexoza khc, galactoza c th ln men thnh cn etylic, arabinoza

v galctoza c th s dng nui cy men thc n gia sc. Mt trong

nhng phng php ch bin trin vng l hyr ha galactoza

thnh ru unxit. Phng php chit arabinogalactan t g thngbng nc nng c a vo ng dng. Cc dung dch

arabinogalactan c kh nng kt dnh tt v c th c s dng

keo giy thay cho tinh bt trong sn xut giy.

Cc ch phm arabinogalactan phc v cc mc ch nghin

cu c iu ch t g thng bng phng php chit xut bng nc

nng v kt ta bi etanol. Arabinogalactan tinh ch c dng bttrng mn.

3.8.3. Arabinan

Cc arabinan l cc polysacarit m i phn t ca chng cu

to ch yu bi cc mt arabinoza. Song vn v s tn ti ca

mt arabinan ng th ring bit trong g cn cha r rng.Arabinan tng i ph bin trong thin nhin v trong cc loi cy

3)--D-Galn-(13)--D-Galn-(13)--D-Galn-(16 6 6

1 1 1-L-Araf -D-Galn -L-Araf

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cha nhiu cht pectin chng c mt s lng tng i ln. C th,

trong cc cht pectin arabinan lin kt vi cc cht khc, song cng t-

ng i d tch ra khi chng. Nh nu trnh by trn, L-

arabinan tham gia vo thnh phn ca mt s polysacarit hp th

nh arabinoglucuronoxylan hay arabinogalactan. Khi thy phn g

ngi ta tm thy gn nh han tan lng arabinan trong s cc sn

phm thy phn phn polysacarit d thy phn. Hiu xut

arabinan chim 1-1,9% t g cy l kim v ti 4,2% t cy l rng.

Di dng tinh khit, arabinan c ch xut t c ci ng, to,

ht d, cam, quyt v t g thng (Pinus pinaster) 0,31%. Ch

phm arabinan c dng bt, d tan trong nc v cc dung dch kim.

Bc trng phn ca arabinan khng cao lm v ph thuc vo

ngun gc ca ch phm.

Arabinan ch xut t c ci ng c bc trng phn khong 40-

50. N l polixacarit mch nhnh c mch chnh cu thnh t cc

mt -L-arabinofuranoza kt ni vi nhau bng cc lin kt 15. Cc

mch nhnh l cc gc n phn -L-arabinofuranoza kt ni vi

mch chnh bng cc lin kt 13.

Cng thc cu to v k hiu ca arabinan c biu th hnh

3.18.

Trong phn tch g cc arabinan c xp vo nhm pentozan.

L-arabinoza trong dch thy phn c xc nh bng cc phng

php sc k.

O

OHHOH2C

OO

OH

OH

O

OHOH

O

HOH2C OH

O

OH

O

OO

CH2O

CH2

CH2

O

OH

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Hnh 3.18. Mng i phn t arabinan

3.9 Poliuronit

Trong phn tch g, ngi ta thng hiu cc poliuronit l nhng

gluxit cao phn t m i phn t ca chng (mch thng, hoc

mch chnh ca cc i phn t mch nhnh) c cu thnh ch

yu t cc mt n phn l cc gc ca a-xit hexuronic. Cc chui

phn t ny c th l chnh cc hexoronan hay cc dn xut

metyl ha. Nh trnh by trn, cc mt ca cc a-xit hexoronic

cn tham gia vo thnh phn cc mch nhnh ca cc polysacarit

mang gc a-xit (glucoronoxylan, galacturonogalactan, ). Cc

hexoronan trong thc vt, k c thc vt thn g, tham gia vo

thnh phn ca cc cht pectin, nha cy v nim dch.

3.9.1. Axit uronic trong g v cch xc nh chng

Khi phn tch dch thy phn phn d thy phn ca g, ngi

ta tm thy mt s a-xit hexoronic, nh a-xit D-glucoronic, 4-O-metyl-

glucoronic v D-galactoronic. A-xit D-glucoronic v ete ca n l 4-

O-metyl-D-glucoronic c to thnh nh qu trnh thy phn cc

xylan mang gc a-xit, l glucoronoxylan trong g cy l rng v

arabinoglucoronoxylan trong g cy l kim, cn a-xit D-galacturonic

ch yu c to thnh sinh ra t cc cht pectin v cc polysacarit

v poliuronit tan trong nc khc. Trong thin nhin ta thng gp a-xitmanouronic, song trong g chng li khng c mt. Hm lng a-xit

49

5)--L-Araf-(15)- -L-Araf -(15)- -L-Araf -(1

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uronic trong g khng cao lm. G cy l rng cha nhiu a-xit

uronic hn (3,5-6%), cn g cy l kim ch cha 1,8-4,9%. Mc d

hm lng trong g nh, song cc a-xit uronic ng vai tr rt quan

trng. Cc cht pectin m bo s lin kt ca cc t bo trong

m. Nh tnh ho nc cao (cao hn so vi cc hemixenlulo) do trong

phn t cha cc nhm cacbxin, cc mt a-xit uronic trong cc

polysacarit tp cht cn li trong xenlul k thut, lm tng kh

nng ht nc ca si v to iu kin thun li cho vic nghin bt

giy v tng bn ca giy.

Phng php xc nh hm lng chung ca a-xit uronic trong g da trn

phn ng kh cacboxyl di tc dng ca cc a-xit v c mnh. Thng thng, ngita s dng dung dch 12-19% HCl. Cng lc din ra cc phn ng thy phn ccpolysacarit. Cc mt a-xit uronic bin i thnh cc pentoza, tip , sau khithy phn cc lin kt glicozit v kh nc chng bin thnh fuafurol (hnh4.19). Tuy nhin, hiu xut fuafurol theo l thuyt ch chim 35-40%.

Bc phn tch tip theo l xc nh -xit cacbon CO2 bng cc phngphp phn tch trng lng v chun da trn phn ng hp th khcacbonic bng KOH, NaOH hoc BaOH. Theo lng kh CO2 thi ra trong phnng ta tnh c lng a-xit uronic trong g, thng quy i theo hexoronan.Thnh phn cc a-xit uronic c xc nh bng cch phn tch dch thyphn bng phng php sc k. Ph bin nht l cc phng php sc k dchhiu qa cao.

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Hnh 3.19. Cc phn ng ha hc trong xc nh a-xit uronic

3.9.2. Cc cht pectin

Cc cht pectin l t hp cc cht ng mang tnh a-xit, cthnh phn ch yu l a-xit pectic, ngoi ra cn c arabinan vgalactan. Cc cht pectin tng i ph bin trong thin nhin,chng c nhiu trong cc lai nc sinh t, qu, r cy, thn cy vl ca nhiu lai tho vt, trong cc m ha g ca cc cy thng.

Trong cy ta thng gp cc cht pectin tan v khng tan trong

nc lnh. Cc cht pectin ha tan cha trong nc tri cy vi s lng t-ng i ln. Cc cht pectin khng tan, hay cn gi l protopectin,cha trong cc m ca r, c (nh c rt, c ci ng, ), qu (to,l, anh o, cam quyt, ), hm lng ca chng c th t 10-25%hoc nhiu hn na. Trong si ca cc cy ly si (cy bng, cylanh) v v ht hm lng cc cht pectin ch t 0,5-2%. Cc chtpectin tan v khng tan c lin quan n nhau, v cc m thc vtchng c th bin i ln nhau. Protopectin c th chuyn thnh

dung dch bng cch un vi nc nhit 100oC. Khi din rathy phn nh. Qu trnh ny din ra khi ta nu cc thc n thcvt.

Tnh cht ca cc cht pec tin c ngun gc khc nhau cngkhc nhau. Chng hn phn t lng ca pectin dao ng khangt 3.103 n 3.105. Cc cht pectin ring r c th phn bit theomc metyl ha cc nhm cacbxin, phn nhnh, Dung dchcc cht pectin trong nc c nht cao v khi phn t lng di20000 chng c th ng t.

Hm lng cc cht pectin trong g khng cao lm. Chng chchim 0,5-1,5% (rt him khi t 3-4%). Pectin c nhiu trong ccm libe (khong 10-15%, i khi nhiu hn), r cy, phn tngtng. Cy l kim cha t pectin hn cy l rng. Cy cng ln lngpectin trong cc m g cng gim, song khng phi do chng bbin i m l do c s thay i v tng quan khi lng ca cc

cht do trong g to thnh cc cht khc nh polysacarit v lignin.

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Trong g, pectin tham gia vo thnh phn ca tm kp gia v cc m gi chng cng vi lignin to ra s bm kt cht ch ca cct bo vo m. Pectin cn cha trong i-nhy ca mng l vin.

Trong thi gian pht trin ca t bo v thc vt ni chung, cc cht

pectin khng ngng thay i. Chng m bo bn v tnh dodai cho cc cy v m non. Tnh ho nc ca cc pectin ng vaitr quan trng trong qu trnh trao i nc trong thi k sinh trngca cy. Nc do cc cht pectin hp th khng b ng lnh v khbay hi, v vy cc cht pectin to cho cc cy v m non kh nngchng lnh v kh ho.

Cc cht pectin khng th tch ra khi g di dng tinh khit vnguyn dng. V vy m thnh phn v cu to ca chng n nay

vn cha c nghin cu y .Thnh phn c bn ca tt c cc cht pectin l a-xit pectic.

Ban u, ngi ta cho rng a-xit ny l a-xit poligalacturonic bmetyl ha tng phn (hnh 3.20a). Ngy nay, ngi ta li gi thitrng n l mt galacturonan d th hay ramnogalacturonan.

OO

O

O

O

O

O O

COOR

OR'

OR' COOR

OR'

OR'

O

OH

O

OH

OHCH3

R''OCOOR

COOR

O

OR''O

OH

CH3

O

OCOOR

OR'

OR'

OR'

OR'

R =H, CH3;

R' =H, CH3CO, chui gluxit;

R" =H, chui gluxit

a

OC

OHCa

OC

O

O

C

O

HO C

OChui galacturonan

b

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Hnh 3.20. Mt on chui a-xit pectic (a) v s hnh thnhcc cu ni

can-xi ngang (b)

i phn t ramnogalacturonan cu to t cc mt a-xit -D-galacturonic di dng vng piran, kt ni vi nhau bi cc lin ktglicozit 14, v cha cc mt -L-ramnopiranoza. Theo kin mts nh nghin cu, cc mt -L-ramnopiranoza to thnh cc antam phn c trng cha mt mt a-xit galacturonic v co scclin kt glicozit 12 v 14 (hnh 3.20b). Gia cc an ny c cc

an galcturonan ng th mch thng cu to t 6-12 mt a-xitgalacturonic. Trung bnh c ti 75% nhm cacbxin b metyl ha.Nh vy a-xit pectic cha cc nhm OCH3 d b tch v l thnh phnca cc nhm este.

Vn lin kt gia a-xit pectic v cc cht trong t hp cccht pectin nh arabinan v galactan n nay cn cha c sng t.Kh nng c thnht, l gia cc chui ramnogalacturonan, arabinanv galactan c cc lin kt ha hc. C th c s kt ni ca ccchui arabinan v galactan vo cc mt ca ramnopiranoza trongmch chnh bng cc lin kt glicozit 14 v vi cc mt ca a-xitgalacturonic bng cc lin kt 12 v 13 to ra mt cy trc phnnhnh mnh. Trong cc cht pectin c ngun gc thc vt khc nhautng quan gia cc cht a-xit v trung tnh, gia cc phn arabinanv galactan dao ng trong khang tng i ln. Cc c tnh vthnh phn v cu to ny ca cc cht pectin to cho chng nhng

tnh cht c o thc hin cc chc nng a dng.Mt phn cc nguyn t hyr ca cc nhm cacbxin t do

trong chui galacturonan c th c thay th bi cc nguyn t can-xi hay ma-nh. Cc Cation Ca2+ v Mg2+ to ra cc cu ni nganggia cc chui a-xit pectic v nh vy lm cho pectin mt kh nngtan trong nc. Thi gian gn y, ngi ta i n kt lun rng,ion can-xi kt ni cc nhm cacbxin ca bn gc a-xit galacturonicca hai chui k nhau nh hong ngc chiu nhau (hnh 14.20b).Cc cu trc c trong gel ca cc cht pectin v trong pectin t

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nhin. Cc kh nng trao i cation ca cc cht pectin to cho cyc kh nng tip nhn cht dinh dng v c t t qua h thng r.

3.9.3. Nha v nim dch

Cc khi nim v ging nhau v bn cht ha hc nh nha vnim dch cn cha c phn bit r rng, i lc rt kh phnbit gia nha v cc cht pectin. Theo quan im ca i a scc nh nghin cu, nha bao gm cc polysacarit v poliuronittatrong nc, s to thnh chng lin quan n cc qu trnh bnh lsy ra qua cc chn thng c hc v sinh hc ca v cy v mca cc tho mc. Nh vy, nha cy ng vai tr bo v. Cy cnha l cc cy nh anh o, mn, ), song ch yu l cc cy

nhit i. c bit n nhiu nht l nha rp tit ra t cy keo rp. Nha l mt hn hp cc polixacarit v polysacarit d thtrung tnh v mang gc a-xit, c cu to phc tp, cha cc mtmonoxacarit v a-xit hecxoronic khc nhau. Thnh phn v cu toca nha cha c nghin cu k.

Nim dch l cc polysacarit ging vi nha, song chng c mttrong cc tho mc khng b chn thng (v, r, l, ht). Khc vinha, nim dch l cc sn phm ca qu trnh trao i cht htsc bnh thng, chng m nhn cc cha nng d tr trong cy,ng thi gi nc trong cc m. Cu to ca nim dch n ginhn nha. Chng hn, trong mt s lai ht nim dch l ccgalactomannan tan trong nc.

3.10 Cc bin i ha hc ca polysacarit trong qu trnh tch loi lignin

Trng thi cc polysacarit trong g khi kh lignin ph thuc vo

iu kin din ra qu trnh ny, cu to ca i phn t v ccc im cu trc siu phn t ca chng. Cc c im nynh hng ti kh nng chu tc dng ha hc ca cc ha cht. Theokh nng chu cc tc dng ha hc, cc polysacarit c th chiathnh ba nhm:

- polysacarit tan trong dch nu;- polysacarit ngm ha cht;- polysacarit khng chu tc dng ca ha cht.Cc polysacarit thuc nhm th nht tan trong ha cht v

chuyn vo dung dich, nhm th hai gm cc polysacarit b trng,

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cn nhm th ba khng b trng trong dch nu. Cc polysacaritca hai nhm u chu tc dng ca dch nu, trong cc phnng ca cc polysacarit nhm th nht din ra ng th, cn nhmth hai - d th. nhm th ba ch c cc i phn t phn b

trn b ca cc cc siu phn t tc dng vi cc ha cht. Nhmth nht bao gm cc polysacarit tan trong nc (arabinogalactan, cccht pectin, tinh bt, ) . Trong dung dch kim, khi nng kimtng, nhm ny c m rng nh phn hemixenlulo. Nhm th haigm phn ln cc hemixenlulo v cc phn v nh hnh caxenlulo. Nhm th ba gm cc phn tinh th ca xenlulo v ccxenlulozan. Trong qu trnh nu ha tan ca cc polysacarit cth thay i, chng hn, do phn t lng v cu to thay i, do

cc lin kt lignin-gluxit b phn hy, thnh phn ca dch nuthay i. Cu trc ca cc polysacarit c th b bp li v khngchu tc dng ca cc ha cht.

Bt k mt qu trnh nu no cng bao gm mt t hp phctp cc qu trnh a dng chu nh hng ca cc yu t ha hc, lhc v hnh thi hc. Khi kh liclin, k c khi nu hoc khi tyxenlulo, khng ch c licnin chu cc tc ng ha hc m c phngluxit cng b bin i. Cc phn ng, ch yu l phn ng phnhy, ca cc polysacarit s quyt nh hiu sut v cht lng cabn thnh phm si thu c. Cn cc phn ng ca cc sn phmphn hy polysacarit trong dung dch s quyt nh phng n sdng chng. Trong sn xut xenlulo lm giy, ngi ta hng tilm sao gi c ti a phn cacbuahydrat ca g, cn khi sn xutxenlulo phc v ch bin ha hc th xenlulo phi c tinh chhon ton khi lignin v cc hemixenlulo.

3.10.1. Cc phn ng ca polysacarit trong mi trng axit

Khi kh lignin trong mi trng a-xit cc polysacarit ca g bthy phn. Bin i ca cc polysacarit trong iu kin thyphn c xc tc a-xit c trnh by phn trc (xem mc 3.6.Nhng nt c trng ca qu trnh nu nh hng n mc cacc bin i ny v din bin ca cc phn ng ph gn lin vi

tc dng ca cc cacbuahydrat vi cc cht ca dch nu. Nt ctrng ny c th xem xt qua trng hp nu sunfit din ra di tc

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dng ca cc dung dch dioxt lu hunh v cc baz (cc dung dchca a-xit sunfur v cc mui ca n). Nu sunfit c thc hintrong iu kin mi trung c tr gi pH khc nhau. Khi pH tngnng cc ion H3O+ gim v dioxit lu hunh thy ha bin thnh

cc ion hydrosunfit v sau to thnh cc ion sunfit khi pH tng. Trong iu kin nu sunfit (pH 3) din ra thy phn ccpolysacarit d thy phn. Trong g cy l rng chng l cc xylan,khi thy phn ch yu to thnh D-xyloza, a-xit xylobiuronic v mtlng nh cc a-xit anobiuronic. Khi nhit cao hn 100oC din ras kh axetyl ca cc xylan. Trong nu sunfit, cc polysacarit cacy l kim bn vng hn. Nguyn do l cc lin kt glicozit ca ccglucomannan thy phn chm hn so vi cc xylan v hm lng

xylan t hn. Kt qu l hiu xut xenlulo sunfit ch xut t gcy l kim ln hn nhiu do khi nu gi c phn ln cchimixenlulo khi phn hy. Song, ngi ta li s dng tnh cht dthy phn ca cc himixenlulo trong cy l kim mt cch hiu qukhi ty chng bng phng php thy phn trc khi nu trong snxut xenlulo sunfat dng cho ch bin ha hc. Cn phi lu rng,cc hemixenlulo cn li trong bn thnh phm xenlulo sunfit chbin t g cy l kim, b bin i rt nhiu.Arabinoglucoronoxylan bin thnh glucoronoxylan, bi cc mchnhnh mang cc gc arabinofuranoza rt d b phn r. Cng nhxylan ca g cy l rng, cc glucomannan ca g cy l kim bkh axetyl. S phn r nhanh cc mt galactoza v cc nhm axetyldn n cc cu trc glucomannan b nn cht li v lm tng bn vng ca chng khi nu. Xenlulo b thy phn mt mc nhtnh v bc trng phn ca n gim xung cn khong 1000-1500,

song khng nh hng ti hiu xut ca n. Cc sn phm thy phnca cc polixacarit di dng cc dextrin, oligoxacarit v monoxacaritchuyn vo dung dch, ti qu trnh thy li tip din v vothi im cui khi nu to thnh dung dch ch yu cha ccmonoxacarit. Cc phn ng ph ca cc polixacarit trong nu sunfitl cc phn ng kh nc, oxi ha, phn ng phn hy v ngng ktca cc sn phm to thnh t cc phn ng ny.

Cc sn phm ca cc phn ng ph l cc ng anhidro-,

fuafurol, hydroxymetylfuafurol, a-xit fomic v levulinic, cc chthumin v cc a-xit aldonic. Cc a-xit aldonic c to thnh khi oxi

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ha cc gluxit. C gi thit cho rng, trong qu trnh ny c stham gia ca ion hydrosunfit. Ban u din ra phn ng kt hpnucleofil vo cc nhm anehit ca cc gluxit.

CR OH

+ HSO3-

H

R C SO3OH-

Sau a-xit -hydroxysunfua to thnh nh kt qu ca cc phnng oxi ha-kh bin thnh a-xit aldonic.

CRO

+

HSO3-

SO3

--

OHSO3CR

H

+ S2O32-

+3H2O + 2

-SO3

OR C H2O CR

O

OH+

-HSO3

Cc ion thiosunfat sinh ra tham gia vo phn ng ngng kt ca

lignin. nhit di 130oC trong dch nu ngi ta ch tm thy

cc sn phm oxi ha ca cc gluxit, cn nhit cao hn xut

hin cc sn phm kh nc ca chng. Tng a-xit ca dung dch

cng to thun li cho din bin ca cc phn ng kh nc ca cc

gluxit.

Khi nu hydrosunfit (bisunfit) khi pH 4-5 vn tc thy phn ca

cc polysacarit gim, v trong iu kin ny c th s dng g

cy l rng ln cy l kim sn xut xenlulo hiu xut cao. Cc

gluxit trong dung dch b thy phn to thnh cc monosacarit t

hn so vi khi nu sunfit. Nng ion hydrosunfit tng cng lm cho

gluxit b oxi ha mnh hn thnh cc a-xit aldonic di dng n

phn v di dng cc mt u ca cc oligosacarit. Ngoi ra cn

din ra s sunfonic ha cc gluxit thnh cc a-xit sunfuagluxit. Trn

hnh 3.21 m t cc kh nng sunfonic ha gluxit c th. Axit

sunfuagluxit l cc a-xit mnh (hng s phn ly ca n 10 -310-1).

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Khi nu sunfit trung tnh (pH6) cc polysacarit trong dung dch hu

nh khng b phn hy. Ngi ta cho rng, cc lin kt glicozit trong

cc gluxit c th b phn r do qu trnh gi l sunfit ha,tc qu

trnh phn hy c s tham gia ca cc ion sunfit v hydrosunfit.

Sunfonic ha gluxit din ra mnh hn so vi khi nu hydrosunfit.

Ngoi cc phn ng d th ra, nguyn nhn ca s phn hy

cc polixacarit c th l cc qu trnh phn ng ca cc gc t do.

Khng ngoi tr c kh nng phn hy ng th cc chui

polixacarit, c bit l nhit cao, v oxi ha m vai tr ca

chng tng ln khi pH tng. Cht khi u ca qu trnh oxi ha

theo c ch phn ng gc c th l -xy phn t cha trong g v

trong dch nu.

O O

O O

OH

OHOH

OH

+H+

OH

OHO

O

H

+ OH

OH

O

+

HO

+:SO3H_

H

O

OOH

OH

, SO3H

H

O

OOH

OH

,OHHSO3

_2

HCH2OH

O

COOH

CH2

SO3H

+

SO3H

CH2

COOH

CH2OH

H

HOH

Hnh 3.21. S to thnh cc a-xit sunfuagluxit

3.10.2. Phn ng ca polysacarit trong mi trng kimPhn ng ca cc polysacarit nhit cao trong mi trng

kim khc bit nhiu so vi trong mi trng a-xit do c ch phn

hy chng din ra theo mt nguyn tc khc, trong xy ra stch cc mt u chui, tc l din ra s kh trng hp, trong khi

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trong mi trng a-xit ch din ra s kh nc m thi. Trong mitrng kim, s phn hy cc polysacarit ca g din ra nh ba qutrnh: s kh trng phn, thy phn kim v oxi ha. Qu trnhcui oxi ha din ra nh xi lun c trong g v trong mi trng

phn ng.Trong cng ngh ha g, din t qu trnh kh trng hp cacc polysacarit ngi ta s dng khi nim pyling. Pyling l nguynnhn ca cc hao tn xenlulo trong qu trnh nu bi trong trnghp ny xenlulo khng phi l polysacarit bn vng nht nh trongmi trng a-xit. Cc mt mt hemixenlulo xy ra do qu trnh hatan chng trong dch nu. Khi nng kim tng v bc trngphn gim qu trnh ha tan cng tng. S kh trng hp din ra ti

khi bin u kh thnh u mt bn vng di tc dng ca kimv l kt qu ca qu trnh n nh, cn gi l stoping.

Qu trnh kh trng hp cc polysacarit din ra theo c chphn ng o thi ti cc v tr cc bon 2 (o thi -alkoxy) kmtheo mt lat cc phn ng. Qu trnh kh nc t cc u kh (othi -hydroxy) ca chui phn t polysacarit din ra song song viqu trnh kh trng hp v dn n s n nh cc polysacarit.Ngai ra, trong mi trng kim cn xy ra cc phn ng h bin haxeton-enol, ng phn ha v phn ng ph hy cc lin ktcacbon trong cc mt kh v cc mt b tch ra khi phn ttheo c ch phn hy tng on.

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Hnh 3.22. Cc bin i ca gluxit trong qu trnhnu kim

Phn ng o thi ti cc v tr cacbon 1,2 (o thi -) sy ra i vi cacgluxit trong m trng kim theo c ch o thi mt phn t t mt baz ktnhm. Theo c ch phn ng ny, ban u proton c tch ra khi nguyn t

cacbon v tr (C) to thnh mt nhm cacbonion, sau mt hydroxit-anion c tch ra (hnh 3.22.a). Trong trng hp ny, vic tch proton tnguyn t cacbon din ra nh hiu ng cm ng ca nhm cacbonyl l mtnhm th ca C ht in t mnh, ng thi cng nh qu trnh n nhnhm cacboanion bng cng hng. Sn phm kh nc to thnh nh qu trnhh bin xeton-enol tn ti di dng hn hp cn bng ca dng enol v dngxeton (hp cht dicacbonyl).

K bin i th nht ca cc polixacarit trong mi trng kim l enol hacc mt u kh to thnh cc 1,2-endiol (hnh 3.22.b) . Chng c th li bin

thnh dng andehit vi s thay i cu hnh ca nguyn t cacbon th hai(epime ha) hoc thnh dng xeton to thnh 2-xeton (ng phn ha). Vpha mnh, 2-xeton c th bin thnh 2,3-endiol v 3-xeton tip . Schuyn dch nhm cacbonyl sang v tr th hai lm cho lin kt glicozit 14b phn r theo phn ng o thi -alkoxy, dn n kh trng hp ccpolysacarit. Cn nhn mnh rng, nhm alkoxyl b tch ra d dng hn so vinhm hydroxyl.

Hnh 3.22, qua th d xenlulo, minh ha cc bin i c bn capolysacarit c cc lin kt glicozit 14 vi cc qu trnh kh trng hp v n

nh chui i phn t. Trn hnh ny khng lit k cc endiol trung gian, cncc phn ng o thi - th b qua cc k trung gian. Chu trnh kh trng hp

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bt u t khu ng phn ha mt kh u chui (I). Nhm cacbonyl vtr cacbon th hai (II) to cho nhm th alkoxyl tch ra khi lin kt vi cacbon v tr C4 [OR, trong R l gc (C6H10O5)n-1], thc t l tch u kh ra khiphn t. Chui phn t polysacarit ngn hn mt mt v hnh thnh umt kh mi, v qu trnh li lp li. Tch ra khi chui phn t, mt (III) di

dng hp cht dicacbonyl, trong mi trng kim, sp xp li phn t bng cchkt hp mt phn t H2O vo a-xit glucoisosacaric (IV). C c nng phn hytheo t ca mt va c tch ra (III) ti lin kt C (3)-C(4) sau khi lin kt mtphn t H2O (hnh 3.22.d) v sp xp li phn t tip ca cc sn phmphn hy to thnh hai phn t a-xit lactic (V).

Mt u kh (I) c th b kh nc theo c ch phn ng nu trn(hnh 3.22.a). Sn phm kh nc di dng hp cht dicacbonyl (IV) sp xp liphn t bng cch lin kt mt phn t H2O vo mt u ca a-xitglucometasacaric (VII). Bin i ny ca u mt lm cho kh trng hp din

ra sau v dn ti n nh chui polysacarit. S n inh chui cn c thc to ra nh s phn hy theo t ca u mt cc lin kt cacbon C (4)-C(5) sau khi ng phn ha thnh 3-xetoza (VIII) to thnh mt u di dng a-xit 2-metylglixeric (IX).

Nh vy, kh trng hp polysacarit din ra nh qu trnh o thi -alkoxyl,cn n nh chng th nh qu trnh o thi -hydroxyl. Do nhm alkoxyll nhm d tch hn so vi nhm hydroxyl, nn trc khi n nh th mt s lncc mt (i vi xenlulo c ti 40-65 mt) b tch ra.

Cn nhn mnh rng, phn ng phn hy theo t din ra trong iu

kin khc nghit nhit cao hn, so vi s sp xp li phn t to thnhcc a-xit sacaric khc nhau. Ngoi cc a-xit sacaric, a-xit lactic v cc hp chtkhc to thnh t phn hy lin kt C3, cn to thnh mt lat cc cht khiphn hy lin kt C1-, C2-,C4- v C5- , nh a-xit fomic, glycolic, V vt, trongmi trng kim cc polysacarit s to thnh mt hn hp cc a-xit hydroxi.

C th n nh cc polysacarit ngc vi qu trnh pyling bng cch oxi hau mt kh, chng hn bng anthraquinone, hoc kh n bng Na 3BO4hoc cc cht kh khc. Oxi ha u kh din ra trong trng hp nupolisunfua v oxi-kim. Thnh phn cc polysacarit v kh nng chu tc dngkim ca chng, nng kim v nhit l nhng yu t nh hng nvn tc qu trnh pyling. Cc mt xylanpiranoza d tch hn so vi cc mtglucopiranoza, mannopiranza kh tch nht. Qu trnh pyling tr nn mnh mngay nhit 80-100oC.

nhit trn 150oC c th din qu trnh suy thoi ca cc chuipolixacarit do thy phn kim. Trn hnh 4.36 minh ha gi thit ca c chphn ng thy phn kim bao gm ion ha hydroxyl c tnh a-xit cao nht v tr C(2) v to thnh ng 1,2-anhydro trung gian. Ngoi ra, thy phn kim

c th din ra theo c ch thay th nucleofil SN1. Thnh phn ha hc ca ccpolixacarit, nng kim v nhit nh ng n vn tc ca qu trnhny. Khc vi thy phn xc tc bng a-xit, trong trng hp ny lin kt -glicozit

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gia mt a-xit glucoronic v chui xylan l lin kt km bn nht, cn lin kt-glicozit gia mt galactopiranoza v mch glucomannan th tng i bn.Lin kt ny bn hn so vi cc lin kt gia cc mt glucopiranoza. Vn tcthy phn kim ca xenlulo nhit 150-190oC rt nh (nh gp 107 lnvn tc pyling), song trong qu trnh ny cc mt u kh mi c to thnh

v chng xc tin cho qu trnh pyling.

Hnh 3.23. Thy phn kim ca cc polysacarit

Cc c tnh phn t lng ca cc lai xenlulo k thut snxut bng cc phng php nu kim chng t, qu trnh suy tho i

ca polysacarit km theo s gim bc trng hp mt cch ng k.iu ny khng th ch l gii bng hin tng thy phn kim cci phn t ca chng. Trong qu trnh ny phn hy oxi hang vai tr tng i ln. Nh bit, g l vt liu xp- mao dn,cha mt lng xi nht nh c kh nng tc dng vi cc thnhphn ca g trong mi trng kim, to thnh cc phn t c hattnh cao nh gc anion siu -xit O2- v gc hydroxyl O.H. xi vc bit l cc dng hat tnh ca n xi ha cc polysacarit. Oxi

ha cc nhm ru thnh cc nhm cacbonyl trong mi trng kimdn n suy thai cc lin kt glicozit theo c ch phn ng othi - (xem hnh 3.26). Nh vy, mt trong nhng yu t tng hiuxut xenlulo khi s dng anthraquinone v cc cht c cu to tngt trong nu kim c th l s c ch cc phn ng oxi hapolixacarit bng cc dng kh ca chng.

Trong mi trng kim, nhit khng cao lm cc lin kt

este rt d thy b phn , v vy cc nhm a xetyl b tch ngay nhng giai an u khi nu kim. Kh axetyl ca cc hemixenluloc ngha rt quan trng, bi v th m chng c rn chc li.

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Yu t ny c s dng trong nu kt hp nhm tng hiu xutxenlulo. Khi nu cc xylan bn vng hn cc glucomannan. Tnhbn ca cc xylan cao l do ti u kh ca cc i phn t cachng c mt a-xit galacturonic, trong ti v tr cacbon th hai

c mt ramnopiranoza kt ni vo bng lin kt -glicozit, mt nykm hm ng phn ha phn t a-xit. Song, s bo v ny chc hiu nghim nhit khng cao lm. cn khi nhit trn95oC th chnh cc mt ny b tch ra khi i phn t (hnh3.24). Cc mt a-xit 4-O-metylglucoronic v tr th hai trong ccmt ca mch chnh xylan cng c nh hng tng t. nhit cao cc mt ca a-xit uronic b tch ra khi i phn t. Nh trnh by trn, cc lin kt -glicozit ca chng vi mch

xylopiranoza km bn vng nht i vi thy phn kim. V vy,s phn hy cc lin kt ny c th din ra nh phn ng o thi-; qu trnh ny bt u bng kh metyl (hnh 3.25). Cc xylanca g cy l kim bn vng hn, so vi xylan ca cy l rng.Chng cha a-xit 4-O-metylglucoronic nhiu hn, song nguynnhn c bn l do v tr th ba ca cc mt trong mchxylopiranoza c cha cc gc arabinofuranoza.Vng xyloza chamt arabinoza bn cnh d dng tch mt ny ra v chuyn ithnh a-xit xylometasacaric c ch qu trnh pyling. Cc nhm th v tr C(6)(chng hn galactoza trong cc galactoglucomannan)khng h gy nh hng n phn ng tch u mt ca ccpolixacarit.

Kh lignin theo phng php oxi ha trong mi trng kim bngxi v cc hp cht cha xi (zon, nc xi gi, ) phn nh qua dinbin ca cc phn ng ch yu ca polysacarit. Trong iu kin

ny, phn ng xi ha to thnh dn xut C(1)-C(2)-dicacbonyl sy ratrc cc phn ng tch cc u mt. Sau dn xut ny hoctham gia vo phn ng o thi dn n qu trnh pyling, hocchuyn i thnh cc a-xit aldonic cn tr qu trnh pyling. Trongtrng hp ny n nh cc u mt t ra c hiu qu hn, so vicc phng php nu thng thng. S n nh bt u ngay saukhi tch cha ti 10 mt ca i phn t. Yu t ny l nguynnhn c bn dn ti tng hiu xut ca xenlulo k thut khi kh

lignin bng phng php kim xi ha.

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Trong mi trng kim din ra qu trnh xi ha cc polysacarit theo cch phn ng gc t do (radical t do). Xenlulo sn xut bng phng php nuxi-kim c hm lng nhm cacbonyl v cacboxyl cao hn. S to thnh ccnhm cacbonyl cc v tr cacbon C(2), C(3), C(6) xc tin qu trnh suy thoIca cc mch polixacarit theo c ch phn ng o thi -alkoxyl. Ion ha

nhm hyroxyl v tr cacbon C(2) to iu kin thun li cho vic xut hinmt tm radical t do ti nguyn t cacbon ny v s to thnh mt nhmcacbonyl sau (xem hnh 4.39). Khi phn hy lin kt glicozit 14 theophn ng o thi to thnh cc u kh v khng kh. S xut hin u khxc tin qu trnh kh trng hp, cn u khng kh di dng dixeton hocchuyn i thnh u cacboxyfuranozit , hoc xi ha lien kt C(2)- C(3) thnhhai nhm cacboxyl.

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