pubs.acs.org/crystalPublished on Web 06/18/2010r 2010 American Chemical Society

DOI: 10.1021/cg100260f

2010, Vol. 103141–3148

Polymorphism, Phase Transitions, and Thermal Stability of

L-Pyroglutamic Acid

Han Wu,† Nik Reeves-McLaren,† Jan Pokorny,†,‡ Jack Yarwood,§ andAnthony R. West*,†

†Department of Engineering Materials, University of Sheffield, Sheffield, S1 3JD, U.K.,‡Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, Prague 8,Czech Republic, and §Materials and Engineering Research Institute, Sheffield Hallam University,Norfolk Building, Howard Street, Sheffield, S1 1WB, U.K.

Received February 25, 2010; Revised Manuscript Received June 3, 2010

ABSTRACT: L-Pyroglutamic acid undergoes a phase transition,Rf β, at∼68 �Con heating, which is reversible with hysteresisdepending on the cooling rate, before melting at ∼162 �C. On cooling, a further reversible transition, R f R0, which showsmartensitic characteristics, occurs at∼-140 �C. The structural changes at the transitions were studied by variable temperatureX-ray powder diffraction and Raman spectroscopy and compared with reports of single crystal structure determinations.Differences in Raman spectra were attributed to differences in intermolecular N-H 3 3 3O interactions in the three enantiotropicpolymorphs. By optical microscopy, crystals frequently jumped around the microscope slide on passing through the R/βtransition; this is attributed to discontinuous changes in the unit cell dimensions leading to a spring effect in the crystals.

1. Introduction

Thepyroglutamic acid, 2-oxopyrrolidone-5-carboxylic acid(P), Figure 1a, is a five-membered lactam of glutamic acid(Glu), Figure 1b.

Both P and Glu are chiral molecules, with the chiral centersmarked by an asterisk in Figure 1. P is the chain-terminatoror N-terminal residue of a number of biologically significantproteins and peptides.1-3 It occurs naturally in fruits, someplant foods, dairy, and fermented products such as soy sauceand can be produced by thermal dehydration and cyclizationof Glu.4,5 In neurology, the brain-boosting effects of P werediscovered in 1984.6 It plays an important role in the preserva-tion and activity of the key neurotransmitters acetylcholine,gamma-aminobutyric acid (GABA) and Glu;6-8 it readilypasses theblood-brainbarrier to stimulate cognitive-enhancingfunction in rats9 and reduce age-associated memory decline inhumans.10 In drug delivery, P esters can be used as dermalpenetration enhancers for therapeutic agents having poor skinpermeation11 or hair growth agents.12 In chemical synthesis,it has been used as a versatile chiral building block in asym-metric synthesis of alkaloids,13 pharmaceuticals, and manyothernaturalproducts;14-18 itsderivativeshavebeenextensivelyapplied for enantioselective synthesis.19,20

L-P is also known as a novel organic nonlinear optical(NLO)material for tunableUVharmonic generation down to266 nm at room temperature.21 Most organic NLOmaterialscontaining nitro groups have the following characteristics.They are not transparent in theUV region; difficult to grow aslarge optical-quality single crystals; susceptible to damageduring processing; and not suitable for commercial applica-tions due to their poor thermal, mechanical, and chemicalstability.22 In spite of this, however, L-P containing an amidegroup instead of a nitro group was reported recently to showgoodNLO performance:23,24 large single crystals grown from

suitable solution are chemically stable andhave relatively highmelting point and good hardness.

Although L-P has numerous applications, little informationis available on its thermal stability and any possible decom-position products or polymorphic transformations. Recently,we discovered that at elevated temperatures, L-P forms bycyclization of L-Glu, even in the absence of a catalyst oraqueous solution.25 The complete transformation sequenceon heating L-Glu appears to be

R L-Glu f β L-Glu f P f polyglutamic acid, PGA

L-Glu is a widely used food additive and flavor enhancer in itssodium salt form and also has many applications for treat-ment of neurological disorders.26-28 The thermal stability ofboth L-Glu and L-P is, therefore, important in the food andpharmaceutical industries.

The crystal structures of L- and DL-forms of P have beenreported.29,30 There is a suggestion that L-P is dimorphic andthat solution-grown single crystals of two polymorphs can beobtained from different solvents,24 but this claim appears tobe based entirely on crystal morphology without any othercharacterization such as crystal structure data. There is also asuggestion of a structural change at 64 �C by Fourier trans-form IR (FTIR) and differential scanning calorimetry (DSC)results,32 but no crystallographic characterization of the hightemperature structure was reported.

In this paper, we report a study of the thermal stability ofL-P and find evidence for both a high temperature polymorph

Figure 1. (a) Pyroglutamic acid, P, and (b) glutamic acid, Glu.

*Author for correspondence: Tel: þ44(0)114 2225501. Fax: þ44(0)1142225943. E-mail: a.r.west@sheffield.ac.uk.

3142 Crystal Growth & Design, Vol. 10, No. 7, 2010 Wu et al.

that we label β, the ambient temperature polymorph, R (note:we use R, β as polymorphic labels and do not refer tosubstituents on specific C atoms) and a low temperaturepolymorph, R0. To study the phase transformations, a set ofcomplementary techniques,X-ray powder diffraction (XRD),Raman spectroscopy, thermogravimetry (TGA), DSC, andoptical microscopy, was used.

2. Experimental Section

All experiments were performed with high-purity L-P (purityreported to be >99%, Sigma-Aldrich, UK). As shown later, how-ever, a small amount (∼1.4%) of racemic DL-P was found to bepresent. Phase characterization and determination of unit cell para-meters of L-P at different temperatures were performed using a STOESTADI P X-ray powder diffractometer, Mo KR1, λ = 0.70926 A,equipped with a linear position sensitive detector (PSD), over the 2θrange 4.5� to 24.0�, in Debye-Scherrer geometry. Temperaturecontrol was obtained by an Oxford Systems Cryo Cooler 700, witha built-in thermocouple measuring the temperature (in the range-180 to 120 �C with a temperature stability of 0.1 �C) at the bottomof the nozzle, underneath which a capillary sample was located.

Thermal transformations of L-P were analyzed by DSC using twodifferent instruments, a Perkin-Elmer DSC 7 under dry Ar andPerkin-Elmer DSC 1 equipped with liquid N2 cooling system underdry He. Heat flow was calibrated using In (MPt = 156.6 �C, ΔH=28.45 J/g). The powderwasweighed into anAl sample pan fittedwitha perforated lid. Samples were heated/cooled between -155 and200 �C. Rates varied from 1 to 50 �C/min. The weight loss duringheating was quantified by thermogravimetric analysis (TGA) using aPerkin-Elmer Pyris1 TGA system. Samples were heated from 30 to300 at 10 �C/min in flowing dry N2.

In situ monitoring of L-P on heating was performed using hot-stageRaman spectroscopy. Unpolarized Raman spectra were excited withthe 514.5 nm line of an Ar laser and recorded in back scatteringgeometry using a Renishaw InVia micro-Raman spectrometer. Laserpower of∼20mWwas focused on a∼2 μmspot. The spectrometer wasequippedwith a Peltier-cooledmultichannel CCDdetector and diffrac-tion grating 2400 L/mmwith slit opening 65 μmand spectral resolution∼1 cm-1. Temperaturewas controlled in the range-190 to 100 �Cwith0.1 �C accuracy by a Linkam THMS 600 heating and cooling stageequipped with a TMS 94 temperature controller and LNP 94 liquidnitrogen cooling system. Samples were also observed directly as afunction of temperature using a hot-stage polarizing microscope.

3. Results

3.1. DSC Results. A selection of DSC data obtained onvarious heat-cool cycles starting with L-P and using data fromtwo different instruments is shown in Figure 2 and SupportingInformation, Figure S1. In summary, five different heat effectsare seen between -155 and 200 �C. On heating from roomtemperature to 200 �C, three endotherms, 1, 2, and 3, are seenat ∼68 (ΔH1=0.53 kJ/mol), ∼ 154 (ΔH2=1.83 kJ/mol), and∼162 �C (ΔH3=16.79 kJ/mol), Figure 2b. The behavior oncooling is, however, more complex. On cooling from tempera-tures above that of the first endotherm, for example, from80 �C, an exotherm (ΔH1

0=-0.55 kJ/mol) is observed at 54 �C,Figure 2c, but which exhibits considerable hysteresis, especi-ally at faster cooling rates, Figure S1, and is observed at, forexample, 3 �Cwith a cooling rate of 120 �C/min. This indicatesthat the associated transformation is sluggish on cooling butnevertheless, is reversible. On cooling from temperatures abovethat of the second endotherm, for example, from 156.5 �C, twoexotherms are observed (not shown) with significant hysteresisfor peak 2 as well as for peak 1. On cooling from temperaturesabove peak 3, for example, from 200 �C, without significantholding time prior to cooling, no exotherms are observed,Figure 2b.

We attribute peak 1 to a solid-solid polymorphic phasetransformation and label the polymorphs as R and β. Thetemperature of peak 3 is very close to the reported melting

Figure 2. (a) TGA curve on heating L-P from 30 to 300 at 10 �C/min.(b) DSC curves on heating/cooling L-P between 30 and 200 �C. DSCheating rate 10 �C/min from30 to 140 �C, then2 �C/min to200 �C. (Thechange of heating rate at 140 �C, shown by the baseline discontinuity,was to avoid potential contamination/damage of the facility by sampleescaping from the container at high temperature.) Cooling rate 10 �C/min. Temperatures marked refer to peak maxima. (c) DSC curves oncooling and heating L-P between 80 and 20 �C. Heating/cooling rate10 �C/min. (d) DSC (using Perkin-Elmer DSC 1 coupled with liquidN2 cooling system) curves on cooling and heating L-P between-10 and-155 �C. Heating/cooling rate 10 �C/min between -100 and -10 �C,5 �C/min between -155 and -100 �C.

Article Crystal Growth & Design, Vol. 10, No. 7, 2010 3143

temperature of P, ∼162-163 �C,31 and, by visual observa-tion, we confirmed that the sample had melted above∼160 �C. We attribute peak 2 to the presence of a smallamount of DL-P impurity which leads to the onset of meltingof L-DL mixtures at the eutectic temperature, ∼154 �C; thiswill be described and discussed in a subsequent publication.

From the literature, there is similar evidence fromDSC forthe existence of polymorph β, although the DSC peak thatwas observed at 64 �C was not attributed to a polymorphictransition.32 There is also a claim for the existence of twopolymorphs at room temperature based on two differentcrystal growth morphologies,24 but no further characteriza-tion studies were reported. We note, however, that althoughthe β f R transformation can be suppressed by rapid cool-ing, Figure S1, the β polymorph does not exhibit any long-time stability at room temperature, andwewould not expect,with ourmaterials and conditions, to observe a second, long-lived polymorph at room temperature.

DSC data between -155 �C and -10 �C are shown inFigure 2d. A reversible phase transition with some hysteresisis seen at ∼ -126 �C, which is attributed to a low-tempera-ture polymorphic transition; we label the low-temperaturepolymorph R0. On cooling, the R/R0 exotherm has sawtoothcharacter, Figure 2d inset, which may indicate that thetransition occurs in bursts, typical of amartensitic transition.In addition, the baseline on subsequent heating shows abroad discontinuity at ∼ -70 �C. We might have attributedthis to a glass transition in the sample, but since the sample iscrystalline and not amorphous, an alternative explanation isrequired. At this stage, we do not know the nature of thechanges responsible for this heat effect.

In summary, the polymorphism of L-Pmay be representedas follows:

R0s∼- 126�C

Rs∼68�C

βs∼162�C

liquid

R0, R, and β are enantiotropically related polymorphs. TheR0/R and R/β transitions are reversible on cooling, with hyster-esis dependent on cooling rate. The transition temperatures

define the temperature at which the stability relationship be-tween each pair of polymorphs becomes inverted (i.e., their freeenergy is equal at the transition temperature).However, crystal-lization of liquid L-P is a relatively slow process and was notobserved under the DSC conditions used in our experiments.

3.2. Thermogravimetry. TGA data on heating L-P betweenroom temperature and 300 �C are shown in Figure 2a.Sample mass is constant to∼190 �C then decreases at highertemperatures. We do not know whether the weight loss isassociated with volatilization of the sample or its decom-position. The TGA results do, however, show that the poly-morphic transitions at lower temperature and the samplemelting do not involve any loss of weight.

3.3. Variable Temperature X-ray Diffraction. A smallsection of the powder XRD pattern of L-P is shown on anexpanded scale for several temperatures in Figure 3. Over therange -100 to 63 �C, small shifts in peak positions to lower2θ values are seen, consistentwith thermal expansion of theRpolymorph. Between 63 and 80 �C, significant changes in thepositions of several peaks are seen (although no new peaksappear and none disappear). The pattern of the β polymorphat 80 �C can therefore be indexed on the same orthorhombicunit cell, space group P212121, as the R polymorph, but withsignificantly different lattice parameters, as shown in Table 1and Figure 4. Basically, a and c expand at the R/β transition,whereas b contracts. This anisotropic change in cell dimen-sions accounts for the change in XRD patterns, in whichsome peaks (e.g., (130), (131), and (230)) displace to higher2θ, others (e.g., (101), (102), (200), and (201)) displace tolower 2θ, and the remainder are essentially unchanged inposition. Although individual cell parameters change by∼1% at the transition, these effects cancel in the unit cellvolume, which shows no significant discontinuity, Figure 4d.There are, presumably, small changes in the molecularpacking details at the transition, but this result shows thatthe overall molar volume is essentially unchanged.

Over the temperature range -180 to -100 �C, discontin-uous changes in the positions of many peaks (e.g., (101),(102), (112), (130), (131), and (104)) occur together with

Figure 3. XRD patterns of L-P at temperatures between -180 and 120 �C.

3144 Crystal Growth & Design, Vol. 10, No. 7, 2010 Wu et al.

significant changes in the intensities of some peaks (e.g.,(112) and (130)), Figure 3. Nevertheless, the XRD patternat-180 �C can also be indexed on an orthorhombic unit cell,space groupP212121, as summarized in Table 1 for the latticeparameters of R0, R, and β polymorphs. Fully indexed X-raypowder diffraction data forR0 (at-180 �C),R (at 40 �C), andβ (at 80 �C) are included in Supporting Information and willalso be submitted to the International Centre for DiffractionData for inclusion in the Powder Diffraction File.

3.4. Raman Spectra of r0, r, and β Polymorphs. Ramanspectra of R0, R, and β polymorphs at -180, 25, and 100 �C,respectively, are shown in Figure 5a. A small number ofpeaks that are multiplets in the R form become singlets in theβ form and triplets in the R0 form. This is seen clearly for thepeaks at 3300-3500 cm-1, attributed to NH vibrations, butalso for numerous other sets of peaks (unassigned) through-out the spectrum, Figure S2.

The changes in Raman spectra associated with NH vibra-tions over the range-180 toþ80 �C are shown in Figure 5btogether with assignment of the spectra to different poly-morphs or polymorphic mixtures. Over the range -160 to-130 �C, mixtures of R0 and R are seen, whose relativeamounts change systematically with temperature. Similarphase mixtures were seen in XRDdata over this temperaturerange (not shown). The R0/R transition is characterized bydiscontinuities in peak positions together with a gradualincrease in line widths. Three broad peaks are seen in the Rspectrum over most of the temperature range, but thesebecome a single broad peak at the R/β transition.

3.5. Hot Stage Optical Microscopy. By hot stage opticalmicroscopy, significant changes to crystals of L-Pwere seenonbothheating and cooling through the RT β transition, Figure 6. Speci-fically, some crystals, especially larger ones, jumped around theglass slide holding the samples, as shownby the sequence ofmicro-graphs on heating (1-4) and subsequent cooling (5-8). Startingwith six crystals in the field of view at 30 and 68 �C, three of thesedisappeared on increasing the temperature to 69 �C and a furtheronedisappearedonholdingat69�C.Onsubsequentcooling,oneofthe two remaining crystals jumpedoutof the fieldof visionat 55 �Cand the other one also disappeared on holding at 55 �C.

These changes were not associated with evaporation of thesample since no weight loss is seen by TGA but with physicaldisplacement of crystals on passing through the transition.This effect can be explained by the anisotropic change in unitcell dimensions shown in Figure 4. Significant change incrystal dimensions effectively provided a spring-board forsome of the crystals to jump off the surface of the glass slide.In these particular examples, the crystals appeared to retaintheir integrity on passing through the transition. In othercases, however, crystals were seen to fragment, presumablybecause those crystals were either unable to withstand thedimensional changes and retain their single crystal integrityor were polycrystalline aggregates. Using polarizing micro-scopy at room temperature in transmission mode, on sam-ples before and after cycling through the R/β transition, itwas clear that there remained an abundance of reasonable-sized single crystals after cycling. This showed that thedimensional changes associated with the R/β transition were

Table 1. LatticeParameters ofβ,R, andR0 Polymorphs of L-PDetermined fromPowderXRDatVariousTemperatures, inComparisonwithReportedR and

R0 Data29,33

lattice parameters

temperature (�C) polymorph a (A) b (A) c (A) V (A3)

80 β 9.086 (6) 13.140 (6) 14.586 (7) 1741 (1)40 R 8.964 (4) 13.349 (6) 14.49 (1) 1733 (1)-180 R0 8.146(2) 14.269 (4) 14.610 (5) 1698.2 (6)2529 R29 9.018 (8)29 13.495 (8)29 14.662 (4)29 1784.3329

-15033 R033 8.1972(10)33 14.340(2)33 14.6661 (15)33 1724.1 (4)33

Figure 4. (a-d) Lattice parameters of L-P versus temperature.

Article Crystal Growth & Design, Vol. 10, No. 7, 2010 3145

not totally disruptive but could be accommodated by a largenumber of the crystals.

4. Discussion

4.1. Crystal Structures of r and r0 Polymorphs. Crystalstructure data are available in the literature on L-P based onsingle crystal X-ray diffraction data collected at both roomtemperature29 and-150 �C.33 The low-temperature data arereported in a crystal structure database, but no comparisonwasmade with the room temperature data. Careful compari-son of the unit cell parameters of the two data sets with thoseobtained from our variable temperature powder diffraction

data, Table 1, shows that almost certainly, the low-tempera-ture structure is that of R0, whereas the room temperaturestructure is that of R. We are therefore able to make adetailed comparison from these literature data of the struc-tures of R, R0 polymorphs.

The R and R0 polymorphs share similar packing arrange-ments, as shown for a segment of the unit cell of each inFigure 7. Each has chains of L-P molecules parallel to c.Within each chain, molecules are linked via O-H 3 3 3Ohydrogen bonding (HB) between the amide carbonyl and thecarboxyl group on neighboring molecules. These chains arethen linked together into a three-dimensional structure byN-H 3 3 3O interactions between amide -NH groups in onechain and sufficiently close acid carbonyl oxygen atoms inneighboring chains. A comparison of the crystal structuresindicates that, in both R and R0, there are three distinct N-Hbonds which may, or may not, take part in HB interactions.These are labeled in Figure 8; to facilitate comparison, theatoms in the original structure reports have been relabeled.

The transition on cooling from R to R0 is accompanied bya ∼3.4% reduction in unit cell volume, and as a result, thecrystal structure of R0 is significantly more compact. There isa concomitant reduction in some of the distances between Nand O atoms in neighboring chains, Table 2. Figure 8 showsthe structures of R and R0 polymorphs as viewed down [001].The N1-H 3 3 3O1 distance is the shortest intermolecu-lar N-H 3 3 3O distance in both polymorphs, but decreasesfrom 3.058 A in R to 2.983 A in R0. Positions of the H atoms

Figure 5. Raman spectra of (a) three L-P polymorphs at -180, 25, and 100 �C; (b) R f β and R0 f R phase transitions on expanded scalefrom -180 to þ80 �C.

Figure 6. Hot-stage polarizing microscope pictures of L-P crystalsat different temperatures on heating (1-4) and subsequent cooling(5-8) (Crystals move at∼69 �C on heating and∼55 �C on cooling).

3146 Crystal Growth & Design, Vol. 10, No. 7, 2010 Wu et al.

were not determined experimentally, and so comments onN-H 3 3 3O angles would be rather speculative. Neutrondiffraction studies to locate H atoms and determine thestructure of the high temperature β polymorph are planned.

4.2. Interpretation of Variable Temperature Raman Spec-

tra. Several bands (from internal modes) in the Raman

spectra of R L-P have a multiplet structure. These includethe ν(NH) stretching bands at 3340 and 3405 cm-1, andbands near 1220 and 530 cm-1, Figures 5a and S2. In eachcase, one or two components of themultiplet disappear at thetransition from R to β at ∼68 �C.

Such multiplets in the spectra of molecular crystals haveseveral possible origins. These include splitting of modedegeneracy and crystal field splitting due to n (here n = 12)molecules per unit cell. However, the most likely scenario, inthis case, is that two (ormore) different “environments” existof, at least, one chemical group. Such different “environ-ments” are usually caused by molecular interactions, forexample, the HB of the -NH group to an oxygen-containinggroup on a neighboring molecule.

The ν(NH) band of R L-P near 3300 cm-1 shows threecomponents, which are probably associated with three crys-tallographically distinct N atoms (in both R and R0), Table 2.These may be influenced to different degrees by interactionsof the -NH groups with the acid CdO groups on theneighboring chains. In R these are at 3400, 3387(sh), and3340 cm-1 at 25 �C, Figure 5b. The low frequency compo-nent (3340 cm-1) of theN-Hband disappears (reversibly) atthe R/β transition temperature (TRfβ). It seems likely, there-fore, that the -NH groups find themselves in at least threedifferent environments, one of which is lost, on conversion tothe β phase.

We do not know how many of the three bands near 3300cm-1 are affected by HB; possibly all three are affected by HB,since ν(NH) bands are notoriously weak and perhaps onlythose vibrations perturbed by HB are seen in the Ramanspectra. It seems likely, however, that at least two of the bandsare influenced by HB. Consideration of the N-H 3 3 3O dis-tances and angles in Table 2 reveals which of these groupingsare likely to show HB. Three criteria may be used as anindicator of possible HB: the H 3 3 3O distance, the N-H 3 3 3Odistance (conventionally set at a maximum value of 3.07 A,which is equal to the sum of the van der Waals radii of N andO35), and theN-H 3 3 3Oangle (general consensus is that linearbonds at 150-180� are structurally significant for the occur-rence of HB; however, 110� is also used as a lower limit36).

Given thatH 3 3 3Odistances andN-H 3 3 3Oangles are notknown with certainty, of the four groupings listed in R,N1-H 3 3 3O1 and N2-H 3 3 3O3 are most likely to showHB. The Raman spectra are therefore in line with thestructural data and out of line with the older literature,32

which claims that the -NH group is “free” in the HB sense:one N-H bond could be free, but not the other two.

The component in R that appears to be most stronglyhydrogen bonded, at 3340 cm-1, is considerably broader

Figure 7. Molecular arrangements for (a) R0 and (b) R L-P vieweddown [010]. The atoms are represented by spheres (H (turquoise), C(red), N (green), O (blue)), while intrachain O-H 3 3 3O HBs areindicated with black dashed lines. Interchain N-H 3 3 3O distancesshorter than 3.4 A aremarked with red lines. Structure visualizationwas obtained using Balls & Sticks.34

Table 2. Selected Bond Distances/Angles for R and R0L-Pa

R29 R033

distance (A) angle (deg) distance (A) angle (deg)

N1-H 3 3 3O1 3.06 140.2 2.98 166.4N2-H 3 3 3O2 3.29 100.8 3.21 142.1N2-H 3 3 3O3 3.09 164.5 3.09 125.7N3-H 3 3 3O3 3.16 120.5 3.07 161.6H(N1) 3 3 3O1 2.32 2.12H(N2) 3 3 3O2 3.04 2.47H(N2) 3 3 3O3 2.30 2.49H(N3) 3 3 3O3 2.71 2.22

aThe atom labels refer to those given in Figure 8, with hydrogensreferred to by theN towhich they are bonded. Bond angles are suggestive,based on proposed, rather than determined, positions for H.29,33

Figure 8. Molecular arrangements for (a) R0 and (b) R L-P, vieweddown [001]. The atoms are represented by spheres (H (turquoise), C(red), N (green), O (blue)).

Article Crystal Growth & Design, Vol. 10, No. 7, 2010 3147

than the more weakly bonded component (half widths 38and 24 cm-1, respectively) and is “red-shifted” (62 cm-1) tolower frequency (as expected). As the sample transformsto β, the band broadens even more (half width e50 cm-1 at100 �C).

On cooling to reach the R0 phase, there are now threewell-established and relatively narrow bands in the Ramanspectra at 3378, 3343, and 3305 cm-1 with half widths4, 7, and 7 cm-1. This again ties in nicely with threeN-H 3 3 3O interactions in the crystal structure, two ofwhich are similar. However, the N-H 3 3 3O interactions arenot necessarily the same in R0 as in R. Specifically,the N1-H 3 3 3O1 distance appears most likely to show HBin both but N3-H 3 3 3O3 appears to be more favorable inR0 whereas N2-H 3 3 3O3 appears to be less likely. The shiftto lower frequency of one component (3305 cm-1), relativeto that in R, may reflect the more compact structure of R0,with shorter N-H 3 3 3O distances and consequent strength-ening of the N-H 3 3 3O interactions. The assignment ofRaman bands to individual N-H 3 3 3O interactions needsto be further explored.

In addition, other bands in the Raman spectra show adoublet structure, and the implication is that these alsoarise, at least partly, from differences in N-H 3 3 3O inter-actions. The γ and δ(NH) bands are predicted to be in the588 and 1467 cm-1 regions,37 where we do not find anyspecial doubling effects. However, there are doublet bands at1220 and 530 cm-1, which again become a single band aboveTRfβ.

4.3. Martensitic Character of the r f r0 Transition. TheR f R0 transition on cooling appears as a DSC exothermat ∼ -140 �C but, instead of a smooth exotherm, it hassawtooth character, Figure 2d. This is characteristic oftransitions of martensitic character that occur in burstsbecause of constraints imposed by changes in shape/volumeat the interface between transformed and untransformedcrystal. Consequently, the complete transition occurs overa range of temperatures. The XRD results on cooling (notshown) also show the transition to occur over a range of tem-peratures since mixtures of R and R0 are seen at both -150and -170 �C.

5. Conclusions

Pyroglutamic acid exists in three enantiotropic poly-morphic forms, each with its own temperature range ofthermodynamic stability. Although the high temperature,β, and low temperature, R0, forms had not been recognizedpreviously, evidence for their existence was provided byearlier IR/DSC data (β) and differences in crystal structuresat low temperature (R0) and room temperature (R). All threepolymorphs have the same space group, P212121, which isperhaps somewhat unusual. Structural differences are asso-ciated with detailed molecular packing arrangements and inparticular, the interchain HBs. Differences in the Ramanspectra between R and R0 polymorphs can be interpretedby their different HB arrangements; from the Raman spectraof the β polymorph, suggestions concerning its HB arrange-ments are made.

The R/R0 transition on cooling appears to beMartensitic incharacter in which strain at the R/R0 interface in partiallytransformed crystals causes the transformation to occur inbursts with increased undercooling. There is considerablehysteresis between the temperatures of the R/β transition on

heat/cool cycles, and, in particular, the β f R transition canbe suppressed significantly at fast cooling rates. Althoughthere is little change in overall volume of crystals on passingthrough the R/β transition, there are significant changes,∼1%, in the individual unit cell parameters and therefore incrystal dimensions on passing through the transition in eitherdirection; this has the effect of causing crystals to jumpoff a solid surface, such as a microscope slide, on passingthrough the transition. The sluggishness of the R/β transitionis probably connected to strong HB between pyroglutamicacid molecules in the crystalline state and may also be res-ponsible for the large undercooling of liquid pyroglutamicacid for which no crystallization exotherm was seen underthe normal DSC conditions.

Acknowledgment. We thank X. Zeng for training in, anduse of, the hot-stage microscope. We also thank Rob Hansonfor his help with low temperature DSC measurements.

Supporting Information Available: DSC data showing Peak 1temperature on heating and cooling at different heating/coolingrates (Figure S1), Raman spectrum of three polymorphs of L-P onexpanded scale (Figure S2) and indexed powder XRD of β L-P(Table S1), R L-P (Table S2), and R’ L-P (Table S3). This material isavailable free of charge via the Internet at http://pubs.acs.org.

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