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Ground-state oxygen (3O2 )
Oxygen is the most important factor on the development of lipid peroxidation.
Ground state oxygen is itself a radical, with two unpaired electrons each located in a π* antibonding orbital
Ground state oxygen has its outermost pair of electrons parallel spins: does not allow them to react with most molecules
Ground-state or triplet oxygen is not very reactive
Can be activated by the addition of energy, and transformed into reactive oxygen species
Singlet oxygen (1O2 )
Formed from triplet oxygen
Conversion of oxygen to singlet state can be accomplished by photosensitization in the presence of suitable sensitizers, such as chlorophyl, or heme pigments myoglobin or hemoglobin or by their derivatives
Has a pair of electrons with opposite spins
Singlet oxygen (1O2 )
1ΣgO2 has no unpaired electrons and thus does not qualify as a radical. 37.5 kCla above the ground state O2
1ΣgO2 usually decay to 1ΔgO2. 22.4 kCla above the ground state O2
1ΔgO2 is not a true radical but is reported to be an important ROS in reactions related to ultraviolet exposition (UVA, 320-400 nm)
Excess singlet oxygen formation can lead to certain diseases: porphyrias
Superoxide anion (O2-)
Monovalent reduction of triplet oxygen produces superoxide
Formed in almost all aerobic cells: mitochondriaOutside of mitochondria: ER through oxidation process of
cytochrome P-450 and NADPH-cytochrome c reductaseCu+2 + ascorbate + O2 --- O2
-
Fenton reaction also produces superoxide
Superoxide anion (O2-)
Not reactive enough to abstract hydrogen from lipids
Cannot enter the hydrophobic interior of membrane because of its charged nature
Can produce hydrogen peroxide (Dismutation)
Involved in hydroxyl radical formation
Can also react with nitric oxide (NO.) to produce peroxynitrate (OONO-)
Hydrogen peroxide (H2 O2 )
Not a radical
Important in biological systems because it can pass readily through cell membranes
Superoxide-generating systems produces H2 O2 by non- enzymatic or SOD-catalyzed dismutation
2 O2 - + 2 H+ -----> H2 O2 + O2 (SOD)
Hydroperoxyl radical (HO2.)
Protonation of O2 - yields the HO2.
Hydroperoxyl radical (HO2.) is more reactive than superoxide
and can enter membrane fairly easily.
The pKa of HO2. is 4.7-4.8, and so only 0.25% of O2 -
generated in physiological conditions is hydroperoxyl radical. Localized pH drop can exist
Hydroxyl radical (OH·)
The most reactive oxygen species known: site specific reaction
Can be produced by high-energy ionizing radiationH2 O -----> ·OH + ·H + H2 + H2 O2 + H3 O+ + e-aq
(ionizing radiation)In vivo production comes from metal-dependent (Fe, Cu)
breakdown of hydrogen peroxideFe2+-dependent decomposition of H2 O2 (Fenton reaction)Fe2+ + H2 O2 -----> Fe3+ + ·OH + OH-Fe2+ + H2 O2 -----> ferryl? -----> Fe3+ + ·OH + OH-
Hydroxyl radical (OH·)
Fe3+-dependent multi-stage decomposition of H2 O2
Fe3+ + H2 O2 -----> ferryl + H2 O2 -----> perferryl + H2 O2 --- --> .OH
Fe2+-EDTA dependent decomposition of H2 O2
Fe2+-EDTA + H2 O2 -----> intermediate species (ferryl) ----- > Fe3+-EDTA + ·OH + OH-
Ozone (O3 )
Ozone is not a free radical
Ionizing radiation of oxygen produces ozone
As singlet oxygen, it stimulates lipid peroxidation
Can induce damages at the lipid and proteins
Lipid (R.), peroxyl (ROO.) and alkoxyl radical (RO.)
Very strong reactivity and can abstract hydrogen atom from lipids
Can be formed from the lipid radicals by iron complexesROOH + Fe3+-complex -----> ROO. + H+ + Fe2+complexROOH + Fe2+-complex -----> RO. + OH- + Fe3+-complex
Iron-Oxygen Complexes
Ferryl (Fe4+) and perferryl (Fe5+) radicals
Powerful oxidants as a component of enzyme or simple iron complex
Ferryl species are generated by the interaction of H2 O2 with metmyoglobin
Fe(II)-complex + ROO•
+ H+ Fe(III)-complex + ROOH Fe(II)-complex + RO•
+ H+ Fe(III)-complex + ROH
Nitric oxide (.NO)
Produced in various types of cells
Is not too reactive (poorly oxidizing function), even antioxidant under physiological concentrations (up to 100 nM)
Reacts rapidly with oxygen to yield nitrogen dioxide (.NO2 ) which in turn may react with .NO to yield nitrogen trioxide (N2 O3 )
Rapidly react with O2- and produce extremely reactive peroxinitrite
(ONOO-) which mediates oxidation, nitrosation, and nitration reactions
ONOO- also decomposes to produce OH. radical
Thiyl radicals (RS.)
Thiol compounds (RSH) are frequently oxidized in the presence of iron or copper ions:
RSH + Cu2+ ----> RS. + Cu+ + H+
These thiyl radicals have strong reactivity in combining with O2
RS. + O2 ---> RSO2 . They are able to oxidize NADH into NAD., ascorbic acid and
to generate various free radicals (.OH and O2-).
Thiyl radicals can also be formed by homolytic fission of disulfide bonds in proteins
Irradiation
Radiation: Energy moving through space in invisible wavesLight, infrared heat, microwaves, TVs all use radiant energy
Ionizing radiation: Shorter wavelengths in radiation. Capable of converting atoms and molecules to ions via the removal of electrons. Destroy DNA bonds in bacteria, pathogens and insects
Types of IrradiationRadio Active Nuclides
X-rayGamma Rays
Cobalt-60Cesium-137
Electron Beam
E-beam Irradiation
Better Consumer Reception for Food Irradiation
X-ray can be generated
Other Industrial Uses
Physical and Chemical Modification and Cross-linking
Effects
Provides increased tensile, impact, and abrasion strength for wire, cable, and tubing
Improves product performance
Gamma Irradiation
The method of choice for
Sterilization of single-use medical supplies such as syringes, catheters, IV sets, gloves, face masks and more
Elimination of organisms from pharmaceuticals such as ointments and solutions
Advantages
Offers superior material penetration
Economical for high and/or low volume operation
Is highly time-efficient
Offers versatile irradiator designs Imposes minimal restrictions on product design and packaging
Irradiated Foods Marketed in the U.S.
Irradiated ground beef and poultry
18-20 M Lb sold in 2004
Fruits & vegetables
2 M lb sold annually
Mango, papaya, guava are currently sold by US retailers
Growing interest in blueberries, cherries, raspberries
The amount of irradiated tropical fruits will increase rapidly in the future
Spices and Botanics
Commercially irradiated since 1986
175 M Lb of (1/3 of commercial spices consumed in the US) are irradiated annually
Maximum Irradiation Dose
year Food Dose (kGy)
Purpose
1963 Wheat flour 0.2-0.5 Control molds
1986 Fresh fruit & vegetables 1.0 Inhibit sprouting Delay ripeningDisinfestation
1990 Poultry meat 3.0
Control pathogens1999 Refrigerated meatFrozen meat
4.57.0
Dehydrated enzymes 10
1986 Dehydrated spices & herbs 30
Shelf-lives of Meat Products after Irradiation
Dose Untreated shelf Irradiated shelf-
Meat Products (kGy ) life (days) life (days)
Beef top round 2.0 8-11 28
Beef burgers 1.54 8-10 26-28
Beef cuts under vacuum 2.0 NA 70
Corned beef 4.0 14-21 35
Whole and minced Lamb 2.5 7 28-35
Andrew et al. (1998)
Exhaustive chemical, biological, and feeding studies have shown irradiated foods to be both safe and wholesome
In 1980, FDA and a Joint FAO/IAEA/WHO Expert Committee had concluded on the wholesomeness of irradiated food that irradiation of any food at <
10 kGy causes no toxicological
hazard and nutritional or microbiological problems
In 1983, a worldwide standard covering irradiated foods was adopted by the Codex Alimentarius Commission
2-ACBs, benzene and methyl benzene (toluene)
Is Irradiated Food Safe?
Lipid Oxidation in Precooked Turkey Breast
0
1
2
3
4
5
TBA
RS
(mg
MD
A/k
g m
eat)
0 kGy 2.5 kGy 5 kGy 0 kGy 2.5 kGy 5 kGy
Aerobic Vacuum
Frozen storage for 3 months
Cholesterol Oxidation in Turkey Thigh
Day 7
Vacuum pkg Aerobic pkg0 kGy 4.5 kGy 0 kGy 4.5 kGy
7α
& 7β-hydroxychol. 36.0c 30.0c 51.9b 86.7a
β-epoxide 0b 0b 0b 7.2a
α-epoxide 0c 0c 6.4b 11.5a
20α-hydroxycholesterol 0b 0b 0b 1.4a
cholestanetriol 0 0 0 1.07-ketocholesterol 2.7c 1.5c 19.0b 27.1a
Total* 38.7c 31.6c 77.3b 134.7a
* μg COPs/g lipid