Chemistry of Singlet Oxygen Oxidation of Foods - ACS Symposium

---

Chapter 2

Chemistry of Singlet Oxygen Oxidation of Foods Wesley T. Yang and David B. Min

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: July 7, 1994 | doi: 10.1021/bk-1994-0558.ch002

Department of Food Science and Technology, Ohio State University, Columbus, OH 43210

The effects ofβ-apo-8'-carotenal,β-carotene, and canthaxanthin as well as α-, γ-, and δ-tocopherols on the chlorophyll photosensitized oxidation of soybean oil were studied by measuring peroxides, headspace oxygen, and conjugated diene content. As the number of conjugated double bonds of carotenoids increased, the antioxidant effects of carotenoids increased. αTocopherol showed the highest antioxidant effect, γ-tocopherol second, and then δ-tocopherol. Quenching mechanisms indicated that carotenoids and tocopherols quenched singlet oxygen to act as antioxidants in the chlorophyll photosensitized oxidation of soybean oil. The singlet oxygen quenching rate constants of β-apo-8'-carotenal, β-carotene, and canthaxanthin were 3.06 x 10 , 4.6 x 10 and 1.12 x 1010 M-1S- , respectively. The quenching rate constants of α-tocopherol were 2.7x107 M-1S- by peroxide value and 2.6 x 10 M-1S- by headspace oxygen. 9

1

9

7

1

1

The oxidation of lipids in food has been mainly responsible for theflavorstability, nutritional quality, and acceptability of lipid foods (115). Lipid oxidation is due to the combination of triple oxygen and singlet oxygen oxidations (3). Triplet oxygen lipid oxidation has been extensively studied to improve the oxidative stability of lipid foods during last 50 years (2, 4, 7,13). However, it does not fully explain the initiation step of lipid oxidation (3, 6, 70). Recently, the role of singlet oxygen at the initiation stage of lipid oxidation was suggested because singlet oxygen can direcdy react with double bonds of fatty acids and its reaction rate with linoleic acid is at least 1,450 times higher than that of triplet oxygen (16). Singlet oxygen can be formed by chemical, enzymatic, photochemical, and physical methods (6, 8,10,17,18) and initiate the oxidation of lipids in foods. Photosensitized reaction, which is initiated by sensitizers, is the simplest and common pathway to generate a substantial amount of singlet oxygen in foods. This, in particular, has great impact on die oxidation of foods which contain sensitizers. Chlorophylls and their decomposition products in vegetable oils are known to be efficient photochemical sensitizers for singlet oxygen formation (1927). Singlet oxygen directly reacts with unsaturated fatty acids of vegetable oils to form a mixture of conjugated and nonconjugated hydroperoxides (21, 24).

0097-6156/94/0558-0015$08.00/0 © 1994 American Chemical Society Ho and Hartman; Lipids in Food Flavors ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

16

LIPIDS IN FOOD FLAVORS

The decomposition of hydroperoxides produces off-flavor volatile compounds and potentially toxic oxidation compounds (28-38). The undesirable singlet oxygen lipid oxidation can be minimized by preventing the formation of singlet oxygen and/or quenching singlet oxygen physically and chemically (23, 39). Physical quenching can be explained by energy and charge transfer mechanisms (39). Carotenoids quench the singlet oxygen through energy transfer from singlet oxygen to carotenoids with 9 or more conjugated double bonds. It is exothermic and these carotenoids are efficient singlet oxygen quenchers (40-47). The physical quenching of singlet oxygen by tocopherols is due to the charge transfer mechanism (43).

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: July 7, 1994 | doi: 10.1021/bk-1994-0558.ch002

Chemical Properties of Triple Oxygen and Singlet Oxygen Molecular oxygen which consists of two oxygen atoms has 5 bonding and 5 antibonding orbitals and 12 valence electrons (45). The electronic configuration of triplet state oxygen molecular orbitals is shown in Figure 1. Pauli exclusion principle states that no two electrons in an atom can have the same set of four quantum numbers. Hund's rule states that electrons with a number of equivalent orbitals first occupy all the orbitals singly with parallel spins before paring in any orbital can occur. The two highest energy electrons of triplet state oxygen are located one each in the two degenerate π*2ρ orbitals with parallel spins but with opposite angular momenta. The spin multiplicity used for spin states is defined as 2S +1, where S is the total spin quantum number. The triplet state oxygen has three closely grouped energy states by the two unpaired electrons under a magnetic field. Therefore, triplet state oxygen has paramagnetic and diradical properties, and gives spin multiplicity of 3. As a result, it is characterized as triplet oxygen. The electronic configuration of singlet state oxygen molecular orbitals is shown in Figure 2, which displays no parallel characteristic of those electrons in π*2ρ· In virtue of cancellation of the opposite spin alignments of the two electrons, the total spin quantum number S in singlet state oxygen becomes 0 and the spin multiplicity is 1; therefore, the oxygen is named as singlet oxygen. Singlet oxygen exhibits diamagnetic and non-free radical nature and thus has no magnetic momentum. There are two types of singlet oxygen. The first is Σ energy state and the second is A energy state. Both types violate Hund's rule which also implicates the maximum multiplicity principle. Electronic repulsion and low multiplicity of electronic arrangement of singlet oxygen result in the increase of potential energy. Consequently, both types of singlet oxygen have the energy levels of 37.5 and 22.4 Kcal/mole above triplet oxygen in ground state, respectively. The Σ state singlet oxygen is so energetic that it rapidly converts to the Δ state singlet oxygen upon being produced. Therefore, singlet oxygen is generally referred to as Δ. The lifetime of singlet oxygen is solvent dependent ranging from 50 to 700 μ8 (49); it is longer enough to initiate oxidation reactions with other molecules in food systems. Moreover, temperature has little effect on the reaction rate of singlet oxygen oxidation due to the low activation energy (0-6 Kcal/mole). Triplet oxygen is reluctant to initiate the oxidation reaction of organic molecules which are naturally in singlet state because of a spin forbidden barrier. In contrast, singlet oxygen can readily and directly react with singlet state lipids in a spin allowed process to generate peroxides. Singlet oxygen with an vacant molecular orbital is highly electrophilic and eager to seek electrons to fill the ι

l

ι

ι

ι

Ho and Hartman; Lipids in Food Flavors ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

Singlet Oxygen Oxidation of Foods

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: July 7, 1994 | doi: 10.1021/bk-1994-0558.ch002

YANG AND MIN

Ho and Hartman; Lipids in Food Flavors ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

LIPIDS IN FOOD FLAVORS

18

orbital. Therefore, electron-rich olefinic and aromatic compounds, amines, and sulfides can easily react with singlet oxygen. Singlet oxygen can be detected by several methods such as chemical traps, luminescence, quenchers, and electron spin resonance. (43, 48,50,51).

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: July 7, 1994 | doi: 10.1021/bk-1994-0558.ch002

Mechanisms of Photosensitized Oxidation The effects of light on the flavor stability of lipid foods is explicable by photolytic autooxidation or photosensitized oxidation (70). Photolytic autooxidation which produces free radicals by ultraviolet irradiation is not the major concern in the oil industry because UV light is unlikely to reach lipids in systems except for inadvertent exposure to sunlight without a protective out-layer or container. However, photosensitized oxidation is important for the flavor stability of lipid foods. Photosensitized oxidation occurs in the presence of light, triplet oxygen, and photosensitizers. Photosensitizers include synthetic dyes such as acridine orange, crystal violet, eosin, erythrosin, methylene blue, proflavin, and rose bengal (52); naturally occurring pigments such as chlorophyll, flavin, and porphyrin (53), coenzymes and biochemicals such as pyridoxals and psoralens (54); metallic salts such as cadium sulfide, zinc oxide, and zinc sulfide (54); polycyclic aromatic hydrocarbons such as anthracene and rubrene (55,56); and transition metal complex such as ruthenium bipyridine (56). A sensitizer can absorb visible or near UV light and becomes an excited singlet state sensitizer ^Sen*) which has a short lifetime. The excited singlet state sensitizer ^Sen*) is rapidly converted to the ground state by emitting the fluorescence light or to the excited triplet state sensitizer ( Sen*) by inter-system crossing (ISC). The excited triplet state sensitizer ( Sen*) which has longer lifetime than the excited singlet state ( Sen*) decays to the ground state slowly by emitting the phosphorescence light. Thus, the efficient sensitizers for the generation of singlet oxygen are longlived excited triplet state sensitizers ( Sen*) in high quantum yield (54). The excitation and deactivation of photosensitizers are illustrated in Figure 3. The excited triplet state sensitizer ( Sen*) is produced via inter-system crossing during photosensitization. There are two reaction pathways, Type I and Type Π, for the excited triplet sensitizer ( Sen*) to proceed (Figure 4). In Type I pathway (sensitizer-substrate), the sensitizer serves as a photochemically activated free-radical initiator. The excited triplet sensitizer ( Sen*) reacts with the substrate (RH) to produce radicals (R-) or radical ions (RH- ) by hydrogen transfer or electron transfer, respectively. The resultant radicals react with the diradical triplet oxygen to produce the oxidized products (ROOH) which can breakdown to induce free radical chain autooxidation. In type II pathway, the excited triplet sensitizer reacts with triplet oxygen to generate the singlet oxygen by triplet-triplet annihilation reaction. The singlet oxygen thus produced reacts with substrates (RH) to form the oxidized products (ROOH) (57-62). There is also a chance of 1% that an electron can be transferred from the excited triplet sensitizer ( Sen*) to triplet oxygen to produce superoxide anion (0 -~) and sensitizer radical ion (Sen- ) (63,64). Singlet oxygen involvement in the photosensitized oxidation is in Type Π which occurs rapidly and thus accounts for almost all photosensitized oxidation (65). The participation of Type I or II and the intermediates depend on the chemical nature and concentrations of the sensitizer, substrate, and oxygen as well as the 3

3

J

3

3

3

3

+

J

2

+

Ho and Hartman; Lipids in Food Flavors ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: July 7, 1994 | doi: 10.1021/bk-1994-0558.ch002

2. YANG AND MIN

Singlet Oxygen Oxidation of Foods

19

reaction conditions such as pH and solvents (65). The change of reaction conditions can shift the pathway from Type I to Type Π, or vice versa. Generally, readily oxidizable (phenols and amines) or readily reducible (quinones) compounds favor Type I pathway, while compounds not readily oxidizable or reducible such as olefins, dienes, and aromatic compounds favor Type Π pathway. However, amines, phenols, and other readily oxidizable compounds undergo Type Π pathway under some conditions. The oxygen solubility in solution is one of the most important factors which determine the predominant type of pathway. Oxygen is much more soluble in most organic solvents than in water (66). The high solubility of oxygen in organic solvents might favor Type II pathway, whereas the low solubility of oxygen in water might favor Type I pathway. The preferential solubility of the sensitizer in the solvents might decide the reaction type as well. Lipid soluble chlorophylls and hematoporphyrins might favor Type II pathway but water soluble riboflavin probably favors Type I pathway (65). The competition between substrate and oxygen for a triplet sensitizer is another influential factor to determine whether Type I or Type Π reaction occurs. As shown in Table I (65), Type I or Type Π pathways compete efficiendy for benzophenone in oxygen-saturated ethanol; however, the type II process predominates for the eosin under the same condition even at very low oxygen concentration. Table I. Competition of Type I and Type Π Processes* Sensitizer Benzophenone Eosin

1

KrfSKsec- )

1

KntSKseo )

7

3.2xl0 1.7x10

s

2xl0 2xl0

7

7

* In O2 saturated ethanol as substrates Reaction Mechanisms of Singlet Oxygen with Lipid Foods Singlet oxygen is very electrophilic and involved in several reactions. These include 1,4-cycloaddition to dienes and heterocyclic compounds, the ene reaction with olefins, the oxidation of sulfides to sulfoxides, and the photosensitized oxidation of phenols to unstable hydroxy-dienones (57, 65, 67). Among these reactions, the ene reaction and photosensitized oxidation of phenols are the most important in singlet oxygen oxidation of foods. For the singlet oxygen ene reaction, there are five possible mechanisms: biradical intermediate, zwitterionic intermediate, concerted ene mechanism, pereperoxide intermediate, and perpendicular approach mechanisms. According to biradical intermediate and zwitterionic intermediate mechanisms, singlet oxygen attacks one end of the olefinic linkage to produce either a biradical (48) or a zwitterion (48,50). These intermediates are changed to allylic hydroperoxides. In the concerted ene mechanism, a six-center transition state is involved in which attack of one end of the singlet oxygen molecule occurs at the α-olefinic carbon while the other end abstracts the γ-allylic hydrogen (58). In contrast, in the pereperoxide intermediate and perpendicular approach mechanisms, approach of the singlet oxygen is along the perpendicular bisector of the plane of ρ orbitals (68). The concerted sixmembered ring formation process might yield a hydroperoxide from the cis addition of singlet oxygen. The ene reaction prefers hydrogen abstraction on the disubstituted side of trisubstituted olefins (48,50). The geometry of possible sixmembered transition state in the reaction between singlet oxygen and linoleate leads

Ho and Hartman; Lipids in Food Flavors ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

20

LIPIDS m FOOD FLAVORS

to the formation of conjugated and nonconjugated cis and trans compounds (18). The ene reaction producing conjugated and nonconjugated products can be used to distinguish singlet oxygen oxidationfromthe free radical autooxidation of lipid which does not produce the nonconjugated products. The pereperoxide mechanism is also responsible for the formation of nonconjugated isomers (68). Another important reaction of singlet oxygen in vegetable oils is photosensitized oxidation of phenols. This reaction is a quenching reaction of singlet oxygen by phenol compounds in the lipid. The electron transfer is the primary step which is followed by rapid proton transfer (13). It suggests the possibility of phenol oxidation by the combination of free radical autooxidation and singlet oxygen oxidation (69).

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: July 7, 1994 | doi: 10.1021/bk-1994-0558.ch002

Photosensitized Oxidation of Lipid Foods Biologically important lipids that are susceptible to photosensitized oxidation include unsaturated fatty acids, phospholipids, triglycerides, cholesterol, vitamin D, steroids, and prostaglandins (54). Photosensitized oxidation of unsaturated fatty acids and olefins is mainly responsible for the photooxidative degradation of lipid foods (54). Erythrosin showed the photosensitizing effect on the oxidation of pork lucheon meat and it accelerated the deterioration of the meat flavor during exposure to fluorescent lamps (55). Soybean phosphatidyl choline and synthetic dilinoleoyl phosphatidyl choline react with singlet oxygen in the presence of light and methylene blue as a sensitizer (24). Soybean and olive oils are photooxidized in the presence of light and chlorophyll as a sensitizer. Cholesterol is readily oxidized by singlet oxygen to form 3-p-hydroxy-5a-hydroperoxy-A -cholestene, the decomposition of which leads to free radical chain oxidation of unsaturated fatty acids (70). 6

Quenching Mechanisms and Kinetics of Singlet Oxygen Lipid Oxidation Quenching of singlet oxygen means both chemical and physical quenchings (43). Singlet oxygen reacts with quenchers to form oxidized quenchers in chemical quenching, but physical quenching degenerates singlet oxygen to triplet oxygen. Although chemical and physical quenchings can occur together, chemical quenching is a reaction rather than a quenching. Physical quenching can be explained by energy transfer and / or charge transfer mechanisms (43). Energy transfer quenching involves the formation of triplet oxygen and triplet quencher as follows: 0

l

+Q

3

3

> 0 + Q

l

2

2

The energy of the quencher in this process is very near or below that of singlet oxygen. The quenching of singlet oxygen by β-carotene is a good example of energy transfer quenching (40-47). The quenching rate constant of β-carotene is 3x

l0

l

l

10 MT S' (42).

The compounds with low oxidation potentials and low triplet energies undergo the charge transfer quenching. In the charge transfer quenching, singlet oxygen reacts with electron donors to form a charge transfer complex as follows (71-74): Q + 1Q

> [ Q — O2'] +

2

1

+

> [Q —

0 '] 2

3

> Q + θ2 3

Ho and Hartman; Lipids in Food Flavors ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

2. YANG AND MIN

21

Singlet Oxygen Oxidation of Foods

The complex of singlet state is relaxed to triplet state by inter-system crossing mechanism and then dissociates. The quenching rate constant of charge transfer quenching is below 10 M" S" . The involvement of electron transfer in this mechanism implies that the more easily oxidizable compounds are the better charge transfer quenchers. These types of quenchers are amines, phenols, sulfides, iodide, and azide (43). The singlet oxygen lipid oxidation can be minimized by singlet oxygen quenching and / or triplet sensitizer quenching as shown in Figure 5 (43). Negligible singlet sensitizer quenching due to its short lifetime gives the following steady state kinetic equation for the lipid oxidation product (AO2): 9

1

d[A0 ]

Kp0 ]

2

K[A]

2

=K( Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: July 7, 1994 | doi: 10.1021/bk-1994-0558.ch002

1

Kpo ] + lyQI

dt

)(

KJA] + (K + K . )[Q] + K

2

x Q

) D

where A0 : oxidized lipid, K : rate constant of triplet sensitizer formation, K : reaction rate constant of lipid with singlet oxygen, A : lipid, kq: reaction rate constant of physical singlet oxygen quenching by quencher Q, k ^ Q : reaction rate constant of chemical singlet oxygen quenching by quencher Q, k^: decaying rate constant of singlet oxygen. In the case where there is only singlet oxygen quenching (kg[Q] « ko[ 02]), the equation is as follows: 2

R

3

d[A0 ]

KJA]

2

=K(

)

dt

K [A] + (K +K . )[Q]+K R

Q

O X

Q

D

where, K : rate constant of singlet oxygen formation 1

1

The plot of (d[A02]/dt)" vs. [ A ] ' at various concentrations of [Q] gives constant y-intercept of K ' which is independent of [Q]. When there is no quencher the slope (S ) becomes K " (k(j/k) and the ratio of S to y-intercept gives k