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Singlet oxygen quenching by synthetic antioxidants (BHA, BHT, and TBHQ) was directly observed by spectroscopic monitoring of luminescence at 1268 nm. The luminescence data showed unambiguous evidence of singlet oxygen quenching by synthetic phenolic antioxidants with the highest activity for TBHQ, followed by BHA and BHT. The protective activities of these synthetic antioxidants on alpha-terpinene oxidation with chemically-induced singlet oxygen under dark further confirmed their singlet oxygen quenching abilities. Total singlet oxygen quenching rate constants (k(r) + k(q)) of BHA, BHT, and TBHQ were determined in a system containing alpha-terpinene (as a singlet oxygen trap) and methylene blue (as a sensitizer) during light irradiation, and the values were 5.14 x 10(7), 3.41 x 10(6), and 1.99 x 10(8) M(-1)s(-1), respectively. After the k(r) value of alpha-terpinene was first determined, the k(r) values of the synthetic antioxidants were calculated by measuring their relative reaction rates with singlet oxygen to that of alpha-terpinene under the identical conditions. The k(r) values of the BHA, BHT, and TBHQ were 3.90 x 10(5), 1.23 x 10(5), and 2.93 x 10(6), M(-1)s(-1). The percent partition of chemical quenching over total singlet oxygen quenching (k(r) x 100)/(k(r) + k(q)) for BHA, BHT, and TBHQ were 0.76%, 3.61%, and 1.47%, respectively. The results showed that the synthetic antioxidants quench singlet oxygen almost exclusively through the mechanism of physical quenching. This represents the first report on the singlet oxygen quenching mechanism of these synthetic antioxidants. Practical Application: The synthetic antioxidants, especially TBHQ, have been found to have a strong singlet oxygen quenching ability. This article also clearly showed that singlet oxygen quenching by synthetic antioxidants was mainly by the physical quenching mechanism. The results suggested that these synthetic antioxidants, especially TBHQ, could be used practically for the protection of the food components such as edible oils and vitamins against singlet oxygen induced oxidations without significant losses of antioxidant activity during storage under light.

Singlet oxygen quenching activity of Trolox, a water-soluble derivative of tocopherol, was studied by electron spin resonance (ESR) spectroscopy in a buffer solution (pH 7.4) containing methylene blue (MB), 2,2,6,6-tetramethyl-4-piperidone (TMPD) after light illumination for 30 min. Trolox at the concentration of 125 microM quenched 89.1% singlet oxygen in the system. Trolox showed significantly higher singlet oxygen quenching activity than ascorbic acid in the buffer solution (P < 0.05). Riboflavin in phosphate buffer solutions was degraded very fast under fluorescent light illumination. The photodegradation rate of riboflavin at pH 8.5 was significantly higher than pHs 4.5 and 6.5 (P < 0.05). Lumiflavin was also degraded under the fluorescent light illumination, but its degradation rate was much lower than that of riboflavin under the same light intensity. Unlike riboflavin, the rate of lumiflavin photodegradation was the greatest at pH 4.5 and followed by pHs 6.5 and 8.5, in a decreasing order. Trolox greatly protected the photodegradation of riboflavin and lumiflavin. The protective activities of Trolox against the photodegradation of riboflavin and lumiflavin were also pH dependent. The treatments of 5 mM Trolox in the buffer solutions of pHs 8.5 and 6.5 exhibited 56.1% and 31.7% protection of riboflavin against degradation during 120 min light illumination, respectively. The treatments of Trolox at the concentrations of 1, 3, and 5 mM in the buffer of 6.5 exhibited 14.8%, 58.4%, and 81.4% protection of lumiflavin against degradation during 24 h light illumination, respectively.

Effects of synthetic phenolic antioxidants (BHA, BHT, and TBHQ) on the methylene blue (MB) sensitized photooxidation of linoleic acid as compared with that of alpha-tocopherol have been studied. Their antioxidative mechanism was studied by both ESR spectroscopy in a 2,2,6,6-tetramethylpiperidone (TMPD)-methylene blue (MB) system and spectroscopic analysis of rubrene oxidation induced by a chemical source of singlet oxygen. Total singlet oxygen quenching rate constants (k(ox-Q)+k(q)) were determined using a steady state kinetic equation. TBHQ showed the strongest protective activity against the MB sensitized photooxidation of linoleic acid, followed by BHA and BHT. TBHQ (1 x 10(-3) M) exhibited 86.5% and 71.4% inhibition of peroxide and conjugated diene formations, respectively, in linoleic acid photooxidation after 60-min light illumination. The protective activity of TBHQ against the photosensitized oxidation of linoleic acid was almost comparable to that of alpha-tocopherol. The data obtained from ESR and rubrene oxidation studies clearly showed the strong singlet oxygen quenching ability of TBHQ. The k(ox-Q)+k(q) of BHA, BHT, and TBHQ were determined to be 3.37 x 10(7), 4.26 x 10(6), and 1.67 x 10(8) M(-1) s(-1), respectively. The k(ox-Q)+k(q) of TBHQ was within the same order of magnitude of that of alpha-tocopherol, a known efficient singlet oxygen quencher. There was a high negative correlation (r(2) = -0.991) between log (k(ox-Q)+k(q)) and reported oxidation potentials for the synthetic antioxidants, indicating their charge-transfer mechanism for singlet oxygen quenching. This is the 1st report on the kinetic study on k(ox-Q)+k(q) of TBHQ in methanol as compared with other commonly used commercial synthetic antioxidants and alpha-tocopherol.

The effects and mechanism of sesamol on the methylene blue- or chlorophyll-sensitized photo-oxidations of soybean oil have been studied. Sesamol showed strong antiphoto-oxidative activity in both methylene blue-and chlorophyll-sensitized photo-oxidations of soybean oil in a dose-dependent manner. The 1.0 x 10(-3) M sesamol treatments showed 84.7 and 43.4% inhibitions of methylene blue- and chlorophyll-sensitized photo-oxidations of soybean oil in methylene chloride. The antiphoto-oxidative activity of sesamol was comparable to that of delta-tocopherol in both methylene blue- and chlorophyll-sensitized photo-oxidations, at the same molar basis. Sesamol effectively inhibited rubrene oxidation with a chemical source of singlet oxygen in microemulsion, showing its strong singlet oxygen quenching ability. The results suggested that the antiphoto-oxidative activity of sesamol in the photo-oxidation of oil was, at least in part, due to its singlet oxygen scavenging activity. The singlet oxygen quenching rate constant (k(ox-Q) + k(q)) of sesamol was determined to be 1.9 +/- 0.3 x 10(7) M(-1) s(-1). This represents the first report on the antiphoto-oxidative activity of sesamol in the sensitized photo-oxidation of oil, and its bimolecular singlet oxygen quenching ability.

The article discusses a study which examined the effects of fruit and vegetable juices on rubrene oxidation induced by a chemical source of oxygen in a microemulsion system. A description of the method for determining the radical scavenging activities of juices is given. Results of the study showed that excessive amount of hydrogen peroxide led to the destruction of juices' antioxidant component. It also found that the highest inhibitory activity on oxidation was shown by the pear juice.