Kinetic study of the quenching reaction of singlet oxygen by common synthetic antioxidants (tert-Butylhydroxyanisol, tert-di-Butylhydroxytoluene, and tert-Butylhydroquinone) as compared with α-Tocopherol

Effects of synthetic phenolic antioxidants (BHA, BHT, and TBHQ) on the methylene blue (MB) sensitized photooxidation of linoleic acid as compared with that of α-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 × 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 α-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 × 10⁷, 4.26 × 10⁶, and 1.67 × 10⁸ M^-1 s^-1, respectively. The k_ox-Q + k_q of TBHQ was within the same order of magnitude of that of α-tocopherol, a known efficient singlet oxygen quencher. There was a high negative correlation (r² = -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 α-tocopherol.