Singlet Oxygen Oxidation Rates of ∝-, γ-, and δ-Tocopherols

ABSTRACT:  The reaction rate constants of 5 × 10^?4 M, 10 × 10^?4 M, and 20 × 10^?4 M α-, γ-, and δ-tocopherols with singlet oxygen in methylene chloride containing 1 × 10^?5 M chlorophyll under light at 25 °C for 60 min were studied. The oxidation of tocopherols determined by a spectrophotometric method showed that the losses of 20 × 10^?4 M α-, γ-, and δ-tocopherols after 60 min under light were 21%, 16%, and 9%, respectively. The degradation of α-, γ-, and δ-tocopherols was undetectable in the absence of chlorophyll under light or in the presence of chlorophyll in dark. The losses of tocopherols under light were mainly due to singlet oxygen oxidation. The degradation rates of 20 × 10^?4 M α-, γ-, and δ-tocopherols were 6.6 ×10^?6 M/min, 5.0 × 10^?6 M/min, and 2.9 × 10^?6 M/min, respectively. The reaction rates between α-, γ-, or δ-tocopherol and singlet oxygen were 4.1 ×10⁶/M s, 3.3 × 10⁶/M s, and 1.4 × 10⁶/M s, respectively. The singlet oxygen oxidation rate of δ-tocopherol was significantly lower than α- or γ-tocopherol at α= 0.05. As the electron density in the chromanol ring of tocopherol increased, the singlet oxygen oxidation was increased.