Notable Effects of the Metal Salts on the Quenching Reaction of Singlet Oxygen by α-Tocopherol in Ethanol Solution

Abstract

Abstract Measurement of the quenching rate constant (kQ) of singlet oxygen (1O2) by α-tocopherol was performed in ethanol solution including four kinds of alkali and alkaline earth metal salts (NaClO4, Mg(ClO4)2, MgCl2, and CaCl2) by varying concentrations of metal salts. A remarkable effect of metal salts on the kQ value of α-tocopherol was observed. The 1O2-quenching rate constant (kQ) decreased notably with increasing concentrations of metal salts, and reached a constant value for each salt at high concentration of the salt. For example, the kQ value (9.48 × 107 M−1 s−1) obtained in the presence of 20 mM of Mg(ClO4)2 was 46% of that (2.06 × 108 M−1 s−1) in the absence of metal salt. The kQ value decreased in the order of no metal salt > NaClO4 > CaCl2 > MgCl2 ≈ Mg(ClO4)2 at the same concentration of the metal salts. The metal salts having a smaller ionic radius and a larger charge of the cation gave a smaller kQ value. Effects of anions were negligible. These metal cations coexist with α-tocopherol in foods and biological systems, suggesting that the metal cations may give a notable effect to the 1O2-quenching rate constant (kQ).