Kinetic study of the quenching reaction of singlet oxygen by tea catechins in ethanol solution

A kinetic study of the quenching reaction of singlet oxygen (1O2) with catechins (catechin (CA), epicatechin (EC), epigallocatechin (EGC), epicatechin gallate (ECG), epigallocatechin gallate (EGCG)) and related compounds (5-methoxyresorcinol (MR), 4-methylcatechol (MC), and n-propyl gallate (PG)) was performed in ethanol at 35°C. MR, MC, and PG are considered to be a model of resorcinol (A)-, catechol (B)-, and gallate (G)-rings in catechins, respectively. The overall rate constants, kQ (= kq + kr, physical quenching + chemical reaction), for the reaction of catechins with 1O2 increased in the order of PG < MR < MC < CA < EC < EGC < ECG < EGCG. In a comparison of the rate constants, the relationship between quenching rates and chemical structures is discussed. The catechins which have lower peak oxidation potentials, EP, show higher reactivities. It was observed that the chemical reaction (kr) is almost negligible in the quenching reaction of 1O2 by catechins. The kQ values of EGCG (1.47 × 108 M−1 s−1) and ECG (7.81 × 107) were found to be larger than those of lipids (1.3 × 105-1.9 × 105 M−1 s−1), amino acids (<3.7 × 107), and DNA (5.1 × 105). Further, these values are similar to those (1.15 × 108-2.06 × 108 M−1 s−1) of α- and γ-tocopherol, ubiquinol-10, and γ-tocopherol hydroquinone (plastoquinol model). The result suggests that catechins may contribute to the protection of oxidative damage in biological systems, by quenching 1O2.