A kinetic study of the quenching reaction of singlet oxygen (O-1(2)) 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 degrees 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, k(Q) ( k(q) + k(r) physical quenching + chemical reaction), for the reaction of catechins with 10, 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, E-P, show higher reactivities. It was observed that the chemical reaction (k(r)) is almost negligible in the quenching reaction Of O-1(2) by catechins. The kQ values of EGCG (1.47 x 10(8) M-1 s(-1)) and ECG (7.81 X 10(7)) were found to be larger than those of lipids (1.3 x 10(5-)1.9 X 10(5) M-1 s(-1)), amino acids (< 3.7 x 10(7)), and DNA (5.1 x 10). Further, these values are similar to those (1.15 x 10(8)-2.06 x 10(8) M-1 s(-1)) of alpha- and gamma-tocopherol, ubiquinol-10, and gamma-tocopherol hydroquitione (plastoquinol model). The result suggests that catechins may contribute to the protection of oxidative damage in biological systems, by quenching O-1(2). (c) 2005 Elsevier Inc. All rights reserved.