Kinetic study of the quenching reaction of singlet oxygen by flavonoids in ethanol solution.

The quenching rate of singlet oxygen (1O2) by seven kinds of flavonoids (flavone, flavonol, chrysin, apigenin, rutin, quercetin, and myricetin) with 2,3-double bonds has been measured spectrophotometrically in ethanol at 35 degrees C. The overall rate constants kQ (= kq + kr, physical quenching + chemical reaction) increased as the number of OH groups substituted to the flavone skeleton (that is, the total electron-donating capacity of flavonoids) increases. The existence of catechol or pyrogallol structure in the B-ring is essential for the 1O2 quenching of flavonoids.

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 ((1)O(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 (1)O(2) increased in the order of PG < MR < MC < CA < EC < EGC < ECG < EGCG.