Interaction of hydrated electron with dietary flavonoids and phenolic acids: Rate constants and transient spectra studied by pulse radiolysis

The reaction rate constants and transient spectra of 11 flavonoids and 4 phenolic acids reacting with e(aq)(-) at neutral pH were measured. Absorption bands of the transients of e(aq)(-) reacting with the above compounds all located at a wavelength shorter than 400 nm. The e(aq)(-) scavenging abilities were divided into three groups: (+)catechin ((1.2 +/- 0.1) x 10(8) M(-1)s(-1)) < 4-chromanol ((4.4 +/- 0.4) x 10(8) M(-1)s(-1)) < genistein ((6.2 +/- 0.4) x 10(9) M(-1)s(-1)) approximate to genistin ((8 +/- 1) x 109 M(-1)s(-1)) approximate to rutin ((7.6 +/- 0.4) x 10(9) M(-1)s(-1)) approximate to caffeic acid ((8.3 +/- 0.5) x 10(9) M(-1)s(-1)) < transcinnamic acid((1.1 +/- 0.1) x 10(10) M(-1)s(-1)) approximate to p-coumaric acid ((1.1 +/- 0.1) x 10(10) M(-1)s(-1)) approximate to 2,4,6-trihydroxylbenzoic acid((1.1 +/- 0.1) x 10(10) M(-1)s(-1)) approximate to baicalein ((1.1 +/- 0.5) x 10(10) M(-1)s(-1)) approximate to baicalin((1.3 +/- 0.1) x 10(10) M(-1)s(-1)) approximate to naringenin ((1.2 +/- 0.1) x 10(10) M(-1)s(-1)) approximate to naringin ((1.0 +/- 0.1) x 10(10) M(-1)s(-1)) approximate to gossypin((1.2 +/- 0.1) x 10(10)M(-1)s(-1)) approximate to quercetin((1.3 +/- 0.5) x 10(10) M(-1)s(-1)). These results suggested that C-4 keto group is the active site for e(aq)(-) to attack on flavonoids and phenolic acids, whereas the o-dihydroxy structure in B ring, the C-2,C-3 double bond, the C-3-OH group, and glucosylation, which are key structures that influence the antioxidant activities of flavonoids and phenolic acids, have little effects on the e(aq)(-) scavenging activities. (C) 1999 Elsevier Science Inc.