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 eaq−at neutral pH were measured. Absorption bands of the transients of eaq− reacting with the above compounds all located at a wavelength shorter than 400 nm. The eaq− scavenging abilities were divided into three groups: (+)catechin ((1.2 ± 0.1) × 108 M−1s−1) < 4-chromanol ((4.4 ± 0.4) × 108 M−1s−1) < genistein ((6.2 ± 0.4) × 109 M−1s−1) ≈ genistin ((8 ± 1) × 109 M−1s−1) ≈ rutin ((7.6 ± 0.4) × 109 M−1s−1) ≈ caffeic acid ((8.3 ± 0.5) × 109M−1s−1) < transcinnamic acid((1.1 ± 0.1) × 1010 M−1s−1) ≈ p-coumaric acid ((1.1 ± 0.1) × 1010 M−1s−1) ≈ 2,4,6-trihydroxylbenzoic acid((1.1 ± 0.1) × 1010 M−1s−1) ≈ baicalein ((1.1 ± 0.5) × 1010 M−1s−1) ≈ baicalin((1.3 ± 0.1) × 1010 M−1s−1) ≈ naringenin ((1.2 ± 0.1) × 1010 M−1s−1) ≈ naringin ((1.0 ± 0.1) × 1010 M−1s−1) ≈ gossypin((1.2 ± 0.1) × 1010M−1s−1) ≈ quercetin((1.3 ± 0.5) × 1010 M−1s−1). These results suggested that C4 keto group is the active site for eaq− to attack on flavonoids and phenolic acids, whereas the o-dihydroxy structure in B ring, the C2,3double bond, the C3-OH group, and glucosylation, which are key structures that influence the antioxidant activities of flavonoids and phenolic acids, have little effects on the eaq− scavenging activities.