Phenol Acidity and Ease of Oxidation in Isoflavonoid/β-Carotene Antioxidant Synergism

Regeneration of beta-carotene from the beta-carotene radical cation by the 4'-propylpuerarin anion (second-order rate constant = 1.5 x 10(9) L mol(-1) s(-1) in methanol/chloroform = 1:9 (v/v) solution at 25 degrees C as determined by laser flash photolysis) was found to be marginally slower than regeneration by the 7-propylpuerarin anion (2.3 x 10(9) L mol(-1) s(-1)), in agreement with the 7-propylpuerarin anion being more reducing (E' = 0.56 V vs NHE) than the 4'-propylpuerarin anion (E' = 1.01 V vs NHE). The potentials were calculated from E degrees = 1.12 and 1.44 V (vs NHE) as determined by cyclic voltametry in aqueous solution and pK(a) = 9.51 and 7.23 obtained previously for 7-propylpuerarin and 4'-propylpuerarin, respectively. The less reducing but more acidic 4'-propylpuerarin showed less antioxidant activity in liposome of pH 7.4, but more significant antioxidant synergism with beta-carotene than the more reducing but less acidic 7-propylpuerarin for oxidation initiated in the liposome lipid phase. Electrostatic effects are concluded to be important in the regeneration of beta-carotene from the radical cation in the water/lipid interface because approximately 50% of 4'-propylpuerarin is present as the anion, whereas only 0.5% of 7-propylpuerarin is present as the anion. In contrast, penetration of the undissociated phenolic group into the lipid phase, more significant for 7-propylpuerarin than for 4'-propylpuerarin according to the calculated water/lipid partition coefficients, becomes important for the chain-breaking action in lipid oxidation of the puerarin derivatives as models for (iso)flavonoids and their glycosides.