title: Phenol acidity and ease of oxidation in isoflavonoid/β-carotene antioxidant synergism.
abstract: Regeneration of β-carotene from the β-carotene radical cation by the 4'-propylpuerarin anion (second-order rate constant=1.5×10(9) L mol(-1) s(-1) in methanol/chloroform=1:9 (v/v) solution at 25 °C as determined by laser flash photolysis) was found to be marginally slower than regeneration by the 7-propylpuerarin anion (2.3×10(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°=1.12 and 1.44 V (vs NHE) as determined by cyclic voltametry in aqueous solution and pKa=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 β-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 β-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.
authors.affiliation: Department of Chemistry, Renmin University of China, Beijing 100872, China. rmhan@chem.ruc.edu.cn
authors.forename: Rui-Min
authors.initials: RM
authors.lastname: Han
authors.forename: Dan-Dan
authors.initials: DD
authors.lastname: Li
authors.forename: Chang-Hui
authors.initials: CH
authors.lastname: Chen
authors.forename: Ran
authors.initials: R
authors.lastname: Liang
authors.forename: Yu-Xi
authors.initials: YX
authors.lastname: Tian
authors.forename: Jian-Ping
authors.initials: JP
authors.lastname: Zhang
authors.forename: Leif H
authors.initials: LH
authors.lastname: Skibsted
mesh_terms: D000975:Antioxidants / Q000494:pharmacology*; D004357:Drug Synergism; D005419:Flavonoids / Q000737:chemistry / Q000494:pharmacology*; D006863:Hydrogen-Ion Concentration; D007529:Isoflavones / Q000737:chemistry / Q000494:pharmacology; D010084:Oxidation-Reduction; D010636:Phenols / Q000737:chemistry*; D019207:beta Carotene / Q000737:chemistry / Q000494:pharmacology*
chemical_list: D000975:Antioxidants; D005419:Flavonoids; D007529:Isoflavones; D010636:Phenols; D019207:beta Carotene; C033607:puerarin