Puerarin and Conjugate Bases as Radical Scavengers and Antioxidants: Molecular Mechanism and Synergism with β-Carotene

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Abstract
The 4‘-hydroxyl group of puerarin, a C-glycoside of the isoflavonoid daidzein, was shown, using 2,2‘-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) radical cation and stopped-flow spectroscopy and by comparison with the 7-propylpuerarin (A ring derivative) and 4‘-propylpuerarin (B ring derivative), to be a more efficient radical scavenger as compared to the 7-hydroxyl group by a factor of 2, a difference increasing upon deprotonation. The difference in radical scavenging agreed with the oxidation potentials (cyclic voltammetry in acetonitrile, 0.1 M Bu4NBF4 at 25 °C): E/mV = 862 ± 3 for puerarin, 905 ± 10 for 7-propylpuerarin, and 1064 ± 2 for 4‘-propylpuerarin relative to ferrocene/ferricenium. In aqueous solution, the reduction potential was shown to decrease for increasing pH, and deprotonation of the 4‘-hydroxyl group increased radical scavenging more than deprotonation of the 7-hydroxyl group. The 7-hydroxyl was found to be more acidic (pKa1 = 7.20 ± 0.01 in puerarin and pKa = 7.23 ± 0.01 in 4‘-propylpuerarin) than the 4‘-hydroxyl group (pKa2 = 9.84 ± 0.08 in puerarin and pKa = 9.51 ± 0.02 in 7-propylpuerarin); aqueous solution, ionic strength of 0.1, and 25 °C. In phosphatidyl choline liposome of pH 7.4, puerarin and β-carotene each showed a modest antioxidant activity measured as prolongation of the lag phase for formation of conjugate dienes and using the water-soluble radical initiator APPH with effects of puerarin and β-carotene being additive. For the lipophilic initiator AMVN, the antioxidative effect decreased for puerarin and increased for β-carotene as compared to APPH and showed a clear synergism. A regeneration of β-carotene, effective in the liposome lipid phase as antioxidant, from the cation radical by deprotonated forms of puerarin was demonstrated in 9:1 chloroform/methanol using laser flash photolysis with k2 = 2.7 × 104 L mol-1 s-1 for the bimolecular process between the cation radical and the puerarin dianion. Keywords: Puerarin; β-carotene; radical scavenging; antioxidant synergism

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