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Zinc(II) enhances radical scavenging of the flavonoid kaempferol (Kaem) most significantly for the 1:1 Zn(II)-Kaem complex in equilibrium with the 1:2 Zn(II)-Kaem complex both with high affinity at 3-hydroxyl and 4carboxyl coordination. In methanol/chloroform (7/3, v/v), 1:1 Zn(II)-Kaem complex reduces β-carotene radical cation, β-Car •+ , with a second-order rate constant, 1.88 × 10 8 L• mol −1 •s −1 , while both Kaem and 1:2 Zn(II)-Kaem complex are nonreactive, as determined by laser flash photolysis. In ethanol, 1:1 Zn(II)-Kaem complex reduces the 2,2-diphenyl-1-picrylhydrazyl radical, DPPH • , with a second-order rate constant, 2.48 × 10 4 L•mol −1 •s −1 , 16 times and 2 times as efficient as Kaem and 1:2 Zn(II)-Kaem complex, respectively, as determined by stopped-flow spectroscopy. Density functional theory calculation results indicate significantly increased acidity of Kaem as ligand in 1:1 Zn(II)-Kaem complex other than in 1:2 Zn(II)-Kaem complex. Kaem in 1:1 Zn(II)-Kaem complex loses two protons (one from 3-hydroxyl and one from phenolic hydroxyl) forming 1:1 Zn(II)-(Kaem−2H) during binding with Zn(II), while Kaem in 1:2 Zn(II)-Kaem complex loses one proton in each ligand forming Zn(II)-(Kaem−H) 2 , as confirmed by UV−vis absorption spectroscopy. Zn(II)-(Kaem−2H) is a far stronger reductant than Kaem and Zn(II)-(Kaem−H) 2 as determined by cyclic voltammetry. Significant rate increases for the 1:1 complex in both β-Car •+ scavenging by electron transfer and DPPH • scavenging by hydrogen atom transfer were ascribed to decreases of ionization potential and of bond dissociation energy of 4′-OH for deprotonated Zn(II)-(Kaem−2H), respectively. Increased phenol acidity of plant polyphenols by 1:1 coordination with Zn(II) may explain the unique function of Zn(II) as a biological antioxidant and may help to design nontoxic metal-based drugs derived from natural bioactive molecules.

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Luteolin differs as a radical scavenger dramatically from apigenin in response to Cu­(II) coordination despite a minor structural difference. Coordination of Cu­(II) increases the radical scavenging efficiency of luteolin, especially at low pH, while decreases the efficiency of apigenin at both low and higher pH as studied by ABTS•+ radical scavenging. Luteolin forms a 1:1 complex with Cu­(II) binding to 4-carbonyl and 5-phenol for pH <6 and to 3′,4′-catechol for pH >6. Apigenin forms a 1:2 complex independent of pH coordinated to 4-carbonyl and 5-hydroxylyl. Cu­(II) coordinated to luteolin, as studied by pH jump stopped-flow, translocates with rate constants of 11.1 ± 0.3 s–1 from 4,5 to 3′,4′ sites and 1.0 ± 0.1 s–1 from 3′,4′ to 4,5 sites independent of Cu­(II) concentration, pointing toward the dissociation of Cu­(II) from an intermediate with two Cu­(II) coordination as rate determining. 3′,4′-Catechol is suggested to be a switch for Cu­(II) translocation with deprotonation initiating 4,5 to 3′,4′ translocation and protonation initiating 3′,4′ to 4,5 translocation. For dicoordinated apigenin, the coordination symmetry balances an electron withdrawal effect of Cu­(II) resulting in a decrease of phenol acidity and less radical scavenging efficiency compared to parent apigenin. Compared to that of parent luteolin, the radical scavenging rate of both 4,5 and 3′,4′ Cu­(II)-coordinated luteolin is enhanced through increased phenol acidity by electron withdrawal by Cu­(II), as confirmed by density functional theory (DFT) calculations. Coordination and translocation of Cu­(II) accordingly increases the antioxidant activity of luteolin at pH approaching the physiological level and is discovered as a novel class of natural molecular machinery derived from plant polyphenols, which seems to be of importance for protection against oxidative stress.