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.

Radical scavenging activity of kaempferol is notably enhanced by Ca(ii) binding.

Sn(II) binds to kaempferol (HKaem, 3,4 ,5,7-tetrahydroxy-2-(4-hydroxyphenyl)-4H-1benzopyran-4-one) at the 3,4-site forming [Sn(II)(Kaem) 2 ] complex in ethanol. DPPH • scavenging efficiency of HKaem is dramatically decreased by SnCl 2 coordination due to formation of acid inhibiting deprotonation of HKaem as ligands and thus reduces the radical scavenging activity of the complex via a sequential proton-loss electron transfer (SPLET) mechanism. Moderate decreases in the radical scavenging of HKaem are observed by Sn(CH 3 COO) 2 coordination and by contact between Sn and HKaem, in agreement with the increase in the oxidation potential of the complex compared to HKaem, leading to a decrease in antioxidant efficiency for fruits and vegetables with Sn as package materials. be increased by the presence of copper (II), iron (III), and zinc (II) by other authors and our recent studies [7,8,[23][24][25][26]. In the present study, HKaem was selected as a typical flavonoid with two possible chelation sites (3,4 and 4,5) for metal ions, and complex formation by Sn(II) coordination to HKaem was investigated together with the complex as a radical scavenger. The effect on radical scavenging efficiency of HKaem by Sn(II) coordination is of relevance as Sn is used for packaging of food, and accordingly deserves more attention.Molecules 2020, 25, x FOR PEER REVIEW