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.

中文翻译：

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木犀草素中铜（II）的配位和转运及其对自由基清除的影响。

木犀草素作为一种自由基清除剂，与芹菜素的反应显着不同，尽管结构上存在微小差异，但它们对铜（II）的配位反应却不同。通过ABTS•+自由基清除研究，Cu（II）的配位可提高木犀草素的自由基清除效率，尤其是在低pH条件下，而降低芹菜素在低和较高pH值下的效率。木犀草素与Cu（II）形成1：1的络合物，在pH <6时与4-羰基和5-苯酚结合，在pH> 6时与3'，4'-邻苯二酚结合。芹菜素形成与pH值无关的1：2复合物，该pH与4-羰基和5-羟基基团配位。通过pH跃迁停止流研究，与木犀草素配位的Cu（II）从4.5到3'，4'的位点的速率常数为11.1±0.3 s-1，从3'的位速率常数为1.0±0.1 s-1，独立于Cu（II）浓度的4'至4,5位，指出从一个中间有两个Cu（II）配位的中间体解离Cu（II）的速率。3'，4'-邻苯二酚被认为是Cu（II）易位的开关，去质子化引发4,5至3'，4'易位，质子化引发3'，4'至4,5易位。对于双配位芹菜素，配位对称性平衡了Cu（II）的电子吸收效应，与亲本芹菜素相比，导致酚酸度的降低和自由基清除效率的降低。与母体木犀草素相比，通过密度泛函理论证实，通过Cu（II）的电子撤离提高了酚酸度，从而提高了4,5和3'，4'Cu（II）配位的木犀草素的自由基清除率。 （DFT）计算。