A chemical behavior of Quercetin as antioxidant and metal chelator has become the subject of intense experimental research. In this paper, we apply a semi empirical approach to study the stability constant of Quercetin with metals like Cd, Pb and Bi. The comparative analysis of the theoretical formation constant of Quercetin-metal complexes with metals with experimental results shows that H-removal from phenolic -OH site during the metal chelation correlated with experimentally determined stability constant by Job's method. The analysis of the theoretical BDE (Bond Dissociation Energy) values, for all OH sites in Quercetin, clearly shows the importance of the B-ring -OH groups. Mulliken spin density distribution for the radicals formed after H-removal on each OH site of Quercetin is also investigated. The results indicate that 3'-O. and 4'-O . (B-ring) Quercetin radical spin density appears to be more delocalized than the other sites present on A-ring.
The electronic properties responsible for free radical scavenging and metal chelation of a set of 7 flavonoids compounds were studied. Electronic parameters were calculated using semi empirical PM3 method. The structure activity relationship of OH substitution, C2-C3 double bond, chelation site of flavonoid and related property of antioxidant efficiency, BDE, mulliken atomic charge, electron current movement among A, B and C-rings were investigated. The relative change in energy (H f ) associated with the formation of phenoxyl radicals and spin density distribution of phenoxyl radicals were determined. The complexation mechanism of seven flavonoids quercetin, morin, apigenin, naringenin, chrysin, taxifolin and fisetin with metal ions, by molecular mechanics (UFF) method was investigated. Flavonoids metal chelation site order on toxic metals Pb, Bi and Cd was investigated. The most likely chelation site for metal ion was 5-OH & 4=O group, followed by 4=O & 3-OH group and the 3'& 4' hydroxyl for all the flavonoid molecules studied. In the absence of C2-C3 double bond (taxifolin) the order of metal chelation site with metal was 3-OH & 4=O, followed by 5-OH & 4=O group and 3' & 4' hydroxyl.
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