| Five galvinoxyl radicals (G) reduction by one polyphenolic myricetin (Myc, 3,30,40,5,50,7-Hexahydroxyflavone) molecule-using EPR method-demonstrated that each Myc should donate at least five H
atoms resulted in multiradicals Myc5 (5 G þ 1Myc / 5 GH þ 1 Myc5). The process that five H atoms
donation occurs from different OH sites of Myc lead to appearing of five unpaired valence electrons of
Myc5 via two possible different mechanisms. First; concerted five H atoms donation from five different
OH groups that directly results in Myc5 radicals (Myc / Myc5). Second; the step-wise radical formation
in five different OH groups of Myc (Myc / Myc / Myc2 / Myc3 / Myc4 / Myc5). Computational
DFT method was used to analyze all the six different OH groups of Myc which involved in free radical
reactions for the purposes of clarification the stable multiradicals Myc5 formation mechanism. The fast
semi-empirical combined quantum method, AM1/DFT, as well as full DFT geometry optimization approaches of B3LYP functional DFT/DFT with different basis sets of 6-31G (d), 6-311 þ G (d,p) and 6-
311 þ G (2d,2p) confirmed the stepwise H atom abstraction trend on the main three hydroxyl sites as 40-
O / 40-O3-O / 40-O3-O-7O both in the gas and water phase. Spin delocalization over the entire Myc,
adding the co-planarity, contributed to the stabilization of respective radical species. The excellent
stability of Myc radicals should give an effective chain-breaking antioxidant activity for Myc in biological
environment which is expected to have far fewer side effects. These findings may be useful to elucidate
the radical scavenging mechanism of other flavonoids regarding to design novel antioxidants. |