Stepwise sequential analysis of stable multiradicals formation in polyphenolic myricetin active OH groups throughout the antioxidant process to scavenge free radicals

Abstract

Five galvinoxyl radicals (GO) reduction by one polyphenolic myricetin (Myc, 3,3 ',4 ',5,5 ',7-Hexahydroxyfiavone) molecule-using EPR method-demonstrated that each Myc should donate at least five H atoms resulted in multiradicals Myc(5 center dot) (5 G(center dot) + 1 Myc -> 5 GH + 1 Myc(5 center dot)). The process that five H atoms donation occurs from different OH sites of Myc lead to appearing of five unpaired valence electrons of Myc(5 center dot) via two possible different mechanisms. First; concerted five H atoms donation from five different OH groups that directly results in Myc(5 center dot) radicals (Myc -> Myc(5 center dot)). Second; the step-wise radical formation in five different OH groups of Myc (Myc -> Myc(center dot) -> Myc(2 center dot) -> Myc(3 center dot) -> Myc(4 center dot) -> Myc(5 center dot)). 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 Myc(5 center dot) formation mechanism. The fast semi-empirical combined quantum method, AM-I/DFT, as well as full DFT geometry optimization approaches of B3LYP functional DFT/DFT with different basis sets of 6-31G (d), 6-311 + G (d,p) and 6-311 + G (2d,2p) confirmed the stepwise H atom abstraction trend on the main three hydroxyl sites as 4 '-O-center dot -> 4 '-O(center dot)3-O-center dot -> 4 '-O(center dot)3-O-center dot -> 7O(center dot) 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. (C) 2017 Elsevier B.V. All rights reserved.