A quantum chemical study on the OH radical quenching by natural antioxidant fisetin

In this work, the antioxidant ability of fisetin was explored toward hydroxyl (center dot OH) radical in aqueous and lipid solution using density functional level of theory. Different reaction mechanisms have been studied: hydrogen atom transfer, single electron transfer followed by proton transfer, and radical adduct formation, and sequential proton loss electron transfer. Rate constants for all possible reaction sites have been calculated using conventional transition state theory in conjunction with the Collins-Kimball theory. Branching ratios for the different channels of reaction are reported for the first time. Results show that the reactivity of fisetin toward hydroxyl ((OH)-O-center dot) radical takes place almost exclusively by radical adduct formation regardless of the polarity of the environment. Also, the single-electron transfer process seems to be thermodynamically unfavorable in both media.