Unraveling the antioxidant potential of thiamine: Thermochemical and kinetics studies in aqueous phase using DFT

Abstract

The scavenging mechanism of thiamine (or vitamin B1) towards HO* and HOO* radicals has been revisited using density functional theory (DFT) at the M05-2X/6-311++G (d,p) level of theory. Enthalpies and Gibbs free energics of four common antioxidant reactions, i.e. formal hydrogen transfer (FHT), proton transfer (PT), single electron transfer (SET) and radical adduct formation (RAF), have been systematically calculated in the aqueous phase. Potential energy profiles accompanied with the optimized structures of all transition states (TS) and intermediate ones were also established. As a result, thiamine represents as a potent antioxidant notably via RAF mechanism with the lowest AH° values at C12 position equal to -37.5 and -9.2 kcal/mol towards HO* and HOO* radicals, respectively. The reactions are also favorable with ΔG° values of -28.8 and 2.8 kcal/mol, respectively. The order of the decrease in preponderance of the studied mechanism is as follows: RAF > FHT > SET > PT. The scavenging reactivity towards HO* radical is considerably higher than the one toward HOO* radical as expected. In comparing with trolox and ascorbic acid, their antioxidant potential could be classified in the decreasing order: ascorbic acid > thiamine > trolox.