Investigation of antioxidant activity of epigallocatechin gallate and epicatechin as compared to resveratrol and ascorbic acid: experimental and theoretical insights

Abstract

Antioxidants are molecules that help the human body to fight oxidative stress caused by excess free radicals that trigger a number of diseases. Understanding the structure–activity relationship of antioxidants and their mechanisms of action is important for designing more powerful antioxidants. Polyphenols from Vitis vinifera L. have shown several biological activities among which antioxidant activity. Among polyphenolic compounds having potent biological activities, we find resveratrol (RSV) which is a stilbene and epigallocatechin gallate (EGCg) and epicatechin (EC) which are catechins or flavanols. We report here a comparative study of the antioxidant potential of EGCg and EC as compared to RSV. The most favorable mechanism by which each molecule exerts the antioxidant activity is determined. Ascorbic acid (AA) was included as a reference. DPPH and FRAP assays were used for experimental evaluation of antioxidant activity and for theoretical calculations, density functional theory (DFT) method was chosen. Three mechanisms were investigated: hydrogen atom transfer (HAT), single electron transfer-proton transfer (SET-PT), and sequential proton loss electron transfer (SPLET). Calculated thermodynamic parameters correlate well with percentage inhibition (I%) and half maximal inhibitory concentration (IC50) values given by the DPPH test. Both experimental and theoretical approaches showed that EGCg is more potent antioxidant than EC and RSV. The most preferential sites are gallate moiety and 4′-OH in EGCg and OH sites of the B ring in EC. The pKa values confirm this finding. All proposed mechanisms are favored for EGCg, and SET-PT is preferred antioxidant mechanism for EC and it is the most suitable in the first step for RSV. Flavanols are more potent antioxidants than the stilbene, RSV.