A comparative investigation on the scavenging of 2,2-diphenyl-1-picrylhydrazyl radical by the natural antioxidants (+) catechin and (-) epicatechin

The scavenging ability of (+) catechin (CT) and (-) epicatechin (ECT) was investigated experimentally using 2,2-diphenyl-1-picrylhydrazyl radical (DPPH^•) assay. The half maximal inhibitory concentration (IC₅₀) values for CT and ECT are found to be 8.83 × 10⁻⁶ M and 8.19 × 10⁻⁶ M respectively. In addition, the interaction of CT/ECT with DPPH^• has been elucidated through calculations based on the density functional theory (DFT) at B3LYP/6–311G(2d,2p) level of theory. The formation of resultant complexes, CT-DPPH^• and ECT-DPPH^• are due to the hydrogen bond between hydroxyl groups of catechol ring (CT and ECT) and the nitrogen atom of DPPH^•. Amongst both complexes, the ECT that interacted with DPPH^• has the highest interaction energy of -3.74 kcal/mol.The calculated interaction energy and thermodynamical parameters show that the interaction between CT/ECT and DPPH^• is taking place spontaneously in an exothermic process. The observed red shift in the normal mode of vibration at active site (4′-OH) of CT/ECT confirmed the hydrogen bond (O-H…N) interaction with DPPH^•. The nature of interaction in CT-DPPH^• and ECT-DPPH^• is also examined by quantum theory of atoms in molecules. It shows that interactions are electrostatic in nature. Topological analysis, namely electron localization function (ELF) and localized orbital locator (LOL) maps reveals that the interaction is non-covalent. Natural bond orbital analysis predicts the intermolecular charge transfer through hydrogen bond interaction. The frontier molecular orbitals of CT-DPPH^• and ECT-DPPH^• complexes show that DPPH^• is subjected to a nucleophilic attack by CT/ECT. Further, this analysis confirmed that catechol with picryl ring of CT-DPPH^• and A ring with picryl ring of ECT-DPPH^• contributes more for a TDOS plot. The above results confirm the antioxidant potential of CT and ECT.