Theoretical investigation on the structure and antioxidant activity of (+) catechin and (−) epicatechin – a comparative study

Abstract

The monomeric flavan-3-ols, namely (+) catechin (CT) and (−) epicatechin (ECT), are two important antioxidants available in nature. Density functional theory method has been used to study the homolytic, heterolytic bond cleavages and the associated radical scavenging activities at B3LYP/6-311G(2d,2p) level of theory. The radical scavenging mechanism of CT and ECT was studied by considering three scavenging mechanisms, namely hydrogen atom transfer (HAT), single-electron transfer-proton transfer (SET-PT), and sequential proton loss electron transfer (SPLET). Parameters related to the above mechanisms, such as bond dissociation enthalpy (BDE), ionisation potential (IP), proton dissociation enthalpy (PDE), proton affinity (PA) and electron transfer enthalpy in gas and solvent medium (benzene, methanol and water) were studied. The results suggest that 4’-OH site from B-ring would play a crucial role in the scavenging activity of both the compounds. We observed that HAT is a thermodynamically favoured mechanism in the gas phase, whereas SPLET mechanism is more preferential in the polar medium for both the compounds. The global descriptors, such as ionisation potential (IPv), electron affinity (EAv), chemical hardness (η), softness (S), electronegativity (χ) and electrophilic index (ω) also confirm the high antioxidant activity of CT and ECT. Reactive site of the electrophilic and nucleophilic attack is confirmed and visualised by molecular electrostatic potential (MEP) map. Highlights The optimised geometrical parameters of (+) catechin (CT) and (−) epicatechin (ECT) have been calculated using B3LYP with 6-311G(2d,2p) level of theory. The antioxidant properties of CT and ECT are compared using three mechanisms, namely hydrogen atom transfer (HAT), single-electron transfer-proton transfer (SET-PT), and sequential proton loss Electron transfer (SPLET) by homolytic and heterolytic bond cleavages. The hydroxyl group present at 4’ site from B-ring is the most preferential site for hydrogen donation in both the compounds. The HAT mechanism is thermodynamically preferred in the gas phase, while SPLET mechanism is more favourable in polar solvents. Identification of reactive site by MEP and global descriptors is also confirming the high antioxidant activity in both compounds at the gas phase