Theoretical studies to understand the \(\textrm{OH}^{\bullet }\) and \(\textrm{CH}_{3}{}^\bullet \) radical scavenging mechanisms of an antioxidant: pheophorbide a (Phe a)

Abstract

Context

Hydroxyl (\(\textrm{OH}^\bullet \)) and methyl (\(\textrm{CH}_{3}{}^\bullet \)) radicals are reactive species that induce oxidative damage to important biomolecules and contribute to various diseases. Exogenous antioxidants and those present in our bodies use various mechanisms, including scavenging of reactive species, to prevent oxidative stress. Therefore, identifying efficient natural antioxidants capable of scavenging these radicals is of considerable interest. In this work, the antioxidant activity of pheophorbide a (Phe a), a chlorophyll a derivative, was comprehensively investigated to understand the mechanisms of radical scavenging, including radical adduct formation (RAF), hydrogen atom transfer (HAT), and single-electron transfer (SET). Thermodynamic analysis indicates that RAF and HAT are the preferred radical-scavenging pathways, whereas the SET pathway is thermodynamically unfavourable. Kinetic analysis further demonstrates that RAF is the kinetically more favourable pathway for both radicals investigated. Frontier molecular orbital analysis, molecular electrostatic potential (MEP) mapping, and global quantum chemical descriptors identified the most reactive sites and further support the antioxidant activity of Phe a.

Methods

Density functional theory (DFT) calculations were performed using the M06-2X functional present in the Gaussian 09 software package. Geometry optimisations and frequency calculations for the RAF, HAT, and SET pathways were carried out with 6-31G(d) basis set in the gaseous phase, followed by single-point energy calculations at the M06-2X/6-311+G(d,p) level of theory in gaseous and aqueous phases. Solvent effects were included using the polarizable continuum model (PCM). The reaction free energies (\(\Delta G_f\)) are calculated to investigate the feasibility of the reactions involved in our study. Activation free energies (\(\Delta G^{b}\)) were calculated for the RAF and HAT mechanisms. The corresponding rate constants were determined using transition state theory, incorporating standard-state, Wigner tunnelling, diffusion, and degeneracy corrections. Molecular structures, molecular orbital surfaces, and the vibrational frequencies of the molecules were visualised using GaussView 5.