Kinetics and Mechanism of the Antioxidant Activities of and by Spectrophotometric and DFT Methods.

The kinetics and mechanism of the antioxidant activities of the methanolic extract of the leaves of two vegetables [ () and ()] have been studied using experimental and theoretical approaches. The kinetics (second order and pseudo-first order) of the 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activities of the leaf extracts at varying times (30-90 min) were determined using the UV-visible spectrophotometry method at λ = 517 nm, whereas the mechanism was studied by density functional theory at two levels of functionals (B3LYP and LC-ωPBE) using bond dissociation enthalpy and adiabatic ionization potential values. Molecular properties such as the highest occupied molecular orbital, lowest unoccupied molecular orbital, electronegativity (χ), electrophilicity (ω), hardness (η), and softness () of the predominant phenolic antioxidants were also compared. The second-order kinetics is favored by both plants rather than pseudo-first order; however, with a second-order rate constant of 0.0152 (mM) min is faster in scavenging DPPH radicals than with a value of 0.0093 (mM)min. Chlorogenic acid and luteolin-7-O-β-glucuronide, which are the most abundant phenolic acid antioxidant in and , both preferably scavenge the DPPH radical via a hydrogen atom transfer mechanism. This is evident from their lower bond dissociation enthalpy values than the adiabatic ionization potential values. Successful molecular docking of these phenolic compounds indicates that both compounds form favorable interactions with the therapeutic target, xanthine oxidase.