LC-MS based validation and DFT investigation on the antioxidant properties of clovamide: •OH and •OOH scavenging and Cu(II) chelation mechanisms

Abstract

An experimental isolation of clovamide molecule from Vernonia fastigiata has been performed followed by characterization of the compound through the use of the liquid chromatography-mass spectrometry (LC-MS) technique, which revealed the presence of both the E and the Z geometrical isomers. Density functional theory (DFT) study was performed in order to rationalize the relative abundance of the two isomers. The results of the theoretical study were then compared with the LC-MS experimental findings. The most abundant isomer was selected in order to investigate the potentiality of clovamide as an antioxidant molecule. The DFT method was also selected for utilisation to elucidate the antioxidant properties of clovamide through •OH and •OOH scavenging and Cu(II) chelation mechanism. Theoretical studies have been performed using the DFT/M06−2X and the DFT/MPWB1K methods for the investigation of the radical scavenging properties of the molecule and the DFT/B3LYP and the DFT/BHANDHLYP methods for the investigation of the Cu(II) chelation mechanism. The results of the study, both from experimental and theoretical perspective, indicate that the E isomer is the most thermodynamically stable isomer. The Z isomer is formed as a consequence of UV exposure of the sample. Amongst the different radical scavenging mechanisms, the hydrogen atom transfer is the most preferred mechanism. Clovamide is a better •OH scavenger than •OOH and its mode of radical scavenging is greater in non-polar medium than in polar medium. The most stable radical species corresponds to the abstraction of the phenolic H atom in para positions. However, the most preferred site for abstraction of the H atom, determined by the bond dissociation enthalpy value, is the chiral carbon atom within the clovamide molecule. The Cu(II) chelation mechanism is influenced by the stoichiometric molar ratio in which clovamide combines with Cu(II), the solvent medium in which the reaction is performed and the site at which the Cu (II) cation is coordinated to clovamide. The binding energy values as well as spin density and the charge on the metal ion after coordination suggest that the Cu(II) chelation mechanism is preferred in non-polar medium to polar medium.