Kinetic solvent effects on the reaction between flavonoid naringenin and 2,2-diphenyl-1-picrylhydrazyl radical in different aqueous solutions of ethanol: An experimental and theoretical study

Abstract

Kinetic study of the reaction of flavonoid naringenin with the stable free radical 2,2-diphenyl-1-picrylhydrazyl (DPPH) was performed in different percentage compositions of aqueous ethanol (50–90% v/v) using spectrophotometric method. The reaction, which follows the mixed second-order rate law, was investigated under pseudo first-order conditions with respect to the DPPH radical, at (25.0±0.1) °C and an ionic strength of 0.1moldm−3. The rate of reaction was found to decrease with increasing organic solvent content in binary mixture. The reaction mechanism was inferred from the stoichiometry, kinetics, and product identification. Furthermore, the effects of solvent composition on the reaction rate in the mixed solvents were analyzed in terms of Reichardt parameter (ETN), and Kamlet, Abboud and Taft (KAT) solvatochromic parameters (α, β, and π*). To further investigate the solvent effects we theoretically studied the three antioxidant action mechanisms of naringenin using density functional theory (DFT) method. Reaction enthalpies related to these mechanisms were calculated in gas-phase, water, ethanol and 50–90% (by v/v) ethanol–water. It was found that theoretical findings are in good agreement with experimental results.