Abstract
Aims: The aim is to search for newer and better antioxidants through kinetic spectroscopic studies in combination with product analysis and computation.
Background: Antioxidant effect of caffeic acid, its derivative, and analogues have been well reported. The anti-oxidative efficiencies are related to their molecular structure, and two reaction pathways are well accepted, H-atom transfer (HAT) or single electron transfer. 1-hydroxy ethyl radical (1-HER) being ethanol-derived free radical might be causing the onset of liver injury detected after alcohol administration. 1-HER has also been reported to react with fatty acids and endogenous antioxidants such as glutathione, ascorbic acid and alpha-tocopherol.
Objective: The present study is an attempt to understand the reaction mechanism of 1- HER with caffeic acid, its derivative and analogues in detail.
Methods: Pulse radiolysis with kinetic absorption spectroscopy has been employed to follow the reaction pathway and identify the intermediates produced in the reaction. The reaction products have been detected using LCMS/MS. Based on these studies, a consolidated mechanism has been proposed. Cyclic voltammetry measurements and computational calculations have been used in support of the proposed mechanism.
Results: In the reaction of 1-hydroxy ethyl radical (1-HER) with caffeic acid and its oligomers, reduction takes place below the pKa1, while oxidation occurs with the deprotonated phenolic moiety. The reduction of caffeic acid generates a carbon-centered radical at the double bond of the side chain with a bimolecular rate constant of 1.4x1010 dm3 mol-1 s-1. Notably, a low concentration of oxygen is able to regenerate a part of the caffeic acid molecules in the reduction process. At pH 10 a phenoxyl radical is formed due to oxidation with a much lower bimolecular rate constant of (4.2x108 dm3 mol-1 s-1). In the case of di-hydrocaffeic acid, only phenoxyl radical is formed at pH 10 and, no reaction could be observed below pH 8.
Conclusion: Change in reactive pattern from reduction to oxidation with change in pH within same set of reactants has been evidently established in the present study. The results point towards the importance of α−β unsaturation in the side chain of caffeic acid oligomers for their reaction with 1-HER at neutral pH. The effect of oxygen concentration on the antioxidative protection offered by this class of molecules might be intriguing for the quest of the effectiveness of antioxidants at low concentration.
Other: It may be inferred that the effect of pH on the reactivity pattern as observed is not 1-HER, but substrate specific, in the present case phenolic acids. This study generates further scope for in-depth studies on other polyphenols where unsaturation exists in the side chain.
Keywords: Pulse radiolysis, kinetics, antioxidant mechanism, caffeic acid, 1-hydroxyethyl radical, pH dependent redox reaction.
Graphical Abstract
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