Computational investigation of the radical-scavenging and Cu(ii)-chelating mechanisms of a tannic acid model.

Abstract

Tannic acid (TA) is a natural polyphenol with well-established antioxidant properties; however, its large and structurally complex nature makes a detailed computational investigation challenging. In this study, a galloyl-based fragment of TA was used as a representative model to investigate its radical-scavenging and Cu(ii)-chelating properties using density functional theory (DFT). The antioxidant activity was evaluated in aqueous and lipid-like media, showing efficient radical scavenging, particularly in water, where the calculated rate constants were higher than those of Trolox and BHT. Mechanistic analysis identified the B ring as the main reactive site and indicated that radical scavenging can occur through hydrogen atom transfer (HAT) from the deprotonated species, particularly the doubly deprotonated form (TA2-). The Cu(ii)-chelating ability of the modeled TA fragment was also investigated under physiological conditions using thermodynamic calculations and Quantum Theory of Atoms in Molecules (QTAIM) analysis. The results showed that Cu(ii) preferentially coordinates with the deprotonated forms through bidentate binding. Among the species considered, TA2- showed the highest Cu(ii)-binding affinity and, based on its calculated abundance under the conditions studied, was identified as a major contributor to Cu(ii) chelation. Overall, these findings provide molecular-level insight into the antioxidant and Cu(ii)-chelating behavior of a model fragment of TA and the molecular factors that may contribute to its radical-scavenging and metal-binding properties under physiologically relevant conditions.

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