Preparation and properties of tannin-histidine metal derivatives at multiple reaction sites

Abstract

Tannic acid (TA) is a green, nontoxic substance that contains multiple phenolic hydroxyls. Thus, it has anticancer, antitumor, antibacterial, and antioxidation properties. Researchers aim to modify TA to ensure its effective utilization and broaden its practice applications. In this work, histidine (His) was used to modify the phenolic hydroxyl of TA to prepare tannin-His, which introduced imidazole and carboxyl groups. Tannin-His was coordinated with four transition metal ions (M = Fe, Mn, Cu, and Zn) after its phenolic hydroxyl was protected by benzyl bromide. Afterward, boron trifluoride ether was utilized to deprotect the benzyl bromide to acquire tannin-histidine metal derivatives (T-His-Ms) that have imidazolyl, metal ions, and phenolic hydroxyl active sites. These active sites jointly exerted the antioxidant activity of T-His-Ms. The derivatives were characterized via IF spectrum, UV-Vis, X-ray diffraction, and TG analyses. Results indicated that the products were successfully synthesized in accordance with the expected structure. Furthermore, the antioxidant activity of T-His-Ms was determined via the xanthine oxidase–cytochrome C method. The order of SOD enzyme-like activity of T-His-Ms could be arranged as follows: T-His-Zn > T-His-Fe > T-His-Cu > SOD > T-His-Mn. This order demonstrated that these derivatives exhibited a remarkable SOD-like activity. These results provide a favorable way for modifying natural TA and a means for expanding the application range of this compound.