Influence of valoneoyl groups on the interactions between Euphorbia tannins and human serum albumin

Abstract Tannins belonging to plant polyphenols, are large group of compounds with diverse biological activity. Many of them are being studied as potential natural medicine due to their antioxidant, antiviral, antibacterial or anticancer properties. However, so far little is known about the structural and functional relations in protein-tannin interactions, in particularly the role of valoneoyl groups in tannin structure. In this study we first investigated the mechanisms of interaction 1,2-di-O-galloyl-4,6-valoneoyl-β-D-glucose (Tannin 1), 2-O-galloyl-4,6-valoneoyl-β-D-glucose (Tannin 2), 3-O-galloyl-1,2-valoneoyl-β-D-glucose (Tannin 3) isolated from Euphorbia plants with human serum albumin (HSA). To get more detailed information about nature of albumin-tannin interactions besides standard Trp fluorescence quenching analysis we used also transmission electron microscopy (TEM), circular dichroism (CD) and fluorescence labelling (ANS dye) techniques. It was shown that all the tannins strongly interacted with HSA and quenched the tryptophan amino acid (Trp) fluorescence but slightly changed protein secondary structure (circular dichroism CD analysis). TEM demonstrated that all used compounds formed complexes with HSA. Tannin 3 most strongly quenched HSA fluorescence and changed protein dynamic as well as had the highest binding constant (12.4 ± 1.1 × 10 13  M −1  s −1 in comparison with 7.0 ± 0.38 × 10 13  M −1  s −1 for tannin 1 and 8.6 ± 1.1 × 10 13  M −1  s −1 for tannin 2). For tannin 1 that was the largest of the studied compounds was observed the weakest influence on the fluorescence parameters, probably due to the effect of steric hindrance limiting interaction with albumin Trp pocket. On the other hand T1 induced the strongest changed secondary structure of HSA in comparison to another studied tannins. Our results demonstrated that all used tannins interact with albumin by complex formation but in the manner depends on chemical structure and flexibility of studied compounds.