Effects of Metal Ions on the Precipitation of Penta-O-galloyl-β-d-glucopyranose by Protein

In this study, the effects of metal ions (Al3+, Fe2+, Cu2+, and Zn2+) on precipitation of a purified gallotannin 1,2,3,4,6-penta-O-galloyl-β-d-glucopyranose (PGG) by bovine serum albumin (BSA) were quantitatively analyzed. The stoichiometric ratios of the complexation of metal ions to PGG and methyl gallate (MeG) which can be defined as gallotannins monomer were also explored. The results showed that the addition of metal ions could reduce the solubility of PGG–protein complex and increase the PGG–protein precipitation. Precipitation studies showed that Al3+ and Fe2+ with a higher stoichiometric ratio to PGG and MeG had greater effects on PGG–protein precipitation than Cu2+ and Zn2+. The results of this study suggested that metal ions could combine with PGG to form PGG–metal complex and interact with protein to form PGG–metal–protein ternary complexes, which resulted in the increase of PGG–protein precipitation. Consequently, a model of interaction between metal ions and PGG–protein precipitation was proposed.