Analysis of the pH-Dependent Fe(III) Ion Chelating Activity of Anthocyanin Extracted from Black Soybean [Glycine max (L.) Merr.] Coats

The Fe(III) chelating activity of anthocyanin extracted from black soybean coats was investigated at pH 3.0, 5.0, 6.5, 7.0, and 7.4 with fluorescence spectroscopy and microscale thermophoresis (MST). Cyanidin-3-glucoside (C3G) was determined to be 98% of the total anthocyanin by high-performance liquid chromatography. The binding affinity (K-a) exhibited significant pH-dependent behavior: K-a was 9.7167 X 10(4), 1.0837 X 10(4), 1.4284 x 10(4), 5.4550 x 10(4), and 3.0269 X 10(4) M-1 at pH 3.0, 5.0, 6.5, 7.0, and 7.4, respectively (p < 0.05). The MST data showed that Delta G < 0 and Delta H < 0, demonstrating that chelation is spontaneous and exothermic. Because both Delta H and Delta S < 0, the chelation involves hydrogen bonds and/or van der Waals forces for pH 3.0, 5.0, and 6.5. Electrostatic interactions contributed to chelation at pH 7.0 and 7.4 with Delta H < 0 and Delta S > 0. With the formation of chelates, C3G improved the solubility of Fe(III) at pH 6.5, 7.0, and 7.4 to enhance the ferric ion bioavailability, except for aggregation observed at pH 5.0.