Metal ion interactions with methyl gallate characterized by UV spectroscopic and computational methods.

A simple biologically active phenolic methyl gallate (MeG) was used as a model compound to identify the nature of the complexes it formed with Al3+, Fe3+, Cu2+, and Sn2+ as analyzed by ultraviolet (UV) spectroscopic and computational methods. The results showed that, among all the metal ions studied in the present study, MeG has the highest formation constant with Al3+, and the complexation between MeG and metal ions is a pH-dependent reaction. At pH 6.0, complexes with a stoichiometry of 1:1 were formed with Al3+, Fe3+, and Sn2+, whereas 2:3 complexes were formed with Cu2+. Using a functional density quantum-chemical method, it was shown that the ligand involved in the complexation of MeG-Al undergoes significant changes in electronic delocalization. Modeling of electronic spectra with water as solvent shows high reproducibility for the experimental features of the complexes. The calculations confirm that chelation of Al3+ occurs at the double-deprotonated phenolic hydroxyls.