Evaluation of Solubility and Complexation Ability of Vanillic, Syringic and Gallic Acids Towards Aluminum Cation

Chelation therapy is currently successfully applied to reduce the aluminum burden and its neurodegenerative consequences. In view of a possible application to aluminum chelation therapy, here we have studied the complexation of hydroxybenzoic acids, namely, vanillic, syringic and gallic acids, towards aluminum ion at physiologically relevant conditions as regards temperature (37 °C) and ionic strength (i.e., 0.16 M NaCl). The solubility values and the protonation constants of the hydroxybenzoic acids were primarily assessed to estimate the competition of these acids towards aluminum and H+ ions. Then, potentiometric titrations were carried out, and the speciation analysis indicated a pH-dependent complexation occurring at a 1:1 hydroxybenzoic acid-to-aluminum ratio for vanillic and syringic, and 1:1, 2:1 and 3:1 ligand-to-Al(III) ratios for gallic. Gallic acid forms more stable complexes with Al(III) ion than vanillic and syringic acids and could therefore represent a good candidate for being used as sequestering agents for Al(III) ion.