Complexation behaviour of caffeic, ferulic and p-coumaric acids towards aluminium cation: a combined experimental and theoretical approach

Here we have studied the complexation of caffeic, ferulic and p-coumaric acids with Al(III) under physiological conditions (i.e. at 37 °C and in 0.16 M NaCl). Solubility and protonation constants of these hydroxycinnamic acids were firstly determined in order to evaluate the competition of the ligands for the Al(III) and H+ ions. Speciation analysis made by potentiometric titrations and supported by UV and 1H-NMR data shows that a pH-dependent complexation occurs at 1[thin space (1/6-em)]:[thin space (1/6-em)]1 and 2[thin space (1/6-em)]:[thin space (1/6-em)]1 ligand-to-Al(III) ratios. Notably, these data combined with computational (DFT) results unequivocally indicate that the complexation site is the carboxylate group of the ligands thus excluding any involvement of the phenolic sites.