This review focuses on the ability of some natural antioxidant molecules (i.e., hydroxycinnamic acids, coumarin-3-carboxylic acid, quercetin, luteolin and curcumin) to form Al(III)- and Fe(III)-complexes with the aim of evaluating the coordination properties from a combined experimental and theoretical point of view. Despite the contributions of previous studies on the chemical properties and biological activity of these metal complexes involving such natural antioxidants, further detailed relationships between the structure and properties are still required. In this context, the investigation on the coordination properties of Al(III) and Fe(III) toward these natural antioxidant molecules might deserve high interest to design water soluble molecule-based metal carriers that can improve the metal’s intake and/or its removal in living organisms.

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 : 1 and 2 : 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.

In this work, the solubility of vanillic, gallic, syringic, p-coumaric, ferulic and caffeic acids was determined at 37 °C under different conditions, namely pure water and two different ionic media, NaCl(aq) and NaClO₄(aq), at different ionic strengths (i.e., 0.16, 0.50, 1.0, 2.0 and 3.0 M). The solubility in water of all the acids was found to be higher than that in both of the ionic media. Moreover, the solubility of hydroxycinnamic acids was lower than that of hydroxybenzoic acids. The activity coefficients of neutral species were calculated from these data; this knowledge is necessary when modeling the dependence of equilibrium constants on the ionic strength. Results obtained in this work can be useful for further studies regarding complex formation equilibria between these ligands and bioavailable metal cations.

Taking into account the importance of aluminium it is interesting to explore the ability of quercetin to coordinate it.