Chemical speciation of caffeic and p-coumaric acids with selected lanthanides

Caffeic (CFA) and p-coumaric (p-CA) acids are biologically active compounds commonly found in plants and food of plant origin. Metal complexes of these acids exhibit diverse bioactivity, sometimes even higher than the free ligands. Lanthanide (Ln) complexes with organic ligands also attract the attention of researchers, due to their potential application as novel potential biologically active agents. The aim of the present study was the evaluation of the interactions of CFA and p-CA with representative lanthanides (Ln3+ = Eu3+, Gd3+, and Dy3+) in aqueous solution. Potentiometric, spectrophotometric, and 1H NMR techniques were used to study the acid-base behavior of CFA and p-CA, as well as their complexing ability towards Ln3+ cations, over a wide range of pH values (2 <= pH <= 8), in KCl(aq) at I = 0.2 mol dm-3 and T = 298.15 +/- 0.1 K. The evaluation of the sequestering ability of both ligands towards the studied lanthanides, by means of pM and pL0.5 parameters, show that CFA is a better chelating agent.