Interaction of l-malic acid with alkaline metals and open chain polyammonium cations in aqueous solution

This work reports a potentiometric, calorimetric and spectropolarimetric ultraviolet circular dichroism (UV/CD) study of the interaction of l-malic acid with alkaline metals or (poly)ammonium (methylamine, ethylenediamine, diethylenetriamine, triethylenetetramine, spermine, tetraethylenepentamine and pentaethylenehexamine) cations. Stability data (log K, ΔG0) were obtained potentiometrically for the l-malic acid with (poly)ammonium cations systems; calorimetric measurements (25°C) made it possible to obtain ΔH0 and TΔS0 values for the complexes formed in the systems under examination. log K values calculated (for the reaction: HiAi++HjL(j−z)=ALHr(i+j−z), with r=i+j) range between 0.8 and 4.6, i.e., the interactions are from weak to fairly strong while maximum stability for each system is given by the species with the highest zanion×zcation (z=charge) value. Enthalpy changes associated with reactions HnAn++L2−=ALHn(n−2) and HnAn++HL−=ALHn+1(n−1) are always positive and increase progressively with n. The same is valid for T ΔS0 values, which indicate that these species are entropically stabilized, as expected for electrostatic interactions. It was verified that the UV/CD signal depends on both ionic medium and ionic strength value; for comparison, we used the l-malic acid signal recorded in tetramethylammonium chloride as baseline background salt (as in potentiometry). UV/CD spectra were recorded for solutions containing both cationic and anionic species. When the cation was a protonated polyamine, CD spectra calculations were performed for most stable ion pairs: the results show remarkable differences in Δε (dm3 mol−1 cm−1) values at 205 nm (which is the l-malate UV/CD λmax) between the chiral ligand and its complex with a polyamine.