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 (logK, DeltaG(0)) were obtained potentiometrically for the l-malic acid with (poly)ammonium cations systems; calorimetric measurements (25 degrees C) made it possible to obtain DeltaH(0) and TDeltaS(0) values for the complexes formed in the systems under examination. logK values calculated (for the reaction: H(i)A(i+)+H(j)L((j-z))=ALH(r)((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 z(anion)xz(cation) (z=charge) value. Enthalpy changes associated with reactions H(n)A(n+)+L(2-)=ALH(n)((n-2)) and H(n)A(n+)+HL(-)=ALH(n+1)((n-1)) are always positive and increase progressively with n. The same is valid for T DeltaS(0) 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 Deltaepsilon (dm(3) mol(-1) cm(-1)) values at 205 nm (which is the l-malate UV/CD lambda(max)) between the chiral ligand and its complex with a polyamine.