Role of water structural properties in interaction of amino acids with metal ions of varying size and electrons in d-block or main group elements:Quantitative thermodynamic analysis

Interactions of a homologous series of amino acids (glycine, L-alanine, DL-α-amino-n-butyric acid, L-valine and L-leucine) with salts (LiCl, NaCl, KCl, CaCl2, MnCl2, CoCl2) having common anion, but cation having varying d-electronic configuration and alkaline earth metals have been studied in aqueous solutions employing ultrasensitive isothermal titration calorimetry. Standard molar enthalpies of amino acid-salt interactions have been interpreted in terms of predominant ionic association between charged centres and the corresponding desolvation penalty. Extra crystal field stabilization experienced by Mn2+(aq) and Ca2+(aq) due to d5 and d10 electronic configuration has been addressed based on results of standard molar enthalpies of interaction. The results indicate significant desolvation cost predominantly from hydration of the cations of the salts rather than from hydration of zwitter-ionic ends of the amino acids irrespective of their chain length. This is also indicated by stronger endothermic interaction of the amino acids with the bivalent cations than that with the monovalent ones. The results obtained in this work provide essential mechanistic insights into nature of amino acid interactions modulated by variation in cationic d-electronic configuration in terms of enthalpies of interaction.