Role of functional groups in determining interactions in ternary aqueous solutions of enantiomeric alpha-amino acids: a calorimetric study at 298 K

Cross-homochiral and cross-heterochiral enthalpic interaction coefficients were determined at 298 K by measuring the enthalpies of dilution in water of ternary solutions containing two different alpha-amino acids of the same or different chirality: phenylalanine, glutamine, leucine, asparagine, cysteine, methionine, citrulline, tryptophan. The interaction of these amino acids with small molecules such as glycine, formamide and urea was also studied in the attempt to separate the contribution of the functional groups to the overall interaction. The data are discussed in terms of the preferential configuration model. It is found that the interaction between the hydrophilic groups, the zwitterions, which is thermochemically attractive, mostly determines the values of the coefficients. In some cases, differences were found between the values of the cross-homochiral and cross-heterochiral coefficients: namely, the variation of the interactions between the side chains significantly influences the energetics of the interaction.