Excess enthalpies of aqueous solutions of amides and saccharides. Glycine, N-acetylglycinamide and aldopentoses at 298.15 K

The excess enthalpies of eight ternary aqueous solutions of four aldopentoses and glycine or N-acetylglycinamide have been determined by microcalorimetry at 298.15 K. The coefficients of virial expansion of the excess enthalpies are compared with those reported in the literature and relative to the ternary aqueous solutions of monosaccharides and urea or amides, as appropriate. The enthalpic contributions to the pair cross-interaction coefficients are negative for solutions containing sugars and glycine or urea, while they are positive for solutions of sugars and N-acetylglycinamide or prevailingly hydrophobic alkylamides. This quite distinct behaviour is confirmed by the deviations of the cross-interaction coefficients with respect to the values of the half-sum of the enthalpic contributions to the self-interactions. The half-sum is assumed as a reference value for the condition of minimal mutual perturbation between the solute molecules. This approach allows us to emphasize the possible important implications of the unfavourable positive contribution of the enthalpy to the interactions between peptides and saccharides. The fact that glycine and urea are not good models for peptide chains is stressed.