Thermodynamics of solvation of some small peptides in water at T = 298.15 K

The enthalpies of solution in water, ΔsolHm, of some small peptides, namely the amides of five N-acetyl substituted amino acids of glycine, l-alanine, l-proline, l-valine, l-leucine and two cyclic anhydrides of glycine and l-sarcosine (diketopiperazines), were measured by isothermal calorimetry at T = (296.84, 306.89, and 316.95) K. The enthalpies of solution at infinite dilution at T = 298.15 K were derived and added to the enthalpies of sublimation, ΔsubHm∘, at the same temperature, to obtain the corresponding solvation enthalpies at infinite dilution, ΔsolvHm∞. Moreover, the partial molar heat capacities at infinite dilution at T = 298.15 K, Cp,2∞, were calculated by adding molar heat capacities of solid small peptides, Cp,m(cr), to the ΔsolCp,m∞ values obtained from our experimental data. CH2 group contributions, in terms of solvation enthalpy and partial molar heat capacity, were −3.2 kJ · mol−1 and 89.3 J · K−1 · mol−1, respectively, in good agreement with the literature data. Simple additive methods were used to estimate the average molar enthalpy of solvation and partial molar heat capacity at infinite dilution for the 1/2CONH⋯CONH functional group in the small peptides. Values obtained were −46.7 kJ · mol−1 for solvation enthalpy and −42.4 J · K−1 · mol−1 for partial molar heat capacity, significantly lower than values obtained for the CONH functional group in monofunctional model compounds.