Thermodynamics Research Center / ThermoML | Journal of Chemical Thermodynamics

ABSTRACT

The enthalpies of solution in water, DsolHm, 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, DsubH?m, at the same temperature, to obtain the corresponding solvation enthalpies at infinite dilution, DsolvH1m . Moreover, the partial molar heat capacities at infinite dilution at T = 298.15 K, C1p ;2, were calculated by adding molar heat capacities of solid small peptides, Cp,m(cr), to the DsolC1p ;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.