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

The enthalpies of solution in water, Δ_solH_m, 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, ${\mathrm{\Delta }}_{\mathrm{sub}}{H}_{\mathrm{m}}^{\circ }$, at the same temperature, to obtain the corresponding solvation enthalpies at infinite dilution, ${\mathrm{\Delta }}_{\mathrm{solv}}{H}_{\mathrm{m}}^{\infty }$. Moreover, the partial molar heat capacities at infinite dilution at T = 298.15 K, $C_{p\text{,}2}^{\infty }$, were calculated by adding molar heat capacities of solid small peptides, C_p,m(cr), to the ${\mathrm{\Delta }}_{\mathrm{sol}}{C}_{p\text{,}\mathrm{m}}^{\infty }$ 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.     

The limiting partial molar volume and transfer partial molar volume of glycylglycine in aqueous sodium halide solutions at 298.15 K and 308.15 K

Apparent volumes V_Φ of glycylglycine in aqueous NaX (X = F⁻, Cl⁻, and Br⁻) solutions have been obtained from densities of their solutions at 298.15 K and 308.15 K measured by using a precise vibrating-tube digital densimeter. These values have been utilized in conjunction with the values in water to deduce partial molar volumes of transfer ${\mathrm{\Delta }}_{\mathrm{trs}}{V}_{\mathrm{\Phi }}^{\circ }$ from water to different aqueous NaX solutions. ${\mathrm{\Delta }}_{\mathrm{trs}}{V}_{\mathrm{\Phi }}^{\circ }$ values are positive. The interpretation is that these results arise from the dominant interaction of the NaX with the charged centers of glycylglycine. The results show that ${\mathrm{\Delta }}_{\mathrm{trs}}{V}_{\mathrm{\Phi }}^{\circ }$ depend less on the temperature. Hydration numbers and interaction coefficients have been calculated from $V_{\mathrm{\Phi }}^{\circ }$ and ${\mathrm{\Delta }}_{\mathrm{trs}}{V}_{\mathrm{\Phi }}^{\circ }$ values and the values have been interpreted in terms of various interactions.