EXCESS-ENTHALPIES OF AQUEOUS-SOLUTIONS OF SMALL PEPTIDES AT 308.15-K AND 318.15-K CHARACTERISTICS OF SOLUTE-SOLUTE INTERACTIONS

Dilution enthalpies of the aqueous solutions of the following peptides: glycine, diglycine, triglycine, beta-alanyl-glycine, beta-alanyl-beta-alanine, L-alpha-alanyl-glycine, DL-alpha-alanyl-glycine, glycyl-L-alpha-alanine, glycyl-DL-alpha-alanine, L-alpha-alanyl-L-alpha-alanine, DL-alpha-alanyl-DL-alpha-alanine, DL-alpha-alanyl-DL-valine, DL-alpha-alanyl-beta-alanine, glycyl-gamma-aminobutyric acid, glycyl-L-leucine and DL-alpha-alanyl-glycyl-glycine have been determined at 308.15 and 318.15 K. Concentration dependences of the excess enthalpies have been analysed using the formalism of McMillan and Mayer theory. The contributions from the side-non-polar groups into the value of enthalpic coefficient h2 have been calculated. The value of the contribution from the side-groups h2(R) has been found to decrease over the temperature interval studied (alanine and leucine groups) or to pass through the maximum at 308.15 K (methylene and valine groups). A substantial influence of the conformational changes on the temperature dependence of the solute-solute interaction has been supposed for the glycyl-containing peptides. The values of Kirkwood-Buff integrals G22(0) their temperature derivatives (partial derivative G22(0)/partial derivative T)p and the attractive contributions phi(A) for the solute-solute interaction in the aqueous solutions of some peptides have been calculated. It has been shown that considerable attractive contributions characterize the solute-solute interactions in the solutions of glycyl-containing peptides, and the introduction of side non-polar groups into a molecule results in the decrease of the attractive affinity of hydrophilic groups. The temperature increase for the peptides containing non-polar side-groups results in the strengthening of the solute-solute attraction, and, vice versa, the hydrophilic interaction becomes weaker with the increasing temperature.