Aqueous solutions containing amino acids and peptides. Part 13.—Enthalpy of dilution and osmotic coefficients of some N-acetyl amino acid amides and some N-acetyl peptide amides at 298.15 K

Abstract

The energetics of the interactions occurring between some N-acetyl amino acid amides and some N-acetyl peptide amides in aqueous solutions at 298.15 K have been investigated. Osmotic coefficients of solutions containing N-acetylglycinamide (G), N-acetyl-L-alaninamide (A) and N-acetyl-L-leucinamide (L) and equimolal solutions of G + A, G + L and A + L have been obtained using the isopiestic vapour pressure technique. Enthalpies of dilution of N-acetylglycylglycinamide (G₂), N-acetyl-L-alanyl-L-alaninamide (A₂), N-acetylglycylglycylglycinamide (G₃), N-acetyl-L-alanyl-L-alanyl-L-alaninamide (A₃) and N-acetyl-L-alanylglycinamide (AG) and equimolal solutions of G + G₂, G + G₃ and A + A₂ were obtained using microcalorimetry. The results obtained were used to calculate the pairwise free energy and enthalpy parameters for like–like and like–unlike solute interactions. The effects of molecular structure and substitution on these parameters are considered and the efficiency of a group interaction approach is investigated. The group interaction idea works well for both the rather limited free energy data set considered and the more extensive enthalpy data set, with the exception of the most hydrophobic molecule A₃. It is suggested that the results for A₃ indicate a degree of intramolecular folding which perturbs the intermolecular interactions.