Enthalpies of interaction of some N-acetyl amino acid amides dissolved in N-methylformamide at 298.15 K

Abstract

Enthalpies of dilution of N-acetyl amides of glycine, L-alanine, L-valine, L-leucine and L-proline, as well as the N-acetyl-N′-methyl acids of the above amino acids dissolved in N-methylformamide (NMF) have been measured calorimetrically at 298.15 K. The results were used to calculate enthalpic pair interaction coefficients B^h₂, which are a measure of the enthalpy of interaction between two solute molecules.

The B^h₂ coefficients of the above peptides are compared with those in N,N-dimethylformamide (DMF) and N-methylacetamide (NMA). The possible role of these solvents to be used as model environments for the inner parts of globular proteins is discussed. With regard to the latter, all three amidic solvents appear to be equally suitable, although DMF seems to be slightly more favourable than the other two. Finally, the B^h₂ coefficients obtained here have been combined with previously reported ones and analysed in terms of the excess group additivity model. By taking into account the intramolecular hydrogen-bond formation of the peptides the predictive power of this model is enhanced.