On the role of the functional group in determining the strength of hydrophobic interactions in aqueous solutions of alpha-amino acid derivatives

Calorimetric measurements were carried out at 25 degrees C on binary aqueous solutions containing N-acetyl derivatives of the following alpha-amino acids: glycine, alanine, valine, leucine, norleucine, aspartic acid, glutamic acid, phenylalanine, methionine, asparagine, glutamine, cysteine, histidine, serine. The N-acetyl derivative of the alpha-amino acid proline was also studied. The results obtained have been used to calculate the pairwise enthalpic interaction coefficients and these are compared with results reported in the literature concerning free alpha-amino acids, and their acetylamide derivatives. The coefficients largely increase passing from free amino acids to N-acetyl derivatives. The derivatives of alpha-amino acids bearing unsubstituted alkyl chains present an increased cooperativity of hydrophobic interactions.