Aqueous solutions containing amino acids and peptides. Part 28.—Enthalpy of interaction of some amides with glycine and α-alanine: interactions of the zwitterionic group of α-amino acids with hydrophobic groups and peptide groups

Abstract

Enthalpies of dilution, in water at 25 °C, of N-ethylformamide and binary solute solutions of glycine with formamide, acetamide, N-methylformamide, N-methylacetamide, N-ethylformamide, N-methylpropionamide, N,N-dimethylformamide, N,N-dimethylacetamide, N,N-diethylformamide and N,N-diethylacetamide, and of α-alanine with all of the above except N,N-dimethylformamide, have been obtained. The information has been discussed in terms of a group additivity approach, and group contributions for the interactions between primary (CONH) and secondary (CON) peptide groups with the zwitterionic head group [NH⁺₃, (CO^–₂)] and the hydrophobic methylene group have been evaluated. It was found that the interaction between the head group and the hydrophobic group is endothermic (i.e. thermochemically repulsive) whereas the two types of peptide group interact with the zwitterionic group exothermically (i.e. in a thermochemically attractive manner). In water, the secondary peptide group interacts more attractively with the zwitterionic group than does the primary peptide group.