Chiral interactions of some N-acetyl-N′-methyl amino acid amides dissolved in N,N-dimethylformamide

Abstract

Enthalpies of dilution of N-acetyl-N′-methyl amides of D-alanine, D-leucine, D,L-alanine, D,L-valine, D,L-leucine, D,L-isoleucine, D,L-phenylalanine and D,L-proline, dissolved in N,N-dimethylformamide have been measured calorimetrically at 298.15 K. In addition, enthalpies of dilution of equimolar combinations of N-acetyl-N′-methyl amides of L- and D-alanine with L- and D-leucine, L-phenylalanine and L-proline have been measured. From these, enthalpic pair interaction coefficients have been obtained using equations which are related to the McMillan–Mayer theory of solutions. Combining the present results with previously obtained data and using the assumption that L,L-interactions are equal to D,D-interactions, enthalpic pair interaction coefficients of homotactic D,L-pairs and heterotactic L,L- and D,L-pairs of peptides could be calculated. It will be shown that D,L-interactions are characterized by a significantly less exothermic interaction coefficient than are L,L-interactions. The results are interpreted by using a model for dimer formation in solution and compared with some results in water from the literature.