Heat capacities of sixteen small peptides ( N-acetyl- N′-methylamino acid amides) measured by differential scanning calorimetry

Abstract

The molar heat capacities at constant pressure of N-acetyl- N′-methylamides of glycine, l-and dl-alanine, dl-2-aminobutyric acid, l- and dl-proline, l- and dl-valine, dl-norvaline, l- and dl-leucine, dl-norleucine, l- and dl-isoleucine and l- and dl-phenylalanine have been measured by differential scanning calorimetry from 318 K to about 20 K below their melting points. Values at 298.15 K have been obtained by extrapolating the smoothed fitting equations. The C* p,m values at 298.15 K of both l- and dl-amino acid derivatives, including that of glycine, increase very linearly as a function of their molar masses. Slopes, giving 25±1 J K −1 mol −1 per methylene group, are coincident with those of the corresponding amino acids. Isomeric compounds (iso- and noramino acid derivatives) show a small departure from linearity, whereas proline and phenylalanine derivatives present somewhat larger and decidedly strong deviations, respectively.