HEAT-CAPACITIES OF 16 SMALL PEPTIDES (N-ACETYL-N'-METHYLAMINO ACID-AMIDES) MEASURED BY DIFFERENTIAL SCANNING CALORIMETRY

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.