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'-methylam ides of glycine, L-and DL-alanine, DL-2-aminobutyric acid, L- and DL-p roline, L- and DL-valine, DL-norvaline, L- and DL-leucine, DL-norleuci ne, L- and DL-isoleucine and L- and DL-phenylalanine have been measure d by differential scanning calorimetry from 318 K to about 20 K below their melting points. Values at 298.15 K have been obtained by extrapo lating 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, i ncrease very linearly as a function of their molar masses. Slopes, giv ing 25 +/- 1 J K-1 mol-1 per methylene group, are coincident with thos e of the corresponding amino acids. Isomeric compounds (iso- and noram ino acid derivatives) show a small departure from linearity, whereas p roline and phenylalanine derivatives present somewhat larger and decid edly strong deviations, respectively.