Heat capacities, and enthalpies and entropies of fusion of some uncharged small peptides (N-acetylamino acid amides and 2,5-diketopiperazines)

Molar heat capacities at constant pressure of solid N-acetyl amides of glycine, L-alanine, L-valine, L-leucine, L-isoleucine, and cyclic anhydrides of glycine (2,5-piperazinedione), glycine, and L-alanine (3-methyl-2,5-piperazinedione), L-alanine (3,6-dimethyl-2,5-piperazinedione), and sarcosine (1,4-dimethyl-2,5-piperazinedione) were measured by differential scanning calorimetry. Calibration methods and experimental procedures adopted are described. Measurements were made from ambient temperature to below the melting temperatures, and values at T = 298.15 K were derived by extrapolating the smoothed fitting equations. The molar heat capacities C-p,C-m at T = 298.15 K increase linearly as a function of molar mass, with an increment of (26 +/- 1) J.K-1.mol(-1) for a CH2 group, for both series of linear and cyclic compounds. Temperatures, enthalpies, and entropies of fusion of both groups of compounds were also determined. They are discussed on the basis of crystal structures given in the literature. (C) 1997 Academic Press Limited.