Heat Capacities and Fusion Enthalpies and Entropies of Some Uncharged Small Peptides (N-acetylamino Acid Amides and 2,5-Diketopiperazine)

Molar heatcapacities at constant pressure of solidN-acetylamides of glycine,l-alanine,l-valine,l-leucine,l-isoleucine, and cyclic anhydrides of glycine (2,5-piperazinedione), glycine, andl-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 atT=298.15 K were derived by extrapolating the smoothed fitting equations. The molar heatcapacitiesCp,matT=298.15 K increase linearly as a function of molar mass, with an increment of (26±1) J · K−1 · mol−1for CH2group, 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.