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

Abstract

Molar heat capacities at constant pressure of solidN-acetyl amides 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 heat capacitiesC_p,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.