L. Abate et al., HEAT-CAPACITIES, AND ENTHALPIES AND ENTROPIES OF FUSION OF SOME UNCHARGED SMALL PEPTIDES (N-ACETYLAMINO ACID-AMIDES AND 2,5-DIKETOPIPERAZINES), Journal of Chemical Thermodynamics, 29(3), 1997, pp. 359-368
Molar heat capacities at constant pressure of solid N-acetyl amides of
glycine, L-alanine, L-valine, L-leucine, L-isoleucine, and cyclic anh
ydrides of glycine (2,5-piperazinedione), glycine, and L-alanine (3-me
thyl-2,5-piperazinedione), L-alanine (3,6-dimethyl-2,5-piperazinedione
), and sarcosine (1,4-dimethyl-2,5-piperazinedione) were measured by d
ifferential 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. Tempe
ratures, enthalpies, and entropies of fusion of both groups of compoun
ds were also determined. They are discussed on the basis of crystal st
ructures given in the literature. (C) 1997 Academic Press Limited.