Heat Capacities of N-Acetyl Amides of Glycine, L-Alanine, L-Valine, L-Isoleucine, and L-Leucine

As a follow-up to our effort to establish reliable thermodynamic data for amino acids, the heat capacity and phase behavior are reported for N-acetyl glycine amide (CAS RN: 2620-63-5), N-acetyl-L-alanine amide (CAS RN: 15962-47-7), N-acetyl-L-valine amide (CAS RN: 37933-88-3), N-acetyl-L-isoleucine amide (CAS RN: 56711-06-9), and N-acetyl-L-leucine amide (CAS RN: 28529-34-2). Prior to heat capacity measurement, thermogravimetric analysis and X-ray powder diffraction were performed to determine decomposition temperatures and initial crystal structures, respectively. The crystal heat capacities of the five N-acetyl amino acid amides were measured by Tian-Calvet calorimetry in the temperature interval (266-350 K), by power compensation DSC in the temperature interval (216-471 K), and by relaxation (heat-pulse) calorimetry in the temperature interval (2-268 K). As a result, reference heat capacities and thermodynamic functions for the crystalline phase from 0 K up to 470 K were developed.