Thermodynamics of Sublimation of Low-Volatility Proteinogenic Amino Acids: l-Histidine and l-Aspartic Acid

Special mild conditions (temperature range and duration) were selected for Knudsen effusion mass spectrometric thermodynamic studies of low-volatility amino acids l-histidine and l-aspartic acid, which made it possible to avoid any significant degradation of the starting materials, as evidenced by IR spectroscopy and visual observation. The saturated vapor pressure data in the studied temperature range are described by a linear equation ln(p/Pa) = −(21983 ± 485)/T + (42.411 ± 1.029) for l-histidine (448–480 K) and ln(p/Pa) = −(22350 ± 600)/T + (43.757 ± 1.351) for l-aspartic acid (420–450 K); the overall uncertainty that follows the ± sign includes statistical (expanded standard uncertainty with k = 2) and systematic components. The pressures of these two compounds are of the same order, but l-histidine is most likely slightly less volatile. The enthalpy of sublimation was determined by the methods of the second and third laws of thermodynamics; the recommended values are 183.5 ± 4.2 kJ·mol–1 for l-histidine and 183.2 ± 3.6 kJ·mol–1 for l-aspartic acid. The enthalpy of formation of the gaseous amino acids was found to be −283.2 ± 5.0 kJ·mol–1 for l-histidine and −791.1 ± 4.6 kJ·mol–1 for l-aspartic acid.