Heat Capacities of <span style="font-variant: small-caps">l</span>-Histidine, <span style="font-variant: small-caps">l</span>-Phenylalanine, <span style="font-variant: small-caps">l</span>-Proline, <span style="font-variant: small-caps">l</span>-Tryptophan and <span style="font-variant: small-caps">l</span>-Tyrosine

• Main Authors: Václav Pokorný, Vojtěch Štejfa, Jakub Havlín, Květoslav Růžička, Michal Fulem
• Format: Article
• Language: English
• Published: MDPI AG 2021-07-01
• Series: Molecules
• Subjects: <span style="font-variant: small-caps">l-</span>histidine; <span style="font-variant: small-caps">l-</span>phenylalanine; <span style="font-variant: small-caps">l</span>-proline; <span style="font-variant: small-caps">l-</span>tryptophan; <span style="font-variant: small-caps">l</span>-tyrosine; crystal heat capacity
• Online Access: https://www.mdpi.com/1420-3049/26/14/4298

In an effort to establish reliable thermodynamic data for proteinogenic amino acids, heat capacities for <span style="font-variant: small-caps;">l</span>-histidine (CAS RN: 71-00-1), <span style="font-variant: small-caps;">l</span>-phenylalanine (CAS RN: 63-91-2), <span style="font-variant: small-caps;">l</span>-proline (CAS RN: 147-85-3), <span style="font-variant: small-caps;">l</span>-tryptophan (CAS RN: 73-22-3), and <span style="font-variant: small-caps;">l</span>-tyrosine (CAS RN: 60-18-4) were measured over a wide temperature range. Prior to heat capacity measurements, thermogravimetric analysis was performed to determine the decomposition temperatures while X-ray powder diffraction (XRPD) and heat-flux differential scanning calorimetry (DSC) were used to identify the initial crystal structures and their possible transformations. Crystal heat capacities of all five amino acids were measured by Tian–Calvet calorimetry in the temperature interval from 262 to 358 K and by power compensation DSC in the temperature interval from 307 to 437 K. Experimental values determined in this work were then combined with the literature data obtained by adiabatic calorimetry. Low temperature heat capacities of <span style="font-variant: small-caps;">l</span>-histidine, for which no literature data were available, were determined in this work using the relaxation (heat pulse) calorimetry from 2 K. As a result, isobaric crystal heat capacities and standard thermodynamic functions up to 430 K for all five crystalline amino acids were developed.