Heat Capacities of <scp>l</scp>-Alanine, <scp>l</scp>-Valine, <scp>l</scp>-Isoleucine, and <scp>l</scp>-Leucine: Experimental and Computational Study

Abstract

This work is part of the effort on establishing reliable thermodynamic data for amino acids and, in a broader context, benchmarking first-principles calculations of thermodynamic properties of molecular crystals against reliable experimental data. In this work, crystal heat capacities of l-alanine (CAS RN: 56-41-7), l-valine (CAS RN: 72-18-4), l-isoleucine (CAS RN: 73-32-5), and l-leucine (CAS RN: 61-90-5) were newly measured in the temperature range 262–450 K by Tian–Calvet calorimetry and power-compensation differential scanning calorimetry (DSC) and combined with the critically assessed literature data to obtain the reference data from near 0 to 450 K. The heat capacity measurements were accompanied by thermogravimetric analysis to determine the decomposition temperatures of the studied amino acids and phase behavior studies by X-ray powder diffraction and heat-flux DSC to identify the initial crystal structures and their possible transformations. The crystal heat capacities calculated by combining the periodic density functional theory and the quasi-harmonic approximation showed an agreement with the developed reference experimental data within 10% which can be considered as success of the employed computational methodology. Quantum chemical calculations further helped interpret the differences in thermodynamic and structural properties of the studied crystalline amino acids.