Heat capacity of solid state proteins by thermal analysis

In an ongoing effort to understand their thermodynamic properties, solid state heat capacities of poly(amino acid)s of all 21 natural amino acids, 4 copoly(amino acid)s and 8 proteins have been determined in our Advanced THermal AnalysiS (ATHAS) laboratory. In this thesis, results are presented for two of the poly(amino acid)s: polymethionine and polyphenylalanine; six globular proteins: bovine insulin, chymotrypsinogen, lactoglobulin, ribonuclease A, chicken lysozyme, ovalbumin based on adiabatic and differential scanning calorimetry over wide temperature ranges. The measured data were analyzed with both the ATHAS empirical addition scheme and the computation scheme with approximate vibrational spectra. Good agreement is found between experiment and calculation with rms errors mostly within ± 3%. This effort links the macroscopic heat capacity to its microscopic cause, the group and skeletal vibrational motion. For each sample, one set of parameters, Θ₁ and Θ₃, of the Tarasov function representing the skeletal vibrational contribution to heat capacity are obtained from a new optimization procedure.

Based on this study, vibrational heat capacity can now be calculated for all proteins with an accuracy comparable to common experiments. Furthermore, gradual transitions, indicative of molecular motion prior to liquidation or decomposition, can be identified.