Heat capacities of solid, globular proteins

In an ongoing effort to understand the thermodynamic properties of proteins, solid‐state heat capacities of poly(amino acid)s of all 20 naturally occurring amino acids and 4 copoly(amino acid)s were previously determined using our Advance Thermal Analysis System (ATHAS). Recently, poly( L ‐methionine) and poly( L ‐phenylalanine) were further studied with new low‐temperature measurements from 10 to 340 K. In addition, an analysis was performed on literature data of a first protein, zinc bovine insulin dimer C 508 H 752 O 150 N 130 S 12 Zn. Good agreement was found between experiment and calculation. In the present work, we have investigated four additional anhydrous globular proteins, α‐chymotrypsinogen, β‐lactoglobulin, ovalbumin, and ribonuclease A. The heat capacity of each protein was measured from 130 to 420 K with differential scanning calorimetry, and the data were analyzed with both the ATHAS empirical addition scheme and a fitting to computations using an approximate vibrational spectrum. For the solid state, agreement between measurement and computation scheme could be accomplished to an average and root mean square percentage error of 0.5 ± 3.2% for α‐chymotrypsinogen, −0.8 ± 2.5% for β‐lactoglobulin, −0.4 ± 1.8% for ovalbumin, and −0.7 ± 2.2% for ribonuclease A. With these calculations, it was possible to link the macroscopic heat capacities to their macroscopic causes, the group and skeletal vibrational motion. For each protein one set of parameters of the Tarasov function, Θ 1 and Θ 3 , represent the skeletal vibrational contributions to the heat capacity. They are obtained from a new optimization procedure [α‐chymotrypsinogen: 631 K and 79 K (number of skeletal vibrators N s = 3005); β‐lactoglobulin: 582 K and (79 K) ( N s = 2188); ovalbumin: 651 K and (79 K) ( N s = 5008) and ribonuclease A: 717 K and (79 K) ( N s = 1574), respectively]. Enthalpy, entropy, and Gibbs free energy can be derived for the solid state.