HEAT-CAPACITIES OF SOLID, GLOBULAR-PROTEINS

In an ongoing effort to understand the thermodynamic properties of pro teins, solid-state heat capacities of poly(amino acid)s of all 20 natu rally 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 wi th new low-temperature measurements from 10 to 340 K. In addition, an analysis was performed on Literature data of a first protein, zinc bov ine insulin dimer C508H752O150N130S12Zn. Good agreement was found betw een experiment and calculation. In the present work, we have investiga ted four additional anhydrous globular proteins, alpha-chymotrypsinoge n, P-lactoglobulin, ovalbumin, and ribonuclease A. The heat capacity o f each protein was measured from 130 to 420 K with differential scanni ng calorimetry, and the data were analyzed with both the ATHAS empiric al addition scheme and a fitting to computations using an approximate vibrational spectrum. For the solid state, agreement between measureme nt and computation scheme could be accomplished to an average and root mean square percentage error of 0.5 +/- 3.2% for alpha-chymotrypsinog en, -0.8 +/- 2.5% for beta-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 microscopic causes, the group and skeletal vibrational motion. For each protein o ne set of parameters of the Tarasov function, Theta(1) and Theta(3), r epresent the skeletal vibrational contributions to the heat capacity. They are obtained from a new optimization procedure [alpha-chymotrypsi nogen: 631 K and 79 K (number of skeletal vibrators N-s = 3005); beta- 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), re spectively]. Enthalpy, entropy, and Gibbs free energy can be derived f or the solid state.