Heat capacities of solid state proteins: implications for protein stability in solution

We show that experimentally measured heat capacities of six different proteins (lysozyme, myoglobin, chymotrypsinogen, lactoglobulin, ovalbumine, and ribonuclease A) in the solid state, in the temperature range from 260 to 420 K, can be well characterized as a sum of one Einstein mode, corresponding to the group vibrations, plus a constant, corresponding to the nearly excited skeletal vibrations. The relative root mean square deviations between experimental data and fitted data are less than 1%. We also show for lysozyme and ribonuclease A that the experimental values of the solid state heat capacities, corrected for intermolecular vibrational modes, deviate less than 15% relative experimental protein data in solution, where the hydration part of the latter data is excluded by a model compound evaluation (J. Mol. Biol. 224 (1992) 715). Thus, analyzing solid state proteins may give important information to energetics and stabilization of proteins in solution.