Comparison of the chemical and thermal denaturation of proteins by a two-state transition model

The conformational stabilities of eight proteins in terms of the free energy differences between the native “folded” state of the protein and its “unfolded” state were determined at 298 K by two methods: chemical denaturation at 298 K and extrapolation to 298 K of the thermal denaturation results at high temperature. The proteins were expressed in Escherichia coli from the Haemophilus influenzae and E. coli genes at different levels of expression, covered a molecular mass range from 13 to 37 kg mol−1 per monomeric unit (some exhibiting unique structural features), and were oligomeric up to four subunits. The free energy differences were determined by application of a two-state transition model to the chemical and thermal denaturation results, ranged from 9.4 to 148 kJ mol−1 at 298 K, and were found to be within the experimental uncertainties of both methods for all of the proteins. Any contributions from intermediate states detectable from chemical and thermal denaturation differences in the unfolding free energy differences in these proteins are within the experimental uncertainties of both methods.