Effect of temperature and guanidine hydrochloride on ferrocytochrome c at neutral pH.

Thermally denatured horse heart ferrocytochrome c (ferrocyt c) has been characterized using absorption spectroscopy, differential scanning calorimetry (DSC) and viscometry at pH 7.0. DSC experiments have yielded the transition temperature of denaturant-free ferrocyt c unfolding as 100.6+/-0.3 degrees C, indicating an extremely high stability of the protein. The presence of guanidine hydrochloride (GdnHCl) facilitated estimation of the structural features of thermally unfolded ferrocyt c. The stability of the protein, expressed by Delta G(D) at 25 degrees C, is 59+/-5 kJ mol(-1) (DSC) and 65+/-6 kJ mol(-1) (absorption spectroscopy). An absorption spectrum of ferrocyt c demonstrates that the heme occurs in the high-spin state at extreme denaturing conditions (94 degrees C, 6.6 M GdnHCl). Absorption spectroscopy, using heme as a probe, shows that thermal denaturation of ferrocyt c occurs as a transition from a native low-spin (Met80/His18) to a high-spin disordered state with involvement of non-native, low-spin (bis-His) species.