Abstract
Absorbanceโdetected thermal denaturation studies of the C102T variant of Saccharomyces cerevisiae isoโ1โferricytochrome c were performed between pH 3 and 5. Thermal denaturation in this pH range is reversible, shows no concentration dependence, and is consistent with a 2โstate model. Values for free energy (ฮG~D~), enthalpy (ฮH~D~), and entropy (ฮS~D~) of denaturation were determined as functions of pH and temperature. The value of ฮG~D~ at 300 K, pH 4.6, is 5.1 ยฑ 0.3 kcal mol^โ1^. The change in molar heat capacity upon denaturation (ฮC~p~), determined by the temperature dependence of ฮH~D~ as a function of pH (1.37 ยฑ 0.06 kcal mol^โ1^ K^โ1^), agrees with the value determined by differential scanning calorimetry. pHโdependent changes in the Soret region indicate that a group or groups in the heme environment of the denatured protein, probably 1 or both heme propionates, ionize with a p__K__ near 4. The C102T variant exhibits both enthalpy and entropy convergence with a ฮH~D~ of 1.30 kcal mol^โ1^ residue^โ1^ at 373.6 K and a ฮS~D~ of 4.24 cal mol^โ1^ K^โ1^ residue^โ1^ at 385.2 K. These values agree with those for other singleโdomain, globular proteins.