Horse heart ferric cytochrome c was investigated by the following three methods: (I) Light absorption spectrophotometry at 23 °C and 77 °K; (II) Electron paramagnetic resonance (EPR) spectroscopy at 20 °K; (III) Precise equilibrium measurements of ferric cytochrome c with azide and imidazole between 14.43 and 30.90 °C. I and II have demonstrated that: (1) Ferric cytochrome c azide and imidazole complexes were in the purely low spin state between 20 °K and 23 °C; (2) The energy for the three t2g orbitals calculated in one hole formalism shows that azide or imidazole bind to the heme iron in a similar manner to met-hemoglobin azide or imidazole complexes, respectively. III has demonstrated that: (1) The change of standard enthalpy and that of standard entropy were -2.3 kcal/mol and -1.6 cal/mol per degree for the azide complex formation, and -1.4 kcal/mol and 2.9 cal/mol per degree for the imidazole complex formation. (2) A linear relationship between the change of entropy and that of enthalpy was observed for the above data and for the cyanide complex formation. The complex formation of ferric cytochrome c was discussed based on the results of X-ray crystallographic studies compared with hemoglobin and myoglobin.
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