The redox couple of the cytochrome c cyanide complex: the
contribution of heme iron ligation to the structural stability,
chemical reactivity, and physiological behavior of horse
cytochrome c  Document Type   Publication Date   Source
Publication  Protein Science   Contrary to most heme proteins,
ferrous cytochrome c does not bind ligands such as cyanide and
CO. In order to quantify this observation, the redox potential
of the ferric/ferrous cytochrome c–cyanide redox couple was
determined for the first time by cyclic voltammetry. Its E0′ was
−240 mV versus SHE, equivalent to −23.2 kJ/mol. The entropy of
reaction for the reduction of the cyanide complex was also
determined. From a thermodynamic cycle that included this new
value for the cyt c cyanide complex E0′, the binding constant of
cyanide to the reduced protein was estimated to be 4.7 × 10−3
LM−1 or 13.4 kJ/mol (3.2 kcal/mol), which is 48.1 kJ/mol (11.5
kcal/mol) less favorable than the binding of cyanide to
ferricytochrome c. For coordination of cyanid