Redox state of the partially reduced cytochrome aa3- cyanide complex.

The low-spin ferric cyanide complex of beef heart cytochrome aa3 can be partially reduced by stoichiometric additions of ferrous cytochrome c or by similar additions of N,N,N',N'-tetramethyl-p-phenylene diamine. In both cases the initial ratio of cytochrome c oxidized: cytochrome a reduced or Wurster's Blue: cytochrome a reduced approximates the value 2. It is concluded that the binding of a single HCN prevents the reduction of both cytochrome a3 and its associated EPR-invisible Cu atom.