The Reactivity of Ferrocytochrome C with Iron-binding Ligands.

The reactions of ferricytochrome c with ionic ligands at neutral pH have favorable free energy changes that arise from positive entropy terms, to which protein chain rearrangements seem to be the main contributing factor (1, 2). On the other hand, in neutral solutions ferrocytochrome c does not react with carbon monoxide or nitric oxide, nor is it oxidized by 02. This lack of reactivity precludes comparison between the thermodynamic parameters for the reactions of reduced and oxidized cytochrome c. It is possible, however, to form the carbon monoxide, nitric oxide, and cyanide ion complexes of ferrocytochrome c at extremely alkaline pH values (3-5). By neutralizing these solutions the kinetics of the dissociation of the complexes can then be measured. In the case of ferrocytochrome c-nitric oxide the dissociation appears to be extremely slow (4), whereas the CO and CNcomplexes have measurable kinetics; the measurements of the dissociation of the latter two under varied conditions of pH and temperature are reported in this paper, and the results are discussed in terms of the structure of cytochrome c in its reduced and oxidized states.