Kinetics and Mechanism of the Outer‐Sphere Oxidation of Horse‐Heart Cytochrome c by an Anionic Chromium(v) Complex – Kinetic Evidence for Precursor Formation and a Late Electron‐Transfer Transition State

The irreversible outer‐sphere electron‐transfer reaction between trans ‐bis(2‐ethyl‐2‐hydroxybutanoato(2–))oxochromate(v) and cytochrome c II was investigated as a function of pH, concentration, temperature and pressure. The plot of the observed pseudo‐first order rate constant as a function of the Cr V concentration shows a clear trend towards saturation at higher Cr V concentrations, from which the precursor formation constant and the electron‐transfer rate constant could be separated ( K = 37 ± 5 M −1 and k ET = 1510 ± 180 s −1 at pH 4.8 and 279 K). In the low Cr V concentration range the second‐order electron‐transfer rate contants were measured as a function of temperature (Δ H # = 20.9 ± 0.6 kJ mol −1 ; Δ S # = –79.9 ± 2.1 J K −1 mol −1 ; Δ G # (279 K) = 43.2 kJ mol −1 ). High‐pressure experiments were performed at two different pH values. The kinetic (stopped‐flow) and thermodynamic (electrochemical) measurements as a function of pressure enabled the construction of a volume profile for the system at 279 K. The activation volumes for the redox process are –9.2 ± 0.2 (pH 5.0) and –11.1 ± 0.8 cm 3 mol −1 (pH 4.7), and the overall reaction volumes were estimated to be –7 ± 2 (pH 5.0) and –10 ± 2 cm 3 mol −1 (pH 4.7) . The transition state of the redox reaction lies to a large extent on the product side and can be described as “late”. The results are discussed in comparison to earlier measurements using cobalt and ruthenium complexes as reaction partners for cytochrome c .