Outer-sphere electron-transfer between horse heart cytochrome c and anionic Cu(ii/i) complexes. Evidence for precursor formation and coordination sphere reorganization for electron transfer

Abstract

The outer-sphere electron-transfer reaction between anionic bis(5,6-bis(4-sulfonatophenyl)-3- (2-pyridyl)-1,2,4-triazine)Cu(II) and cytochrome c^II was investigated as a function of pH, ionic strength, concentration, temperature and pressure. The plot of the observed pseudo-first-order rate constant as a function of the Cu(II) complex concentration showed saturation at higher Cu(II) concentrations, from which the precursor formation constant and the electron transfer rate constant could be separated (K = (7.7 ± 0.5) × 10³ M⁻¹ and k_ET = 6.2 ± 0.4 s⁻¹ at I = 0.2 M, pH 7.4 and 288 K). The pseudo-first-order electron-transfer rate constant was measured as a function of temperature and pressure at (low and) high Cu(II) concentrations (ΔH^‡ = (85 ± 4) 89 ± 4 kJ mol⁻¹; ΔS^‡ = (−61 ± 13) −79 ± 15 J K⁻¹ mol⁻¹; ΔG^‡ (288 K) = (67.6) 66.1 kJ mol⁻¹; ΔV^‡ = (+8.8 ± 0.6) +8.0 ± 0.7 cm³ mol⁻¹). Within the volume change for the overall reaction, the volume profile for the electron transfer step is almost symmetrical. The redox process and the change in coordination of the copper centre are proposed to be clearly separated. The back reaction between the Cu(I) complex and cytochrome c^III was investigated as a function of Cu(I) concentration at pH 7.4 at 1 bar. The observed pseudo-first-order rate constant reaches a saturation at high Cu(I) concentrations from which the precursor formation constant and the electron-transfer rate constant were estimated (K′ = (2.0 ± 0.2) × 10³ M⁻¹ and k′_ET = 0.014 ± 0.001 s⁻¹ at I = 0.2 M, pH 7.4 and 288 K). Simulations of the measured cyclovoltammogramms applying an EC mechanism with two redox systems and two homogeneous chemical reactions were performed. The results are discussed with reference to earlier studies involving Co, Ru and Cr complexes as redox partners for cytochrome c.