Figure 7.

Reduction of yK79H iso-1-Cytc as a function of Co(terpy)₂²⁺ concentration. (A) Reduction of oxidized yK79H iso-1-Cytc by Co(terpy)₂²⁺ monitored as a function of time at 550 nm, A₅₅₀. The time axis is logarithmic. Data are shown at 5 concentrations of Co(terpy)₂²⁺. The absolute magnitudes of A₅₅₀ have been adjusted so that the data at different Co(terpy)₂²⁺ concentrations can be compared readily. For data collected with a 10 mm pathlength, the following adjustments were made: 0.48 mM, no adjustment to A₅₅₀; 1.02 mM, observed A₅₅₀ – 0.265; 2.03 mM, observed A₅₅₀ – 0.79. For data collected with a 2 mm pathlength, the following adjustments were made: 3.00 mM, observed A₅₅₀ – 0.06; 4.51 mM, observed A₅₅₀ – 0.21. The solid black curves are fits of each data set to a triple exponential rise to maximum equation. (B) Rate constants for the fast phase, k_obs,1, and intermediate phase, k_obs,2, for reduction of oxidized yK79H by Co(terpy)₂²⁺ as a function of Co(terpy)₂²⁺ concentration. The main panel shows k_obs,2 as a function of Co(terpy)₂²⁺ concentration at pH 5.0 and 7.5, with fits to eq 2 (dashed curves) and eq 4 (solid curves). The inset shows the dependence of k_obs,1 on Co(terpy)₂²⁺ concentration. The slopes of the lines in the inset yield k_ET. The values of k_ET obtained were used in the fits of the k_obs,2 versus Co(terpy)₂²⁺ concentration data in the main panel to eq 2 and eq 4. The rate constants k_ET, k_ET(alk), k_b and k_f in the square-scheme kinetic model (Figure 3), derived from these fits, are collected in Table 1.