Photoinduced electron transfer in cytochrome bc: Dynamics of rotation of the Iron-sulfur protein during bifurcated electron transfer from ubiquinol to cytochrome c and cytochrome b.

The electron transfer reactions within wild-type Rhodobacter sphaeroides cytochrome bc (cyt bc) were studied using a binuclear ruthenium complex to rapidly photooxidize cyt c. When cyt c, the iron‑sulfur center FeS, and cyt b were reduced before the reaction, photooxidation of cyt c led to electron transfer from FeS to cyt c with a rate constant of k = 80,000 s, followed by bifurcated reduction of both FeS and cyt b by QH in the Q site with a rate constant of k = 3000 s. The resulting Q then traveled from the Q site to the Q site and oxidized one equivalent each of cyt b and cyt b with a rate constant of k = 340 s. The rate constant k was decreased in a nonlinear fashion by a factor of 53 as the viscosity was increased to 13.7. A mechanism that is consistent with the effect of viscosity involves rotational diffusion of the iron‑sulfur protein from the b state with reduced FeS close to cyt b to one or more intermediate states, followed by rotation to the final c state with FeS close to cyt c, and rapid electron transfer to cyt c.