Viscosity dependence of the electron transfer rate from bound cytochrome c to P840 in the photosynthetic reaction center of the green sulfur bacterium Chlorobium tepidum.

Anomalous high viscosity dependence was found in the rate of reaction between the bound cytochrome c and the primary donor bacteriochlorophyll dimer (P840) of the reaction center complex purified from the green sulfur bacterium Chlorobium tepidum. The cytochrome has a primary structure with the N-terminal three membrane-spanning helices connected to the extended C-terminal heme-containing hydrophilic moiety. The rate constant of the reaction decreased from 5.0 x 10(3) s-1 to 1.0 x 10 s-1 as the glycerol concentration increased from 0 to 60% (v/v) at 295 K, showing a linear dependence on the -2.4th power of the specific viscosity. The glycerol effect was fully reversible. The extraordinary high viscosity dependence cannot be explained by the simple diffusive Brownian fluctuation model and suggests that the electron transfer mechanism is dependent on the unique conformational fluctuations of the heme-containing moiety of cytochrome c.