Free-energy dependence of the rate of electron transfer to the primary quinone in beta-type reaction centers

Reaction centers of the beta-type mutants, (M)L214H and (M)L214H/(L)E104V, which contain a bacteriochlorophyll (denoted β) in place of the photoactive bacteriopheophytin, have been depleted of the native ubiquinone and reconstituted with a number of quinones. This system has allowed investigation of the rate versus free-energy relationship for electron transfer to the primary quinone in the activationless and inverted regions. The dependence of the rate on driving force is found to be much weaker in both mutants than in wild-type RCs. Analysis of the data using electron transfer theory shows that the essentially flat dependence of rate on free energy for the quinone-reconstituted beta-type mutants cannot be explained simply on the basis of increased driving force, but additionally requires a decrease in the reorganization energy. A decreased reorganization energy most likely derives from a change in the nature of the intermediary electron carrier in the mutants compared to wild-type RCs, in particular the involvement of the accessory bacteriochlorophyll molecule BChl_L. The weak free-energy and temperature dependence of the electron transfer process are consistent with coupling to a range of high- to low-frequency vibrational modes of the cofactors and protein.