Probing Excited-State Electron Transfer by Resonance Stark Spectroscopy: 4. Mutations near BL in Photosynthetic Reaction Centers Perturb Multiple Factors that Affect →

Stark spectra have been obtained in the region of absorption by the accessory bacteriochlorophylls (the B-band region around 800 nm) in Rhodobacter sphaeroides reaction centers. The Stark spectra in this region are dominated by the resonance Stark effect of B L, the accessory bacteriochlorophyll on the functional side; results in Part 1 of this series demonstrated that these effects are associated with the alternative electron-transfer pathway →, where H L is the bacteriopheophytin acceptor on the functional side. Low-temperature absorption and Stark spectra for unmodified, Q A -reduced and P-oxidized samples of wild-type and three mutant reaction centers (M203GD, M210YF, and M210YW) are presented. This combination samples many perturbations to the state that can be quantified by fitting the data using the theory of resonance Stark effects developed in Parts 2 and 3 of this series. It is found that the mutations perturb not only the energy of relative to that of but also the electronic coupling between these states and the effective distance of charge transfer for the reaction → . Rates for this alternative charge-separation pathway are estimated from spectral analysis and compared with time-resolved measurements reported by several groups. By comparison of the spectra from the Q A -reduced and P-oxidized samples, it is found that the dielectric screening is larger in the region around Q A and the probe than in the region around P and . The mean frequency of the vibrational modes that are coupled to this charge-transfer process is also estimated. In the context of fitting the Stark spectra, evidence is presented for a strong excitonic interaction between the localized transitions at the B L and H L binding sites in reaction centers containing the M214LH mutation and among the localized transitions at the B L, B M, and P binding sites in wild-type reaction centers.