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

Abstract

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 BL, 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 HL is the bacteriopheophytin acceptor on the functional side. Low-temperature absorption and Stark spectra for unmodified, QA-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...