Picosecond-photodichroism studies of the transient states in Rhodopseudomonas sphaeroides reaction centers at 5 K. Effects of electron transfer on the six bacteriochlorin pigments

We have examined the dichroism of the visible and near-infrared absorption changes due to the early transient states in Rhodopseudomonas sphaeroides reaction centers imbedded in polyvinyl alcohol films at 5 K. The transient-state, ground-state and derivative spectra acquired under these conditions are highly resolved. Spectral features have been assigned to the bacteriochlorophyll (BChl) dimer (P) that serves as the primary electron donor, to each of the two additional BChls, and to the two bacteriopheophytin (BPh) molecules. The dichroism of the absorption changes, taken together with earlier results including our observation of a detection-wavelength dependence of the kinetics, argues that only one of the BPhs is a clearly resolved electron carrier prior to ubiquinone. The second BPh and the two BChls not constituting P display electrochromic effects and/or nuclear relaxations, possibly involving the protein, in response to the charge-separation process.