A THEORETICAL-EXAMINATION OF THE ELECTRONIC-STRUCTURE AND SPECTROSCOPY OF THE PHOTOSYNTHETIC REACTION CENTER FROM RHODOPSEUDOMONAS-VIRIDIS

We report electronic structure calculations of the excited states of a model of the photosynthetic reaction center (RC) from the photosynthetic bacterium Rhodopseudomonas viridis. These calculations are performed using the intermediate neglect of differential overlap model (INDO/S) and include the effect of the polarizable protein by utilizing the self-consistent reaction field (SCRF) solvent model. These calculations reproduce the main characteristics of the experimental optical spectra. We are able to identify the two lowest excited states as the lower P*(-) and upper P*(+) exciton states of the bacteriochlorophyll b (BChlb) dimer (P) separated by about 1300 cm-1, as well as several charge-transfer (CT) states. In both the gas-phase and solvent calculations, the charge-transfer (CT) states on the photoactive L branch are lower than the corresponding CT bands for the M branch. Without including the relaxation that results from the polarization of the media, the CT states are predicted to be some 8000 cm-1 higher in energy than the lowest excited state of the BChlb dimer (P*(-)). Including the effects of the protein, even in an averaged way, dramatically lowers the energy of all the CT states. P+H(L)-, the state responsible for the observed charge separation, where H(L) designates the bacteriopheophytin b on the L branch, is the lowest CT state with a calculated energy only 1200 cm-1 above the lowest excitonic state P*(-), and we argue that geometric relaxation of the RC or of the surrounding protein would lower this CT state even further. These calculations demonstrate the preference for CT along the L branch as well as supplying a rationale for the absence of an explicit intermediate involving the auxiliary BChlb (B(L)) that lies between P and H(L) in the structure of the RC.