Abstract
Spin density calculations were performed on the oxidized primary electron donor in reaction centers (RC’s) from Rhodobacter sphaeroides R-26. The calculations were based on the all-valence-electron MO method RHF-INDO/SP and on the three-dimensional structure determined by X-ray diffraction. Results are compared with experimental isotropic hyperfine couplings (hf c’s) obtained from ENDOR/TRIPLE magnetic resonance spectra and with previous studies on in RC’s from Rhodopseudomonas viridis. Both bacteriochlorophyll dimer species and show marked deviations of their spin distributions from C2-symmetry. However, is more symmetric than with respect to sums of spin densities on the A and B halves of the dimers. The effects of various environmental (inclusion of surrounding amino acid residues) and structural refinements (e.g. rotation of acetyl groups) on hfc’s and excited state properties were studied. The latter were obtained by additional INDO/S calculations. In both species, P865 and P960, the excited electron in the lowest unoccupied molecular orbital (LUMO) is predicted to be preferentially on the B half of the dimer, which is close to the accessory bacteriochlorophyll monomer on the photoactive A branch. Thus the preferential electron transfer along the A branch originates in the excited state of the dimer.
Published Version
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