THE ELECTRONIC-STRUCTURE OF THE PRIMARY DONOR CATION-RADICAL IN RHODOBACTER-SPHAEROIDES R-26 - ENDOR AND TRIPLE-RESONANCE STUDIES IN SINGLE-CRYSTALS OF REACTION CENTERS

The electron spin density distribution of the cation radical of the pr imary donor, D+., a bacteriochlorophyll a dimer was determined by ENDO R and TRIPLE resonance experiments performed on single crystals of rea ction centers (RCs) of Rhodobacter sphaeroides R-26. Nine isotropic pr oton hyperfine coupling constants (hfc's) were obtained and from the a ngular dependence of the hfc's in three crystallographic planes, five complete hyperfine (hf) tensors were determined. Theoretical hf tensor s were calculated by the all-valence-electron SCF molecular orbital me thod RHF-INDO/SP using the X-ray structure data of the dimer D and its amino acid environment. A comparison of the directions of the princip al axes of the experimental and calculated hf tensors enabled us to id entify the hfc's with specific protons on the two bacteriochlorophyll halves D(L) and D(M) of the dimer. The result shows that the unpaired valence electron is unequally distributed over the dimer halves, favor ing D(L) by approx. 2:1. This ratio has been obtained from the proton hfc's of rotating methyl groups, which directly reflect the pi-spin de nsities at the corresponding positions in the two macrocycles, D(L) an d D(M). It was further confirmed by recent N-15-ENDOR experiments on R C single crystals (Lendzian, F., Bonigk, B., Plato, M., Mobius, K. and Lubitz, W. (1992) in The Photosynthetic Bacterial Reaction Center II (Breton, J. and Vermeglio, A., eds.), pp. 89-97, Plenum Press, New Yor k). The observed asymmetry of D+. is attributed to the difference in e nergies of the highest filled molecular pi-orbitals of the monomeric h alves, D(L) and D(M), which is caused by differences in the structure of the two bacteriochlorophylls and/or their environment. Possible imp lications of this asymmetry for the electron transfer in the RC are di scussed.