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

Abstract The electron spin density distribution of the cation radical of the primary donor, D + , a bacteriochlorophyll a dimer was determined by ENDOR and TRIPLE resonance experiments performed on single crystals of reaction centers (RCs) of Rhodobacter sphaeroides R-26. Nine isotropic proton hyperfine coupling constants (hfc's) were obtained and from the angular dependence of the hfc's in three crystallographic planes, five complete hyperfine (hf) tensors were determined. Theoretical hf tensors were calculated by the all-valence-electron SCF molecular orbital method 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 principal axes of the experimental and calculated hf tensors enabled us to identify 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, favoring D L by approx. 2:1. This ratio has been obtained from the proton hfc's of rotating methyl groups, which directly reflect the π-spin densities at the corresponding positions in the two macrocycles, D L and D M . It was further confirmed by recent 15 N-ENDOR experiments on RC 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 York). The observed asymmetry of D + is attributed to the difference in energies of the highest filled molecular π-orbitals of the monomeric halves, D L and D M , which is caused by differences in the structure of the two bacteriochlorophylls and/or their environment. Possible implications of this asymmetry for the electron transfer in the RC are discussed.