Magnetic Resonance and Molecular Orbital Studies of the Primary Donor Cation Radical P+.960in the Photosynthetic BacteriumRhodopseudomonas viridis

The light-induced radical cation of the primary electron donor, P+.960, in photosynthetic reaction centers (RCs) from Rhodopseudomonas viridis has been investigated by various magnetic resonance techniques (ESR, ENDOR, TRIPLE). Comparison of the measured hyperfine couplings with those of the cation radical of monomeric bacteriochlorophyll b (BChl b) and with RHF-INDO/SP molecular orbital calculations, performed on P+.960 using coordinates of an X-ray structure analysis, consistently show an asymmetric distribution of the unpaired electron over the two BChl b molecules which constitute P960. The derived spin density ratio is 2.8 in favor of the dimer half bound to the L protein subunit. The possible relevance of this result for the primary electron transfer step in the RC is briefly discussed.