The light-induced radical cation of the primary electron donor P&b in photosynthetic reaction centers from Rhodopseudomonus viridis has been investigated by ESR, ENDOR and TRIPLE techniques. Both the comparison with the cation radical of monomeric bacteriochlorophyll b (BChl b) and with molecular-orbital calculations performed on P& using the results of an X-ray structure analysis, consistently show an asymmetric distribution of the unpaired electron over the two BChl b molecules which constitute P&b. The possible relevance of this result for the primary electron transfer step in the reaction center is briefly discussed.

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The radical cation of bacteriochlorophyll b (BChl b) is investigated by ENDOR and TRIPLE resonance in liquid solution. The experimental hyperfine coupling constants, ten proton and three. nitrogen couplings, are compared with the predictions from advanced molecular-orbital calculations (RHF INDO/SP). The detailed picture obtained of the spin density distribution is a prerequisite for the investigation of the primary electron donor radical cation in BChl b containing photosynthetic bacteria.

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