Bacteriochlorophyll dimers in photosynthesis

The X-ray crysallagraphic study of reaction center (RC) single crystals of the photosynthetic bacteria Rps.Viridis and Rb. sphaeroides confirms the existence of bacteriochlorophyll (BChl) dimers which were postulated earlier from EPR and ENDOR studies at low temperature to be the primary electron donors P960 and P870. Apart from the spatial structure of these dimers a knowlegde of the electron density distribution in various electronic stated in indispensable for an understanding of their functional properties. For P870+ and P960+ under physiological ocnditions the electron spin density distrubutions were obtained by ENDOR-in-solution via the hyperifne couplings. The comparison between the EPR/ENDOR data of P870+ and P960+ in RC's and of onomeric BChl a. and BChl b. shows that the primary donors are pie conjugated supermolecules with more or less asymmetric spin dnesity distrubutions over the dimer halves. Theoretical spian and charge densities were calculated by an all-valence electron SCF MO method, RHF-INDO/SP, using coordinates from refined X-ray data. These calculations yield asymmetry ratios similar to those observed.Consequences of the asymmetries in the charge distribution with respect to the observed unidirectionality of the electron transfer are discussed. (author). 29 refs.; 4 figs