ENDOR on the triplet state of the primary electron donor in the photosynthetic bacterium Rhodobacter sphaeroides - One step forward in a still unfinished story

The electronic structure of the primary electron donor (D) of the photosynthetic bacterium Rhodobacter sphaeroides is investigated by ENDOR in the photoinduced triplet state of D. Hyperfine (hf) splittings of the triplet state measured on frozen solutions are given and compared to the results obtained earlier (Lendzian F., van Willigen H., Sastry S., Mobius K., Scheer H., Feick R.: Chem. Phys. Lett. 118, 145 (1985)). The hf splittings found are consistent with a model of an asymmetric spin density distribution over the two bacteriochlorophyll molecules which constitute D, suggesting a mirror image symmetry of HOMO and LUMO coefficients. This could be relevant for electron transfer, in particular unidirectionality in the reaction center (RC). The first triplet state ENDOR experiments on single crystals of RC's are also reported.