ELECTRONIC-STRUCTURE OF THE OXIDIZED PRIMARY ELECTRON-DONOR OF THE HL(M202) AND HL (L173) HETERODIMER MUTANTS OF THE PHOTOSYNTHETIC BACTERIUM RHODOBACTER-SPHAEROIDES - ENDOR ON SINGLE-CRYSTALS OF REACTION CENTERS

The electronic structure of the primary electron donor (D) in the hete rodimer mutants mutants HL (M202) and HL (L173) of the photosynthetic bacterium Rhodobacter sphaeroides was investigated using EPR and ENDOR (electron nuclear double resonance) methods on single crystals of rea ction centers. In the mutants, one of the two bacteriochlorophyll (BCh l) molecules of D is replaced by a bacteriopheophytin. The assignment of the ENDOR lines to specific methyl and non-methyl protons was accom plished by comparing the directions of the principal axes of the hyper fine tensors with the directions predicted from the X-ray structure an d theory, We showed that the unpaired electron is localized on the BCh l in the heterodimers, i.e., on the L-side (D-L) in the HL (M202) and on the M-side (D-M) in the HL (L173) mutant. Significant differences i n the electronic structure of D-L and D-M were observed; they are attr ibuted to the protein and/or pigment environment, Possible consequence s of these differences for electron transfer, e.g., unidirectionality are discussed. The inequivalence of D-L and D-M also shows up in the a symmetry of the electronic structure of D in the native homodimer, who se electronic structure was reinterpreted using the heterodimers as mo nomer models.