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

AbstractThe electronic structure of the primary electron donor (D) in the heterodimer 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 reaction centers. In the mutants, one of the two bacteriochlorophyll (BChl) molecules of D is replaced by a bacteriopheophytin. The assignment of the ENDOR lines to specific methyl and non-methyl protons was accomplished by comparing that directions of the principal axes of the hyperfine tensors with the directions predicted from the X-ray structure and theory. We showed that the unpaired electron is localized on the BChl in the heterodimers, i.e., on the L-side (DL) in the HL (M202) and on the M-side (DM) in the HL (L173) mutant. Significant differences in the electronic structure of DL and DM were observed; they are attributed to the protein and/or pigment environment. Possible consequences of these differences for electron transfer, e.g., unidirectionality are discussed. The inequivalence of DL and DM also shows up in the asymmetry of the electronic structure of D in the native homodimer, whose electronic structure was reinterpreted using the heterodimers as monomer models