High-field EPR on the primary electron donor cation radical in single crystals of heterodimer mutant reaction centers of photosynthetic bacteria — first characterization of the G-tensor Original Research Article

First high-field/high-frequency (95 GHz) EPR experiments on the cation radical of the primary electron donor (D) in single crystals of the HL (M202) heterodimer mutant of the photosynthetic bacterium Rhodobacter sphaeroides are reported. The measurements have been performed in order to determine the electronic G-tensor of D+ in the mutant and to compare it to the G-tensor of D+ in wild type (R-26). In the heterodimer mutant of the two bacteriochlorophyll (BCHl) molecules constituting D in R-26 is replaced by a bacteriopheophytin. In D2+ of the mutant the unpaired electron is localized on the BChl, thus, its G-tensor should reflect monomer properties, whereas in D+ of R-26 the unpaired electron is delocalized over the two BChl molecules. The preliminary results reported here show that directions of the principal axes of the G-tensor of D+ in HL (M202) are similar to those of R-26 investigated previously. Possible origins of this result are discussed.