EPR and ENDOR Studies of the Oxidized Primary Donor in Single Crystals of Reaction Centers of Rhodobacter sphaeroides R-26

Abstract

To understand electron transfer rates in bacterial reaction centers (RCs) a knowledge of the spatial as well as the electronic structure of the pigments is required. From the X-ray structure analysis of RC’s of two bacterial species, Rhodopseudomonas viridis [1,2] and Rhodobacter sphaeroides [3–5], the spatial arrangement of the protein and the pigments have been determined. EPR and ENDOR investigations of paramagnetic states of the pigments provide information on the electronic structure, in particular the spin density distribution in the valence orbitals. Furthermore, studies of the anisotropic hypcrfine (hf) interaction can ultimately yield the position of protons, which cannot be accurately determined from X-ray structure analyses. They also may facilitate the identification of ENDOR lines with specific protons. The anisotropic interactions are preferably studied in single crystals. Here we present preliminary results of ENDOR investigations on the cation radical of the primary donor D in single crystals of RC’s from Rb. sphaeroides R-26 near room temperature (284K). The resolution of the spectra at 284K is better than that observed at cryogenic temperatures reported earlier [6]. A full rotation study of the hf interaction in one crystal plane was performed. A partial interpretation of the observed angular dependence of the hf splittings based on investigations of D⁺ in solution [7] and the arrangement of D in the crystal as obtained by X-ray diffraction [5] is presented. Experiments, similar to the ones reported here, have been performed independently by F. Lendzian et al. and are presented in an accompanying paper [8].