Conserved electronic structure of the primary donor in reaction centres of sulphur and non-sulphur purple bacteria

The photosynthetic membrane has been isolated from 20 species of sulfur and non-sulfur purple bacteria that contain bacteriochlorophyll (BChl) a. The oxidized state of the primary electron donor of the reaction centres (RC), P·+, was created by illumination with actinic light and stabilized by rapid freezing. ENDOR spectroscopy at 160 K of the formed radical cation reveals an overall similarity of the electronic structure of P·+ in all investigated species, although the solid state spectra show distinct variations of the hyperfine splittings. In order to understand these differences, site-directed mutants of Rhodobacter sphaeroides were constructed on the basis of sequence alignments. P·+ in membranes and RC of the mutants was investigated by ENDOR spectroscopy at 160 K and Special TRIPLE resonance spectroscopy at 288 K, respectively. A comparison of the spectra indicates that P is a BChl a dimer in all wild type species with an approximate 2:1 distribution of the unpaired spin density of P·+ over the two dimer halves. It is concluded that the asymmetry of the spin/charge density distribution is a conserved property of the primary donor in the RC of purple bacteria and could therefore be of functional significance. Supported by Alexander-von-Humboldt-Stiftung and DFG (Sfb 312 and 498).