title: 15N electron nuclear double resonance of the primary donor cation radical P+.865 in reaction centers of Rhodopseudomonas sphaeroides: additional evidence for the dimer model.
abstract: Four 15N hyperfine coupling constants, including signs, have been measured by electron nuclear double resonance (ENDOR) and electron nuclear nuclear triple resonance (TRIPLE) for the bacteriochlorophyll a radical cation, BChla+., in vitro and for the light-induced primary donor radical cation, P+.865, in reaction centers of Rhodopseudomonas sphaeroides R-26. A comparison of the data shows that the hyperfine coupling constants have the same sign in both radicals and are, on the average, smaller by a factor of 2 in P+.865. These results provide additional evidence that P+.865 is a bacteriochlorophyll dimer and are in contradiction with the monomer structure of P+.865 recently proposed by O'Malley and Babcock. The reduction factors of the individual 15N couplings, together with the evidence from proton ENDOR data and molecular orbital calculations, indicate a dimer structure in which only two rings (either I and I or III and III) of the bacteriochlorophyll macrocycles overlap.
mesh_terms: D001426:Bacterial Proteins / Q000378:metabolism*; D002412:Cations / Q000378:metabolism*; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods; D005609:Free Radicals; D045342:Light-Harvesting Protein Complexes; D008968:Molecular Conformation; D009587:Nitrogen Isotopes; D045322:Photosynthetic Reaction Center Complex Proteins; D012242:Rhodobacter sphaeroides / Q000378:metabolism*
chemical_list: D001426:Bacterial Proteins; D002412:Cations; D005609:Free Radicals; D045342:Light-Harvesting Protein Complexes; D009587:Nitrogen Isotopes; D045322:Photosynthetic Reaction Center Complex Proteins