title: The electronic structure of Fe2+ in reaction centers from Rhodopseudomonas sphaeroides. II. Extended x-ray fine structure studies.
abstract: Extended x-ray absorption fine structure (EXAFS) studies were performed on reaction centers (RC) of the photosynthetic bacterium Rhodopseudomonas sphaeroides R-26. RC containing two, one, and no quinones (2Q, 1Q, 0Q) samples were studied. The average ligand distance of the first coordination shell was determined to be 2.10 +/- 0.02 A with a more distant shell at 4.14 +/- 0.05 A. The Fe2+ site in RC was found to have a very large structural disorder parameter, from which a spread in ligand distance per iron site of approximately +/- 0.1 A was deduced. The most likely coordination number of the first shell is six, with a mixture of oxygens and nitrogens as ligands. The edge absorption results are consistent with the Fe2+ being in distorted octahedral environment. The EXAFS spectra of the 2Q and 1Q samples with and without O-phenanthroline were found to be the same. This indicates that either the secondary quinone and o-phenanthroline do not bind to Fe2+ or that they replace an equivalent ligand. The 0Q sample showed a 12% decrease in the EXAFS amplitude, which was restored upon addition of o-phenanthroline. These results can be explained by either a loss of a ligand or a severe conformational change when the primary quinone was removed.
mesh_terms: D055598:Chemical Phenomena; D002627:Chemistry, Physical; D005296:Ferrous Compounds / Q000378:metabolism; D005583:Fourier Analysis; D007501:Iron / Q000378:metabolism; D010618:Phenanthrolines / Q000378:metabolism; D010788:Photosynthesis; D011809:Quinones / Q000378:metabolism; D012242:Rhodobacter sphaeroides / Q000032:analysis*; D013053:Spectrophotometry; D013057:Spectrum Analysis; D014965:X-Rays
chemical_list: D005296:Ferrous Compounds; D010618:Phenanthrolines; D011809:Quinones; D007501:Iron; C025205:1,10-phenanthroline