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.

The Fe2+ Site of Photosynthetic Reaction Centers Probed by Multiple Scattering X-Ray Absorption Fine Structure Spectroscopy: Improving Structure Resolution in Dry Matrices

“…Nevertheless, relatively few XAFS studies have been performed on the RC Fe 21 site. Two pioneering works (21,22) revealed the presence of six atoms in the Fe 21 coordination shell, a result subsequently confirmed by XRD. These early analyses were based on a comparison with the spectra of model Fe compounds, and provided an average distance between Fe 21 and first-shell atoms.…”

“…We first studied RCs incorporated into a dried polyvinyl alcohol (PVA) film. The choice of this matrix, besides providing a practical, stable system well-suited for XAFS measurements in RCs (27), also allowed direct comparison with the early XAFS study by Eisenberger et al (21), who introduced this approach. Moreover, local structures determined in PVA most likely well-approximate solution structures, as indicated by a comparative XAFS study performed on cytochrome (cyt) c in a liquid solution and in a dehydrated PVA film (28).…”

Evidence for Delocalized Anticooperative Flash Induced Proton Binding as Revealed by Mutants at the M266His Iron Ligand in Bacterial Reaction Centers

“…From a study of time-resolved IR spectroscopy, it has been suggested that the oxidation of Q A -was delayed with respect to the formation of Q B -, implying the occurrence of an intermediary component in the ET process (13). The redox state of the non-heme iron is unchanged (Fe 2+ ) in the overall transition from Q A -Q B to Q A Q B - (14,15), but the Fe atom still appeared as an attractive candidate for the kinetic intermediate postulated by Remy and Gerwert (13). A recent EXAFS study (16) has disproved the possibility that it could act as an electron transfer intermediate in a "push-pull" mechanism initiated by the reduction of Q A (13).…”

My Road to Biophysics: Picking Flowers on the Way to Photosynthesis

“…Although the results were interesting, working at an accelerator with a large supporting group and a grueling schedule did not appeal to me. Twenty-five years later I had the same feeling doing extended X-ray absorption fine structure (EXAFS) experiments on reaction centers at the Stanford Linear Accelerator (37).…”