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

Abstract

We report on the x-ray absorption fine structure of the Fe(2+) site in photosynthetic reaction centers from Rhodobacter sphaeroides. Crystallographic studies show that Fe(2+) is ligated with four N(epsilon) atoms from four histidine (His) residues and two O(epsilon) atoms from a Glu residue. By considering multiple scattering contributions to the x-ray absorption fine structure function, we improved the structural resolution of the site: His residues were split into two groups, characterized by different Fe-N(epsilon) distances, and two distinct Fe-O(epsilon) bond lengths resolved. The effect of the environment was studied by embedding the reaction centers into a polyvinyl alcohol film and into a dehydrated trehalose matrix. Incorporation into trehalose caused elongation in one of the two Fe-N(epsilon) distances, and in one Fe-O(epsilon) bond length, compared with the polyvinyl alcohol film. The asymmetry detected in the cluster of His residues and its response to incorporation into trehalose are ascribed to the hydrogen bonds between two His residues and the quinone acceptors. The structural distortions observed in the trehalose matrix indicate a strong interaction between the reaction-centers surface and the water-trehalose matrix, which propagates deeply into the interior of the protein. The absence of matrix effects on the Debye-Waller factors is brought back to the static heterogeneity and rigidity of the ligand cluster.

Mutational control of bioenergetics of bacterial reaction center probed by delayed fluorescence

“…The observed increase of the entropy contribution in the acceptor side mutants is in agreement with softening of the H-bond structure. It is consistent with the results from neutron scattering [69], X-ray absorption fine structure spectroscopy [70] and Mössbauer spectroscopy [71] measurements which suggested the rigidity of the native protein core around the Fe ligand that could be specifically softened by point mutations. In addition to conformational flexibility, polar side chains and proton distribution at the cytoplasmic side of the protein might play some role in entropy increase in these mutants if they are different in the ground and charge separated states.…”

Carboxylate Shifts Steer Interquinone Electron Transfer in Photosynthesis

“…For oxidized quinones and a bidentate carboxyl the coordination of the Fe(II) is distorted octahedral, with the z axis along D2His 214 -Fe-D1His 227 in PSII, as also found in model compounds (49) and by XAS analysis (50). For Q A Ϫ and breaking of one basal Fe-O bond, the geometry is trigonal-bipyramidal.…”