The Fe²⁺ site of photosynthetic reaction centers probed by multiple scattering XAFS spectroscopy: improving structure resolution in dry matrices

¹ University of Bologna
² CNR, Bari
³ University of Bari

^* To whom correspondence should be addressed. E-mail: ventur{at}alma.unibo.it.

Submitted on March 1, 2008
Revised on March 25, 2008
Accepted on 9 April 2008

	Abstract

We report on the X-ray absorption fine structure (XAFS) of the Fe²⁺ site in photosynthetic reaction centers (RC) from Rhodobacter sphaeroides. Crystallographic studies show that Fe²⁺ is ligated with four N atoms from four His residues and two O atoms from a Glu residue. By considering multiple scattering contributions to the XAFS function we improved the structural resolution of the site: His residues were split in two groups, characterized by different Fe-N distances, and two distinct Fe-O bond lengths resolved. The effect of the environment was studied by embedding the RC into a polyvinyl alcohol (PVA) film and into a dehydrated trehalose matrix. Incorporation into trehalose caused elongation in one of the two Fe-N distances, and in one Fe-O bond length, as compared to the PVA 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 RC surface and the water-trehalose matrix, which propagates deeply to 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.

Key Words: Rhodobacter sphaeroides, XAFS, iron site, reaction center, trehalose