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Spectroscopic characterization of a semi-stable, charge-separated state in Cu(2+)-substituted reaction centers from Rhodobacter sphaeroides
Authors:Andréasson Ulf  Carlsson Theres  Andréasson Lars Erik
Institution:Department of Biochemistry and Biophysics, University of G?teborg, Medicinaregatan 9E, S-413 90 G?teborg, Sweden.
Abstract:In reaction centers from Rhodobacter sphaeroides exposed to continuous illumination in the presence of an inhibitor of the Q(A)(-) to Q(B) electron transfer, a semi-stable, charge-separated state was formed with halftimes of formation and decay of several minutes. When the non-heme iron was replaced by Cu(2+), the decay of the semi-stable, charge-separated state became much slower than in centers with bound Fe(2+) with about the same rate constant for formation. In Cu(2+)-substituted reaction centers, the semi-stable state was associated with an EPR signal, significantly different from that observed after chemical reduction of the acceptor-side quinone or after illumination at low temperature, but similar to that of an isolated Cu(2+) in the absence of magnetic interaction. The EPR results, obtained with Cu(2+)-substituted reaction centers, suggest that the slow kinetics of formation and decay of the charge-separated, semi-stable state is associated with a structural rearrangement of the acceptor side and the immediate environment of the metal-binding site.
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