Spectroscopic characterization of a semi-stable, charge-separated state in Cu2+-substituted reaction centers from Rhodobacter sphaeroides

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 Cu2+, the decay of the semi-stable, charge-separated state became much slower than in centers with bound Fe 21 with about the same rate constant for formation. In Cu2+ 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 Cu2+ in the absence of magnetic interaction. The EPR results, obtained with Cu2+ 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. (C) 2003 Elsevier B.V. All rights reserved.