Structure of Radical Pairs D^•+ Q ^•−_A in Bacterial Photosynthetic Reaction Centers Cooled to Cryogenic Temperatures in the Dark and Under Illumination: A High-Field EPR/PELDOR Study

Abstract

Reaction centers (RCs) from the photosynthetic bacterium Rhodobacter (Rb.) sphaeroides R-26 exhibit drastic changes in the recombination kinetics of the charge separated radical pair state, D^•+ Q_A ^•+, depending on whether the RCs are cooled to cryogenic temperatures in the dark or under continuous illumination (Kleinfeld et al. 1984). Here we investigated light-induced structural changes associated with chargeseparation, using time-resolved electron paramagnetic resonance (EPR) and pulsed electron-electron double resonance (PELDOR) techniques at 95 GHz, which yielded the distance and the relative orientation between donor and acceptor ions (D^•+ Q_A ^•−) in RCs frozen in the dark and frozen under illumination. The results showed that the difference in the charge recombination kinetics of D^•+Q_A ^•− is not due to changes in distance or orientation of the radical ions.