Orientation-Resolving Pulsed Electron Dipolar High-Field EPR Spectroscopy on Disordered Solids:  I. Structure of Spin-Correlated Radical Pairs in Bacterial Photosynthetic Reaction Centers

Distance and relative orientation of functional groups within protein domains and their changes during chemical reactions determine the efficiency of biological processes. In this work on disordered solid-state electron-transfer proteins, it is demonstrated that the combination of pulsed high-field EPR spectroscopy at the W band (95 GHz, 3.4 T) with its extensions to PELDOR (pulsed electron−electron double resonance) and RIDME (relaxation-induced dipolar modulation enhancement) offers a powerful tool for obtaining not only information on the electronic structure of the redox partners but also on the three-dimensional structure of radical-pair systems with large interspin distances (up to about 5 nm). Strategies are discussed both in terms of data collection and data analysis to extract unique solutions for the full radical-pair structure with only a minimum of additional independent structural information. By this novel approach, the three-dimensional structure of laser-flash-induced transient radical pairs in frozen-solution reaction centers (RCs) from the photosynthetic bacterium <i>Rhodobacter (Rb.) sphaeroides</i> is solved. The measured positions and relative orientations of the weakly coupled ion radicals and are compared with those of the precursor cofactors P<sub>865</sub> and Q<sub>A</sub> known from X-ray crystallography. A small but significant reorientation of the reduced ubiquinone Q<sub>A</sub> is revealed and interpreted as being due to the photosynthetic electron transfer. In contrast to the large conformational change of upon light illumination of the RCs, the small light-induced reorientation of had escaped previous attempts to detect structural changes of photosynthetic cofactors upon charge separation. Although small, they still may be of functional importance for optimizing the electronic coupling of the redox partners in bacterial photosynthesis both for the charge-separation and charge-recombination processes.