High-field dipolar electron paramagnetic resonance (EPR) spectroscopy of nitroxide biradicals for determining three-dimensional structures of biomacromolecules in disordered solids.

We consider the state-of-the-art capabilities and future perspectives of electron-spin triangulation by high-field/high-frequency dipolar electron paramagnetic resonance (EPR) techniques designed for determining the three-dimensional structure of large supra-molecular complexes dissolved in disordered solids. These techniques combine double site-directed spin labeling (SDSL) with orientation-resolving pulsed electron-electron double resonance (PELDOR) spectroscopy. In particular, we appraise the prospects of angular triangulation, which extends the more familiar distance triangulation.

Electron-nuclear and electron-electron double resonance spectroscopies show that the primary quinone acceptor QA in reaction centers from photosynthetic bacteria Rhodobacter sphaeroides remains in the same orientation upon light-induced reduction.

Reaction centers (RCs) from the photosynthetic bacterium Rhodobacter (Rb.) sphaeroides R-26 exhibit changes in the recombination kinetics of the charge-separated radical-pair state, P(·+) Q(A)(·-), composed of the dimeric bacteriochlorophyll donor P and the ubiquinone-10 acceptor Q(A), depending on whether the RCs are cooled to cryogenic temperatures in the dark or under continuous illumination (Kleinfeld et al. Biochemistry 1984, 23, 5780-5786).