Molecular distances from dipolar coupled spin-labels: the global analysis of multifrequency continuous wave electron paramagnetic resonance data.

For immobilized nitroxide spin-labels with a well-defined interprobe geometry, resolved dipolar splittings can be observed in continuous wave electron paramagnetic resonance (CW-EPR) spectra for interelectron distances as large as 30 A using perdeuterated probes. In this work, algorithms are developed for calculating CW-EPR spectra of immobilized, dipolar coupled nitroxides, and then used to define the limits of sensitivity to the interelectron distance as a function of geometry and microwave frequency. Secondly, the CW-EPR spectra of N epsilon-spin-labeled coenzyme NAD+ bound to microcrystalline, tetrameric glyceraldehyde-3-phosphate dehydrogenase (GAPDH) have been collected at 9.8, 34, and 94 GHz. These data have been analyzed, using a combination of simulated annealing and global analysis, to obtain a unique fit to the data. The values of the intermitroxide distance and the five angles defining the relative orientation of the two nitroxides are in reasonable agreement with a molecular model built from the known crystal structure. Finally, the effect of rigid body isotropic rotational diffusion on the CW-EPR spectra of dipolar coupled nitroxides has been investigated using an algorithm based on Brownian dynamics trajectories. These calculations demonstrate the sensitivity of CW-EPR spectra to dipolar coupling in the presence of rigid body rotational diffusion.