An algorithm to analyze PELDOR data of rigid spin label pairs

Pulsed Electron–electron Double Resonance (PELDOR) is a method frequently used to determine the distances between paramagnetic centers in biomacromolecules on the nanometer scale. A standard algorithm for determination of distances from the experimental data assumes that all possible mutual orientations of the spin labels are equally probable. However, in many applications the mobility of the spin labels attached to large molecules can be significantly restricted. In order to determine the total PELDOR signal in this case, the individual contributions of each rigid biradical should be explicitly calculated for the given frequencies of the probe and pump pulses. The solution of the inverse problem of determination of the ensemble of molecular structures that fit the experimental PELDOR data acquired at multiple microwave frequencies and magnetic fields has proven to be a non-trivial task, especially, when no information about the molecular structure under study is available. In this work we present a fitting algorithm that reconstructs experimental data by searching for an optimal combination of presimulated PELDOR time traces for nitroxide biradicals with all relative orientations and with inter-spin distances in the experimentally accessible range. The generated library of PELDOR time traces has been employed to excellently fit experimental data containing orientation selection effects gathered on model biradical systems.