Rapid Analysis of DEER Signals Including Short Distances

Double\nelectron electron resonance (DEER) spectroscopy is an important\ntechnique to measure distance distributions <i>P</i>(<i>r</i>) for studying protein structures and protein–protein\ninteractions. DEER data analysis can at times become challenging due\nto the lack of a detailed analytical signal expression or numerical\nmethods with rapid computation time. We have derived an analytical\nexpression κ<sub>FULL</sub>, which includes both the pseudo-secular\ndipolar coupling (PSDC) and the finite pulse effects, especially important\nfor shorter distances. Analyses of experiments by κ<sub>FULL</sub> yield accurate and consistent <i>P</i>(<i>r</i>) values for three DEER nitroxide-rulers with distances (<i>r</i><sub>AVG</sub>) in the range of 15 to 32 Å, while\nthe current standard analysis produces erroneous results for <i>r</i><sub>AVG</sub> < 20 Å. Computation times for deriving <i>P</i>(<i>r</i>) vary between 1 min and 4 min, which\nis usually much shorter than previous methods that include pseudo-secular\nand other effects. The expression can be applied to all types of DEER\nspin probes with little or no modifications.