Double Electron–Electron Resonance vs. Instantaneous Diffusion Effect on Spin-Echo for Nitroxide Spins Labels

Double electron–electron resonance (DEER, also known as PELDOR) is an efficient tool to study nanoscale distances between paramagnetic species forming oligomers or arranged in clusters. DEER also may be applied to study heterogeneous systems in which large clusters of spin labels may be considered as randomly distributed species of enhanced local concentration. For these systems, information of the same kind could be obtained with a simple two-pulse Hahn’s spin-echo sequence, if a so-called instantaneous diffusion (ID) effect is separated from other dephasing processes. Comparison of DEER and ID decays performed here at X-band EPR for model systems of nitroxides dissolved in molecular glasses showed good agreement between DEER and ID data, as well as with theory for randomly distributed spins. For spin-labeled stearic acids in a model biological membrane, the obtained DEER and ID data indicate on cluster formation with enhanced local concentration. For high stearic acid concentration, the ID data were found to strongly deviate from the DEER data, which were interpreted as an evidence for correlation of mutual orientations of spin labels in the clusters.