Practical Aspects of Copper Ion-Based Double Electron Electron Resonance Distance Measurements

Abstract

We present experimental conditions that lead to high-quality Cu2+-based double electron electron resonance (DEER) data. Such experiments are feasible at temperature of about 20 K, and sample concentrations in the range of 0.15–1.5 mM. By systematically investigating the effects of pulse lengths, we find that observer π pulse lengths of 20–48 ns provide reasonable modulation depths as well as signals. The length of the pump pulse needs to be minimized (16 ns in our case). For a Cu2+–Cu2+ DEER measurement, the optimal frequency offset is about 100 MHz. For a Cu2+–nitroxide DEER measurement, the frequency offset is often varied in the range of 100–500 MHz, to probe orientational selectivity. For both cases, the frequency of the pump pulse should be smaller than the observer pulse in order to obtain a larger modulation depth.