Using PELDOR to count spins on multi-nitroxides

We investigated the accuracy and limitation of using the modulation depth of pulsed electron-electron double resonance experiments to count the number of coupled spins. For this purpose, synthesized multi-nitroxide molecules with 2-6 spins were used. We could show that the main limitation on accurately counting larger number of coupled spins at Q-band frequencies is determined by the reproducibility of adjusting and calibrating the pump pulse excitation efficiency. Using broadband sech/tanh or short 10 ns rectangular pump pulses modulation depth suppression effects arising from non-ideal coverage of the dipolar-split signals can be avoided for molecules with intra-molecular spin distances larger than 2 nm. The transverse relaxation times for our model compounds with one to six spins did not depend on the spin number and were all the same. Nevertheless, the signal intensity of the primary Hahn echo signal in a 4-pulse PELDOR sequence decreased strongly with the number of coupled spins. This is due to the dipolar defocusing if more than one spin is excited by the first two pulses at the detection frequency, resulting in a loss of refocused echo intensity of the PELDOR experiments. This effect further reduces the accuracy of using the PELDOR modulation depth for spin counting. Altogether, our results demonstrate that this method is potentially applicable up to hexameric complexes with nitroxides.