Electron dipole–dipole interaction in ESEEM of nitroxide biradicals

Fourier transform of the standard primary electron spin echo envelope modulation (ESEEM) of nitroxide biradicals in organic glass shows a peak ascribed to electron–electron dipole–dipole interaction. The dipolar couplings obtained in imidazoline biradical and double spin-labeled peptide Trichogin GA IV are in good agreement with pulsed electron–electron double resonance (PELDOR) data. The intensity of this peak is increased substantially in a new three-pulse relaxation-induced dipolar modulation enhancement (RIDME) experiment. This experiment involves the non-resonant partner spin in the formation of ESEEM via a relaxation-induced change of the local dipolar field experienced by the resonant spin.