Pulsed electron–electron double resonance spectroscopy between a high-spin Mn2+ ion and a nitroxide spin label

Pulsed Electron–Electron Double Resonance (PELDOR) has attracted considerable attention for biomolecular applications, as it affords precise measurements of distances between pairs of spin labels in the range of 1.5–8 nm. Usually nitroxide moieties incorporated by site-directed spin labelling with cysteine residues are used as spin probes in protein systems. Recently, naturally occurring cofactors and metal ions have also been explored as paramagnetic spin species for such measurements. In this work we investigate the performance of PELDOR between a nitroxide spin label and a high-spin Mn2+ ion in a synthetic model compound at Q-band (34 GHz) and G-band (180 GHz). We demonstrate that the distances obtained with high-frequency PELDOR are in good agreement with structural predictions. At Q-band frequencies experiments have been performed by probing either the high-spin Mn2+ ion or the nitroxide spin label. At G-band frequencies we have been able to detect changes in the dipolar oscillation frequency, depending on the pump–probe positions across the g-tensor resolved nitroxide EPR spectrum. These changes result from the restricted mobility of the nitroxide spin label in the model compound. Our results demonstrate that the high-spin Mn2+ ion can be used for precise distance measurements and open the doors for many biological applications, as naturally occurring Mg2+ sites can be readily exchanged for Mn2+.