EPR characterization of Mn(II) complexes for distance determination with pulsed dipolar spectroscopy

The four Mn(II) complexes Mn-DOTA, Mn-TAHA, Mn-PyMTA, and Mn-NO3Py were characterized by electron paramagnetic resonance (EPR), electron-nuclear double resonance (ENDOR), and relaxation measurements, to predict their relative performance in the EPR pulse dipolar spectroscopy (PDS) experiments. High spin density localization on the metal ions was proven by ENDOR on H-1, D, N-14, and Mn-55 nuclei. The transverse relaxation of the Mn(II) complexes appears to be slow enough for PDS-based spin-spin distance determination. Rather advantageous ratios of T-1/T-m were measured allowing for good relaxation induced dipolar modulation enhancement (RIDME) performance and, in general, fast shot repetitions in any PDS experiment. Relaxation properties of the Mn(II) complexes correlate with the strengths of their zero field splitting (ZFS). Further, a comparison of Mn(II)-DOTA and Gd(III)-DOTA based spin labels is presented. The RIDME technique to measure nanometer-range Mn(II)-Mn(II) distances in biomolecules is discussed as an alternative to the well-known DEER technique that often appears challenging in cases of metal-metal distance measurements. The use of a modified kernel function that includes dipolar harmonic overtones allows model-free computation of the Mn(II)-Mn(II) distance distributions. Mn(II)-Mn(II) distances are computed from RIDME data of Mn-rulers consisting of two Mn-PyMTA complexes connected by a rodlike spacer of defined length. Level crossing effects seem to have only a weak influence on the distance distributions computed from this set of Mn(II)-Mn(II) RIDME data.