Sensitive Cu2+-Cu2+ distance measurements in a protein-DNA complex by double-quantum coherence ESR.

Double quantum coherence (DQC) ESR spectroscopy is applied to measure the Cu(2+)-Cu(2+) distance in the EcoRI-DNA complex. A simple method is proposed to reduce the contribution of nuclear hyperfine and quadrupole interactions to such data. The effects of such interactions between the electron spin of Cu(2+) and neighboring nuclei on the DQC data make it difficult to measure the nanometer range interspin distance. The DQC data is in good agreement with results obtained by double electron electron resonance (DEER) spectroscopy. At the same time, the signal-to-noise ratio per shot in DQC is high. Taken together, these results provide impetus for further development of paramagnetic metal ion-based DQC techniques.