Paramagnetic metal ions in pulsed ESR distance distribution measurements

The use of pulsed electron spin resonance (ESR) to measure interspin distance distributions has advanced biophysical research. The three major techniques that use pulsed ESR are relaxation rate based distance measurements, double quantum coherence (DQC), and double electron electron resonance (DEER). Among these methods, the DEER technique has become particularly popular largely because it is easy to implement on commercial instruments and because programs are available to analyze experimental data. Researchers have widely used DEER to measure the structure and conformational dynamics of molecules labeled with the methanethiosulfonate spin label (MTSSL). Recently, researchers have exploited endogenously bound paramagnetic metal ions as spin probes as a way to determine structural constraints in metalloproteins. In this context Cu(2+) has served as a useful paramagnetic metal probe at X-band for DEER based distance measurements. Sample preparation is simple, and a coordinated-Cu(2+) ion offers limited spatial flexibility, making it an attractive probe for DEER experiments. On the other hand, Cu(2+) has a broad absorption ESR spectrum at low temperature, which leads to two potential complications. First, the Cu(2+)-based DEER time domain data has lower signal to noise ratio compared with MTSSL. Second, accurate distance distribution analysis often requires high-quality experimental data at different external magnetic fields or with different frequency offsets. In this Account, we summarize characteristics of Cu(2+)-based DEER distance distribution measurements and data analysis methods. We highlight a novel application of such measurements in a protein-DNA complex to identify the metal ion binding site and to elucidate its chemical mechanism of function. We also survey the progress of research on other metal ions in high frequency DEER experiments.