Paramagnetic NMR study of Cu2+–IDA complex localization on a protein surface and its application to elucidate long distance information

The paramagnetic metal chelate complex Cu2+-iminodiacetic acid (Cu2+–IDA) was mixed with ubiquitin, a small globular protein. Quantitative analyses of 1H and 15N chemical shift changes and line broadenings induced by the paramagnetic effects indicated that Cu2+–IDA was localized to a histidine residue (His68) on the ubiquitin surface. The distances between the backbone amide proton and the Cu2+ relaxation center were evaluated from the proton transverse relaxation rates enhanced by the paramagnetic effect. These correlated well with the distances calculated from the crystal structure up to 20 A. Here, we show that a Cu2+–IDA is the first paramagnetic reagent that specifically localizes to a histidine residue on the protein surface and gives the long-range distance information.