Fast assignments of 15N-HSQC spectra of proteins by paramagnetic labeling
15N-HSQC spectra of 15N-labeled proteins are widely used to identify ligand-binding sites on protein surfaces, providing a way to assess protein-protein interactions as well as to screen for the binding of small chemical compounds. The method is particularly powerful if the cross peaks in the 15N-HSQC spectrum have been sequence - specifically assigned. The present article reviews the use of paramagnetic labeling to obtain these resonance as- signments quickly with the use of 15N-labeled protein, provided the crystal structure of the protein is known and the protein can be labeled with a paramagnetic ion. The method also yields the anisotropy tensor of the magnetic susceptibility, which can be used to model the orientation of the ligand with respect to the protein.