Practical Aspects of Paramagnetic Relaxation Enhancement in Biological Macromolecules.

In this brief review, we summarize various aspects of NMR paramagnetic relaxation enhancement (PRE). We discuss the types of spin labels used in NMR studies, describe the relevant theory used to accurately calculate PREs from coordinates, including how to take into account the fact that paramagnetic labels tend to be highly mobile and sample a wide range of conformational space, and outline methods to refine structures or ensembles of structures directly against PRE data using simulated annealing. Finally, we show how the PRE can be used to detect, characterize, and visualize sparsely populated states of proteins and their complexes that are invisible to all other biophysical techniques.