The theory, practice, and applications of paramagnetic relaxation enhancement for the characterization of transient low-population states of biological macromolecules and their complexes were studied. Quantitative refinement against PRE relaxation rates not only affords a powerful tool in three-dimensional solution structure determination that also permits structural information on populated states. The unambiguous identification of a PRE effect is entirely straightforward, since the assignment of cross-peaks in 2D or 3D correlation spectra is already known from through-bond scalar triple resonance experiments. The introduction of extrinsic PRE labels can potentially alter the interaction energy, structure, or kinetics of the interacting molecules, where the absence of significant structural perturbations involve high-populated state of the labeled molecule and the complex can be easily assessed by NMR spectroscopy using chemical shifts as an indicator.
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