title: Visualizing transient dark states by NMR spectroscopy.
abstract: Myriad biological processes proceed through states that defy characterization by conventional atomic-resolution structural biological methods. The invisibility of these 'dark' states can arise from their transient nature, low equilibrium population, large molecular weight, and/or heterogeneity. Although they are invisible, these dark states underlie a range of processes, acting as encounter complexes between proteins and as intermediates in protein folding and aggregation. New methods have made these states accessible to high-resolution analysis by nuclear magnetic resonance (NMR) spectroscopy, as long as the dark state is in dynamic equilibrium with an NMR-visible species. These methods - paramagnetic NMR, relaxation dispersion, saturation transfer, lifetime line broadening, and hydrogen exchange - allow the exploration of otherwise invisible states in exchange with a visible species over a range of timescales, each taking advantage of some unique property of the dark state to amplify its effect on a particular NMR observable. In this review, we introduce these methods and explore two specific techniques - paramagnetic relaxation enhancement and dark state exchange saturation transfer - in greater detail.
authors.affiliation: Laboratory of Chemical Physics,National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health,Bethesda,Maryland 20892-0520,USA.
authors.forename: Nicholas J
authors.initials: NJ
authors.lastname: Anthis
authors.affiliation: Laboratory of Chemical Physics,National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health,Bethesda,Maryland 20892-0520,USA.
authors.forename: G Marius
authors.initials: GM
authors.lastname: Clore
mesh_terms: D060328:Magnetic Phenomena; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D013997:Time Factors