Measuring hydrogen exchange in proteins by selective water saturation in (1)H- (15)N SOFAST/BEST-type experiments: advantages and limitations.

HET(ex)-SOFAST NMR (Schanda et al. in J Biomol NMR 33:199-211, 2006) has been proposed some years ago as a fast and sensitive method for semi-quantitative measurement of site-specific amide-water hydrogen exchange effects along the backbone of proteins. Here we extend this concept to BEST readout sequences that provide a better resolution at the expense of some loss in sensitivity. We discuss the theoretical background and implementation of the experiment, and demonstrate its performance for an intrinsically disordered protein, 2 well folded globular proteins, and a transiently populated folding intermediate state. We also provide a critical evaluation of the level of accuracy that can be obtained when extracting quantitative exchange rates from HET(ex) NMR measurements.