Abstract
Small-molecule NMR studies often have a strict need to work at the limits of both signal detection and optimal resolution. Obtaining complete assignments of complex small molecules is usually tractable only with the aid of two-dimensional techniques such as heteronuclear correlation spectroscopy. There is often little freedom to sacrifice resolution in favor of improving sensitivity or vice versa. Increasingly, investigators have turned to non-uniform sampling (NUS, herein defined as the practice of acquiring a subset of samples on the Nyquist grid) to sample data in indirect dimensions to very long evolution times to obtain high-resolution nD-NMR spectra. We have recently described the exact theory for the sensitivity improvement of raw time domain NUS data in multi-dimensional NMR compared with uniform sampling in the same total experiment time. The sensitivity improvement obtained by NUS is an enhancement of the signal relative to the noise in the raw time domain data themselves, prior to the use of any post-acquisition processing schemes. Specifically, NUS yields enhancements of up to about twofold versus uniform sampling when the following criteria can be met for an indirect evolution period: (i) the signal decays with an exponential time constant and (ii) the signal is sampled to times on the order of (2–3)T2. At present, it appears that sensitivity improvement by NUS is most compellingly applicable in two arenas: (i) small-molecule work in liquids and (ii) solid-state NMR of small-molecule and macromolecular biological solids, where it has also been shown that NUS enhancements in two indirect dimensions may be compounded to obtain enhancements of over threefold. We review the origin of the sensitivity enhancement in the time domain, discuss guidelines for implementing NUS for sensitivity enhancement, and conclude with a number of example applications focusing on challenging small-molecule heteronuclear 2D-NMR.
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