The acquisition of multidimensional NMR spectra within a single scan

Abstract

A scheme enabling the complete sampling of multidimensional NMR domains within a single continuous acquisition is introduced and exemplified. Provided that an analyte's signal is sufficiently strong, the acquisition time of multidimensional NMR experiments can thus be shortened by orders of magnitude. This could enable the characterization of transient events such as proteins folding, 2D NMR experiments on samples being chromatographed, bring the duration of higher dimensional experiments (e.g., 4D NMR) into the lifetime of most proteins under physiological conditions, and facilitate the incorporation of spectroscopic 2D sequences into in vivo imaging investigations. The protocol is compatible with existing multidimensional pulse sequences and can be implemented by using conventional hardware; its performance is exemplified here with a variety of homonuclear 2D NMR acquisitions. F ew analytical techniques in science match in either breadth or depth the impact achieved by nuclear magnetic resonance (NMR) (1). After establishing itself as a tool for the characterization of organic molecules (2), the use of NMR has spread throughout the decades, reaching into areas as diverse as pharmaceutics, metabolic studies, structural biology, solid state chemistry, condensed matter physics, rheology, medical diagnosis (where it is known as MRI), and more recently neurobiology (1-8). In addition to assuming in all these disciplines a position of preeminence, the principles of NMR have served as paradigm to other physical methods that also rely on the interaction between radiation and matter (9, 10). Lying at the core of the expansion of NMR are powerful protocols developed over the years for characterizing the nuclear spin environment; in particular, the Fourier transform (FT) and the multidimensional method of NMR analysis (11-13). Contemporary NMR applications in all the scientific disciplines mentioned above rely routinely on Fourier and multidimensional spectroscopies for their implementation.Unidimensional Fourier spectroscopy brings the so-called multiplex advantage into NMR, enabling a shortening in the data-scanning time by orders of magnitude and bringing about a concomitant increase in the signal-to-noise ratio (S͞N) achievable per unit time. FT-NMR methods, the principles of which eventually extended to many other forms of spectroscopy (14), encode the complete spectral distribution I() being sought by acquiring a single time-domain transient S(t), from which I() is reconstructed via a discrete version of the FT calculation I() ϭ (1͞2)͐ 0 AT S(t)eϪit dt. By contrast, the goal of multidimensional spectroscopy is not just to measure but to correlate NMR frequencies over several spectral dimensions. For instance in 2D NMR, the scheme (13)provides a time-domain set S(t 1 , t 2 ), from which correlations between frequencies during the evolution and acquisition periods can be extracted asdt 1 dt 2 . Collecting the 2D S(t 1 , t 2 ) NMR data set over sufficiently large regions of the time domain is usually carried out by a ...

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