Fast Multidimensional NMR Spectroscopy by the Projection – Reconstruction Technique

16 Spectroscopy 19(10) October 2004 Great strides were made in high-resolution nuclear magnetic resonance (NMR) spectroscopy with the introduction of two-dimensional (2-D) Fourier transformation (1, 2), which spreads the important information into two frequency dimensions. For the first time, spectroscopists were presented with a simple graphical picture of the correlations among different nuclear sites within a molecule — evidence for spin–spin coupling, cross-relaxation effects, chemical exchange, and many other internuclear interactions. This pictorial approach proved popular particularly with organic chemists and biochemists whose prime concern is molecular structure determination, because the 2-D charts provide a clear and essentially unambiguous map of the interesting interactions. With the advent of higher and higher magnetic fields the quest was extended to spectra of greater complexity, notably those of biomolecules, and, in particular, proteins. However, it soon became clear that very crowded spectra required extension to three or even four frequency dimensions to resolve ambiguities. These additional frequency dimensions inevitably increase the extent of data gathering and hence the duration of a threedimensional (3-D) experiment, often translating to several hours of spectrometer time. This is because they monitor all the elements of a 3-D array one step at a time, rather like examining every book on every shelf of every stack in a library. Higher-dimensional spectroscopy demands even more extensive sampling, involving measurements that can take many days to complete. Soon NMR spectroscopists began to ask whether there might be a quicker way to gather the data without compromising resolution or degrading the quality of the spectra in some other manner. Suppose we think of a 3-D NMR spectrum as being made up of tiny bright “stars” sparsely distributed in otherwise empty space. This certainly is how the NMR results look when displayed on a monitor. If we view this array from two or three different perspectives we should be able to pinpoint all these “stars” and get a clear map of all their positions, relying upon the human eye and brain to complete the reconstruction. Indeed, we sometimes display a 3-D NMR spectrum on the pages of a scientific journal as two stereoscopic views to create the 3-D effect. This “image reconstruction” concept already has proved its worth in other scientific fields where the object is continuous and therefore far more demanding — for example, a patient in an x-ray scanner. Even the very complex spectra of proteins should be susceptible to this approach, but when we take into account the possibility that some resonances might be eclipsed by others in both perspective views, we need to acquire some Fast Multidimensional NMR Spectroscopy by the Projection– Reconstruction Technique