Isotopically Enriched Systems
Most solid-state NMR measurements employ rare-spin nuclei, such as 13C or 15N, for detection. However, the low natural abundance of those spins limits the possibility of obtaining multidimensional homo- or hetero-nuclear solid-state NMR-spectra, which rely on internuclear correlations between those rare spins, unless signal enhancement or isotopic labelling is applied. In this chapter, we first give an overview of different techniques for selective and uniform isotope labelling of biomolecules. In the following sections, we describe different homo- and hetero-nuclear recoupling techniques and their use in multidimensional NMR spectroscopy. In particular, we emphasize the difference between zeroth-order recoupling techniques, which are well-suited for dipolar transfers between spins close in space, and second- and higher-order recoupling schemes, which allow the detection of long-range correlations. We also provide some examples how these techniques are applied towards structure elucidation of biomolecules. Finally, we briefly outline the technique of signal enhancement by dynamic nuclear polarization, a method that may in part help to overcome the need for isotopic enrichment.