ABSTRACT
Although three-dimensional nuclear magnetic resonance (NMR) experiments can be homonuclear, most often they are used to identify heteronuclear connectivity unambiguously, particularly in large macromolecules such as proteins or nucleic acids, and this chapter discusses these experiments. Three-dimensional NMR experiments are most often applied to large molecules that are at low concentration relative to solvent (usually water); solvent signal and artifact suppression are therefore particularly important. This chapter specifies selective excitation schemes that are necessary in some experiments. It summarizes some of the most common components of multidimensional multinuclear NMR experiments. Generally, these components can be classified as polarization transfers, evolution (frequency labeling) steps, isotropic mixing, refocusing steps or filters. In addition to all of the other problems that are encountered in biological NMR, such as large solvent signals, chemical exchange, low sample concentrations and signal overlap, the very nature of the observed NMR signals from biological macromolecules hampers the experiment.
