In a magnetic field, certain magnetic nuclei, such as 15N, 13C or 1H, can exist in nuclear-spin states of different energy and can be induced to flip between these different energy levels by the application of radio-frequency radiation. The frequency of radiation required to effect this energy-level transition is determined by the chemical environment of the magnetic nucleus in a given applied magnetic field. The magnetic environment of a nucleus is affected through chemical bonds (spin-spin coupling) and through space (dipoledipole coupling), and so the resonance signal for a particular nucleus is directly related to its position within a molecule. When a molecule interacts with another, for example upon ligand binding to a protein, additional intermolecular through-space interactions may become important, affecting the resonance signal for the nuclei involved in the interaction.
In theory, any measurable NMR spectroscopic parameter may be used to investigate binding of a ligand to a protein, but in practice they are limited to measurements that can be determined easily and with a sufficient degree of sensitivity. There are two main approaches used in drug discovery today. The first approach examines chemical shift changes of the protein upon binding of a ligand and the second examines the NMR signals of the ligand and their change upon binding to a protein. In addition, there are also two complementary approaches that examine changes in relaxation or diffusion behaviour of a ligand upon binding to a protein [34, 35].
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