| Definition |
Nuclear magnetic resonance (NMR) is an effect whereby magnetic nuclei in a magnetic field absorb and re-emit electromagnetic (EM) energy. Certain atomic nuclei, and in particular hydrogen, have a magnetic moment or spin; i.e., they have an intrinsic magnetisation, like a bar magnet. The spin aligns along the strong magnetic field, but can be changed to a misaligned excited state in response to applied radio frequency (RF) pulses of electromagnetic radiation. When the excited hydrogen nuclei relax to their aligned state, they emit RF radiation, which can be measured and displayed as a spectrum. The nature of the emitted radiation depends on the environment of each hydrogen nucleus, and if one nucleus is excited, it will influence the absorption and emission of radiation by other nuclei that lie close to it. It is consequently possible, by an ingenious elaboration of the basic NMR technique known as two-dimensional NMR, to distinguish the signals from hydrogen nuclei in different amino acid residues and to identify and measure the small shifts in these signals that occur when these hydrogen nuclei lie close enough to interact: the size of such a shift reveals the distance between the interacting pair of hydrogen atoms. In this way NMR can give information about the distances between the parts of the interacting molecule. NMR provides information about interacting atoms thereby permitting to obtain information about macromolecular structure and molecular interactions.[ PubMed:12120505 ] |
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