NMR spectroscopy makes it possible to discriminate nuclei, typically protons, in different chemical environments. The electron distribution gives rise to a chemical shift of the resonance frequency. The chemical shift, 
, of a nucleus is expressed in parts per million (ppm) by its frequency, 
n, relative to a standard, ref, and defined as = 106 (n - ref)/o, where o is the operating frequency of the spectrometer. It is an indication of the chemical state of the group containing the nucleus. More information is derived from the spin-spin couplings between nuclei, which give rise to multiplet patterns. Greater detail may be derived from two- or three-dimensional techniques. These use pulses of radiation at different nuclear frequencies, after which the response of the spin system is recorded as a free-induction decay (FID). Multi-dimensional techniques, such as COSY and NOESY, make it possible to deduce the structure of a relatively complex molecule such as a small protein (molecular weight up to 25 000). In proteins containing paramagnetic centres, nuclear hyperfine interactions can give rise to relatively large shifts of resonant frequencies known as contact and pseudo-contact (dipolar) shifts, and considerable increases in the nuclear spin relaxation rates. From this type of measurement, structural information can be obtained about the paramagnetic s
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