Protein NMR spectroscopy

Nuclear magnetic resonance (NMR) spectroscopy uses the magnetic spin properties of atomic nuclei within a molecule to identify atoms that are close together in space (either because they are bonded together or because folds of a protein chain bring them together). This information is used to derive a three-dimensional model of a protein in solution (a ‘solution structure’). Only certain isotopes, such as 1H, 13C, 15N or 31P, have a magnetic spin; isotopes such as 12C or 16O are NMR inactive. Protein samples are now routinely enriched with 13C and 15N isotopes to simplify the NMR data by making it possible to identify particular isotopes and chemical groups. During NMR spectroscopy, protein molecules in solution are placed in a magnetic field, so that the magnetic moments of individual nuclei can align with the field. When the sample is irradiated with pulses of radio frequency electromagnetic radiation, NMR-active nuclei will resonate at characteristic frequencies. These different frequencies are obtained as NMR peaks — relative to a reference signal — and are called chemical shifts. The chemical shift of an atomic nucleus depends on its molecular environment and is different for each atom in a protein molecule, unless two atoms are magnetically equivalent. For example, protons within individual side chain CH3 groups of valine are magnetically equivalent but the protons of adjacent side chain CH3 groups of valine are not, because of their stereochemical positioning.