Overhauser effect

The nuclear Overhauser effect (NOE or nOe) is the transfer of nuclear spin polarization from one spin bath to another spin bath via cross-relaxation. In nuclear magnetic resonance spectroscopy, sometimes NOEs can be used to help elucidate the structures of organic compounds.

The NOE is useful in NMR spectroscopy for assigning structures. In this application, the NOE differs from the application of spin-spin coupling in that the NOE occurs through space, not through chemical bonds. Thus, atoms that are in close proximity to each other can give a NOE, whereas spin coupling is observed only when the atoms are connected by 2–3 chemical bonds. The inter-atomic distances derived from the observed NOE can often help to confirm the three-dimensional structure of a molecule. In 2002, Kurt Wüthrich was awarded the Nobel Prize in Chemistry for demonstrating that the NOE could be exploited using two-dimensional NMR spectroscopy to determine the three-dimensional structures of biological macromolecules in solution.

Some examples of two-dimensional NMR experimental techniques exploiting the NOE include nuclear Overhauser effect spectroscopy (NOESY), heteronuclear Overhauser effect spectroscopy (HOESY), rotational frame nuclear Overhauser effect spectroscopy (ROESY), transferred nuclear Overhauser effect (TRNOE), and double pulsed field gradient spin echo NOE (DPFGSE-NOE). NOESY is the determination of the relative orientations of atoms in a molecule, producing a three-dimensional structure. HOESY is NOESY cross-correlation between atoms of different elements. ROESY involves spin-locking the magnetization to prevent it from going to zero, applied for molecules for which regular NOESY is not applicable. TRNOE measures the NOE between two different molecules interacting in the same solution, as in a ligand binding to a protein. In a DPFGSE-NOE experiment, a transient experiment that allows for suppression of strong signals and thus detection of very small NOEs.

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