Nuclear 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. When observed by nuclear magnetic resonance spectroscopy, the technique is used to 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, 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.
The theoretical basis for the NOE was described and experimentally verified by Anderson and Freeman in 1962. The NOE is an extension of the seminal work of American physicist Albert Overhauser who in 1953 proposed that nuclear spin polarization could be enhanced by the microwave irradiation of the conduction electrons in certain metals. The general Overhauser effect was first demonstrated experimentally by T. R. Carver and C. P. Slichter, also in 1953. Another early explanation and experimental observation of the NOE was by Kaiser in 1963  in an NMR experiment where the spin polarization was transferred from one population of nuclear spins to another, rather than from electron spins to nuclear spins. However, the theoretical basis and the applicable Solomon equations had already been published by Ionel Solomon in 1955. Soon after its discovery, NOE was applied to the elucidation of structures of organic compounds.
- Horst Friebolin "Basic One- and Two-Dimensional NMR Spectroscopy", 5th Edition, 2010, Wiley-VCH, Weinhiem. ISBN 978-3-527-32782-9.
- "The Nobel Prize in Chemistry 2002". Nobelprize.org. Retrieved 2011-03-24.
- Ni, Feng; Scheraga, Harold A. (1994). "Use of the Transferred Nuclear Overhauser Effect To Determine the Conformations of Ligands Bound to Proteins". Accounts of Chemical Research. 27 (9): 257–264. doi:10.1021/ar00045a001. ISSN 0001-4842.
- Anderson, W. A.; Freeman, R. (1962). "Influence of a Second Radiofrequency Field on High-Resolution Nuclear Magnetic Resonance Spectra". The Journal of Chemical Physics. 37 (1): 411–5. Bibcode:1962JChPh..37...85A. doi:10.1063/1.1732980.
- Overhauser, Albert W. (1953). "Polarization of Nuclei in Metals". Physical Review. 92 (2): 411–5. Bibcode:1953PhRv...92..411O. doi:10.1103/PhysRev.92.411.
- Carver, T. R.; Slichter, C. P. (1953). "Polarization of Nuclear Spins in Metals". Physical Review. 92 (1): 212–213. Bibcode:1953PhRv...92..212C. doi:10.1103/PhysRev.92.212.2.
- Kaiser, R. (1962). "Use of the Nuclear Overhauser Effect in the Analysis of High‐Resolution Nuclear Magnetic Resonance Spectra". The Journal of Chemical Physics. 39 (1): 2435. Bibcode:1963JChPh..39.2435K. doi:10.1063/1.1734045.
- The Solomon Equations and NOE. chem.iitm.ac.in
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- Anet, F. A. L.; Bourn, A. J. R (1965). "Nuclear Magnetic Resonance Spectral Assignments from Nuclear Overhauser Effects". Journal of the American Chemical Society. 87 (22): 5250–5251. doi:10.1021/ja00950a048.