Nuclear Overhauser effect

The Nuclear Overhauser Effect (NOE) is the transfer of nuclear spin polarization from one nuclear spin population to another via cross-relaxation. It is a common phenomenon observed by nuclear magnetic resonance (NMR) spectroscopy. The theoretical basis for the NOE was described and experimentally verified by Anderson and Freeman in 1962.^[1] 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.^[2] The general Overhauser effect was first demonstrated experimentally by T. R. Carver and C. P. Slichter, also in 1953.^[3] Another early explanation and experimental observation of the NOE was by Kaiser in 1963 ^[4] 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^[5] had already been published by Ionel Solomon in 1955.^[6]

Subsequent to its discovery, the NOE was shown to be highly useful in NMR spectroscopy for characterizing and refining organic chemical structures.^[7] 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 a precise molecular conformation, i.e. 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.^[8]

Some examples of two-dimensional NMR experimental techniques exploiting the NOE include:

• NOESY, Nuclear Overhauser Effect Spectroscopy, determination of the relative orientations of atoms in a molecule, producing a three-dimensional structure
• HOESY, Heteronuclear Overhauser Effect Spectroscopy, or NOESY cross-correlation between atoms of different elements
• ROESY, Rotational Frame Nuclear Overhauser Effect Spectroscopy, spin-locking the magnetization to prevent it from going to zero, applied for molecules for which regular NOESY is not applicable
• TRNOE, Transferred Nuclear Overhauser Effect, measuring the NOE between two different molecules interacting in the same solution, as in a ligand binding to a protein^[9]
• DPFGSE-NOE, Double Pulsed Field Gradient Spin Echo NOE experiment, a transient experiment that allows for suppression of strong signals and thus detection of very small NOEs.

References