Nuclear Overhauser effect

The NOE is particularly important in the assignment of NMR resonances, and the elucidation and confirmation of the structures or configurations of organic and biological molecules.

[4] The NOE developed from the theoretical 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.

[6] A general theoretical basis and experimental observation of an Overhauser effect involving only nuclear spins in the HF molecule was published by Ionel Solomon in 1955.

[8] The application of the NOE was used by Anet and Bourn in 1965 to confirm the assignments of the NMR resonances for β,β-dimethylacrylic acid and dimethyl formamide, thereby showing that conformation and configuration information about organic molecules in solution can be obtained.

[9] Bell and Saunders reported direct correlation between NOE enhancements and internuclear distances in 1970[10] while quantitative measurements of internuclear distances in molecules with three or more spins was reported by Schirmer et al.[11] Richard R. Ernst was awarded the 1991 Nobel Prize in Chemistry for developing Fourier transform and two-dimensional NMR spectroscopy, which was soon adapted to the measurement of the NOE, particularly in large biological molecules.

[12] In 2002, Kurt Wuthrich won the Nobel Prize in Chemistry for the development of nuclear magnetic resonance spectroscopy for determining the three-dimensional structure of biological macromolecules in solution, demonstrating how the 2D NOE method (NOESY) can be used to constrain the three-dimensional structures of large biological macromolecules.

For a single spin-1⁄2 nucleus in a magnetic field there are two energy levels that are often labeled α and β, which correspond to the two possible spin quantum states, +1⁄2 and -1⁄2, respectively.

While rf irradiation can only induce single-quantum transitions (due to so-called quantum mechanical selection rules) giving rise to observable spectral lines, dipolar relaxation may take place through any of the pathways.

Saturation of the degenerate W1S transitions disturbs the equilibrium populations so that Pαα = Pαβ and Pβα = Pββ.

This expression shows that for the homonuclear case where I = S, most notably for 1H NMR, the maximum NOE that can be observed is 1\2 irrespective of the proximity of the nuclei.

In the heteronuclear case where I ≠ S, the maximum NOE is given by 1\2 (γS/γI), which, when observing heteronuclei under conditions of broadband proton decoupling, can produce major sensitivity improvements.

It is usually advantageous to take such spectra with pulse techniques that involve polarization transfer from protons to the 15N to minimize the negative NOE.

While the relationship of the steady-state NOE to internuclear distance is complex, depending on relaxation rates and molecular motion, in many instances for small rapidly tumbling molecules in the extreme-narrowing limit, the semiquantitative nature of positive NOE's is useful for many structural applications often in combination with the measurement of J-coupling constants.

For example, NOE enhancements can be used to confirm NMR resonance assignments, distinguish between structural isomers, identify aromatic ring substitution patterns and aliphatic substituent configurations, and determine conformational preferences.

However, the relation ηIS(max)=1⁄2 obscures how the NOE is related to internuclear distances because it applies only for the idealized case where the relaxation is 100% dominated by dipole-dipole interactions between two nuclei I and S. In practice, the value of ρI contains contributions from other competing mechanisms, which serve only to reduce the influence of W0 and W2 by increasing W1.

Sometimes, for example, relaxation due to electron-nuclear interactions with dissolved oxygen or paramagnetic metal ion impurities in the solvent can prohibit the observation of weak NOE enhancements.

Bell and Saunders showed that following strict assumptions ρ⋇/τc is nearly constant for similar molecules in the extreme narrowing limit.

On the other hand, the initial rate at which the NOE grows is proportional to rIS−6, which provides other more sophisticated alternatives for obtaining structural information via transient experiments such as 2D-NOESY.

Inter-proton distances can be determined from unambiguously assigned, well-resolved, high signal-to-noise NOESY spectra by analysis of cross peak intensities.

This shows that the lower range of the NOESY volume can be shown and that the upper bound is Such fixed distances depend on the system studied.

[16] RNAs, however, have sugars that are much more conformationally flexible, and require wider estimations of low and high bounds.

This simple approach is reasonably insensitive to the effects of spin diffusion or non-uniform correlation times and can usually lead to definition of the global fold of the protein, provided a sufficiently large number of NOEs have been identified.

NOESY cross peaks can be classified as strong, medium or weak and can be translated into upper distance restraints of around 2.5, 3.5 and 5.0 Å, respectively.

Such constraints can then be used in molecular mechanics optimizations to provide a picture of the solution state conformation of the protein.

[18] Full structure determination relies on a variety of NMR experiments and optimization methods utilizing both chemical shift and NOESY constraints.

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.

The figure (top) displays how Nuclear Overhauser Effect Spectroscopy can elucidate the structure of a switchable compound.

In this example,[20] the proton designated as {H} shows two different sets of NOEs depending on the isomerization state (cis or trans) of the switchable azo groups.

In the trans state proton {H} is far from the phenyl group showing blue coloured NOEs; while the cis state holds proton {H} in the vicinity of the phenyl group resulting in the emergence of new NOEs (show in red).