Nuclear Overhauser effect

The nuclear Overhauser effect (NOE) is the transfer of nuclear spin polarization from one population of spin-active nuclei (e.g. ¹H, ¹³C, ¹⁵N etc.) to another via cross-relaxation. A phenomenological definition of the NOE in nuclear magnetic resonance spectroscopy (NMR) is the change in the integrated intensity (positive or negative) of one NMR resonance that occurs when another is saturated by irradiation with an RF field. The change in resonance intensity of a nucleus is a consequence of the nucleus being close in space to those directly affected by the RF perturbation.

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. The ¹H two-dimensional NOE spectroscopy (NOESY) experiment and its extensions are important tools to identify stereochemistry of proteins and other biomolecules in solution, whereas in solid form crystal x-ray diffraction typically used to identify stereochemistry. The heteronuclear NOE is particularly important in ¹³C NMR spectroscopy to identify carbons bonded to protons, to provide polarization enhancements to such carbons to increase signal-to-noise, and to ascertain the extent the relaxation of these carbons is controlled by the dipole-dipole relaxation mechanism.

History

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. The electron-nuclear enhancement predicted by Overhauser was experimentally demonstrated in ⁷Li metal by T. R. Carver and C. P. Slichter also in 1953. 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. Another early experimental observation of the NOE was used by Kaiser in 1963 to show how the NOE may be used to determine the relative signs of scalar coupling constants, and to assign spectral lines in NMR spectra to transitions between energy levels. In this study, the resonance of one population of protons (¹H) in an organic molecule was enhanced when a second distinct population of protons in the same organic molecule was saturated by RF irradiation. 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. Bell and Saunders reported direct correlation between NOE enhancements and internuclear distances in 1970 while quantitative measurements of internuclear distances in molecules with three or more spins was reported by Schirmer et al.

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. In 2002, Kurt Wüthrich 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. Professor Anil Kumar was the first to apply the two-dimensional Nuclear Overhauser Effect (2D-NOE now known as NOESY) experiment to a biomolecule, which opened the field for the determination of three-dimensional structures of biomolecules in solution by NMR spectroscopy.

Relaxation

thumb|upright=1.3|Nuclear spin energy level diagram for two spin nuclei. In many cases, carbon atoms have an attached proton, which causes the relaxation to be dominated by dipolar relaxation and the NOE to be near maximum. For non-protonated carbon atoms the NOE enhancement is small while for carbons that relax by relaxation mechanisms by other than dipole-dipole interactions the NOE enhancement can be significantly reduced. This is one motivation for using deuteriated solvents (e.g. [[deuterated chloroform|CDCl₃) in ¹³C NMR. Since deuterium relaxes by the quadrupolar mechanism, there are no cross-relaxation pathways and NOE is non-existent. Another important case is ¹⁵N, an example where the value of its magnetogyric ratio is negative. Often ¹⁵N resonances are reduced or the NOE may actually null out the resonance when ¹H nuclei are decoupled. 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.

Structure elucidation

[[File:Noe examples.png|thumb|First NOEs reported by Anet and Bourne RNAs, however, have sugars that are much more conformationally flexible, and require wider estimations of low and high bounds.

In protein structural characterization, NOEs are used to create constraints on intramolecular distances. In this method, each proton pair is considered in isolation and NOESY cross peak intensities are compared with a reference cross peak from a proton pair of fixed distance, such as a geminal methylene proton pair or aromatic ring protons. 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. Full structure determination relies on a variety of NMR experiments and optimization methods utilizing both chemical shift and NOESY constraints.

Heteronuclear NOE

<br />

Some experimental methods

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

• NOESY, Nuclear Overhauser effect Spectroscopy
• HOESY, Heteronuclear Overhauser effect spectroscopy
• ROESY, Rotational frame nuclear Overhauser effect spectroscopy
• TRNOE, Transferred nuclear Overhauser effect
• DPFGSE-NOE, Double pulsed field gradient spin echo NOE experiment

NOESY is the determination of the relative orientations of atoms in a molecule, for example a protein or other large biological 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.

Examples of nuclear Overhauser effect

thumb|upright=1.3|Nuclear Overhauser effect

The figure (top) displays how Nuclear Overhauser Effect Spectroscopy can elucidate the structure of a switchable compound. In this example, However, NOESY is not alone, but always combined with generation of theoretical molecular ensembles, which must be deconvoluted, e.g. with the help of NAMFIS.

References

External links

• Hans J. Reich: The Nuclear Overhauser Effect
• Eugene E. Kwan: Lecture12: The Nuclear Overhauser Effect
• Williams, Martin and Rovnyak Vol 2: R. R. Gil and A. Navarro-Vázquez: Chapter 1 Application of the Nuclear Overhauser Effect to the Structural Elucidation of Natural Products
• James Keeler: 8 Relaxation
• YouTube: James Keeler, Lecture 10, Relaxation II. 2013 Cambridge lecture on NOE