Nuclear Overhauser effect description

In Chapter 3 (Section 3.16), there is a description of the nuclear Overhauser effect difference experiment, an experiment that provides information about H— H through-space proximity. Review of this section is helpful before proceeding here. The ROESY experiment, rotating-frame Overhauser effect spectroscopy, is a useful 2-D analogue of the nuclear Overhauser effect difference experiment. This experiment is useful for molecules of all sizes whereas the related experiment, NOESY (nuclear Overhauser effect spectroscopy), is not very useful with small molecules. NOESY is used primarily with biological macromolecules. Both NOESY and ROESY experiments correlate protons that are close to each other in space, typically 4.5 A or less. 

The nuclear Overhauser effect is the most widely measured of the relaxation phenomena in structural studies. The nOe experiments directly measure the dipole-dipole relaxation between nuclei. The great utility of the nOe is its potential to determine internuclear distances. The magnitude of the nOe is proportional to (ris) where rjs is the distance between spins I and S. The nOe effects an increase or decrease in the intensity of a particular signal, based on the spacial proximity of its neighbors and the dynamics of the molecule. For a more rigorous description of the physical basis of the nOe the reader is referred to the excellent text by Neuhaus and Williamson. ... 

These experiments use pulse sequences similar to that of the spin-echo experiment. There are a number of experiments with different pulse sequences. The different experiments are commonly named with acronyms. For example, COSY (Correlation SpectroscopY) was the original two-dimensional NMR experiment. Some other experiments are NOESY (Nuclear Overhauser Effect SpectroscopY) and HETCOR (HETeronuclear CORrelation). The usual goal of a COSY experiment is to determine which lines belong to which multiplet in a complicated spectrum with overlapping multiplets. We will give a simplified description of the COSY experiment for proton NMR (this is sometimes called HH-COSY). 

Gerig has evaluated force field models for trifluoroethanol-water mixture in order to perform a reliable MD simulation of the system. In this study it was very important to obtain good description of solvent fluorine/solute hydrogen NMR cross-relaxation obtained from solute-solvent nuclear Overhauser effects (NOE). An additional requirement was good agreement between experimental and simulated diffusion coefficients as NMR cross relaxation depends on the diffusion of the components in the mfacture. It was found that OPLS-AA (TFE) and TIP5P-E (H2O) did do a reasonably good work. The mixture will be used as a solvent for small peptides in the future. 

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