NOESY experiment

The 2-D nuclear Overhauser effect spectroscopy (2-D-NOESY) experiment resembles the COSY however, the cross-peaks arise from... 

A large number of ID nOe experiments may have to be performed if the spatial relationships among many protons in a molecule are to be determined. In such cases, instead of employing multi pulse ID nOe experiments, we can opt for the nOe spectroscopy (NOESY) experiment. If many protons have close chemical shifts, then NOESY may be particularly advanta-... 

Figure 5.42 Pulse sequence and vector representation of a NOESY experiment.

Both homonuclear and heteronuclear versions of relayed nOe experiments are known. The homonuclear relayed NOESY experiment involves both an incoherent transfer of magnetization between two spins H and H/ that are not coupled but close in space, and a coherent transfer of magnetization between two spins H(and H that are /-coupled together. The magnetization pathway may be depicted as... 

Figure 6.10 Pulse sequences for (a) the HMQOCOSY experiments and (b) the HMQC-NOESY experiments. (Reprinted from J. Mag. Reson. 78, S. W. Fesik and E. R. P. Zuiderweg,. 588, copyright (1988), with permission from Academic Press, Inc.)...

The pulse sequences for HMQC-COSY and HMQC-NOESY experiments are presented in Fig. 6.10. The 3D HMQC-COSY spectrum of a N labeled tripeptide is shown in Fig. 6.11. Since the coherence transfer involved in this experiment is N([Pg.362]

Two-dimensional relayed NOESY experiments (Wagner, 1984 Kessler et al., 1988) give cross-peaks that could be a superposition of nOe s resulting from different relay nuclei. This complicates the extracting of crossrelaxation rates. The 1D NOESY experiment (Kessler et al., 1989a), however, allows the path of the magnetization transfer to be clarified. 

Figure 7.5 Pulses sequences for ID NOESY and ID relayed NOESY experiments, (a) A ID NOESY sequence with full Gaussian pulses is inferior to the ID NOESY sequence (b) with half-Gaussian excitation, (c) A 90° Gaussian pulse in the pulse sequence of 1D relayed NOESYis appropriate for excitation, since antiphase magnetization is required for the first mixing step. (Reprinted from Mag. Reson. Chem. 29, H. Kessler el al, 527, copyright (1991), with permission from John Wiley and Sons Limited, Baffins Lane, Chichester, Sussex P019 lUD, England.)...

Fig. 9.1 The internuclear transfer of magnetization via NOE cross-relaxation in an isolated spin-pair. (A) Build-up curves for the cross-peak intensity in a 2D NOESY experiment for various internuclear distances r. The dashed line indicates a typical mixing time tm = 300rns used for drug-like molecules.

The basic experimental setup for etNOESY is practically identical to the conventional NOESY experiment shown in Eig. 9.2(B). For suppression of residual receptor signals a relaxation filter can be introduced and the mixing time has to be corrected according to the scaling factor a. 

We shall use compound 3 to demonstrate the results obtained from a 2D NOESY experiment, and for comparison we shall use the COSY spectrum obtained from the same compound. Figure 25 shows the COSY (a) and 2D NOESY (b) spectra of compound 3. 

Homonuclear 19F-TOCSY and 19F-NOESY experiments are utilized much less frequently than 19F-COSY, but as discussed in the Battiste and Newmark review,23 they are very much underutilized experiments, which when implemented can provide unique structural and particularly stereochemical information. 

One potential problem that can occur with slightly larger molecules (typically of m.w. > 600) is that the NOE response in both NOE and 2-D (NOESY) experiments is related to the tumbling rate of molecules in solution. The larger the molecule, the slower it will tumble and at a certain point, all expected enhancements will be nullified. This null point depends not only on the tumbling rate (and therefore the size, or more accurately, the shape of the molecule) but also on the field strength of the... 

Chemical shift anisotropy evidenced the helical structure of the complexes. NOESY experiment also suggested the presence of a stable secondary structure. On the basis of the NOESY experiments, the... 

Nuclear Overhauser enhancement spectroscopy (NOESY) experiments play a very important role in structural studies in quinolizidine derivatives. For instance, the endo-type structure of compound 28 was proven by the steric proximity of the H-3a and H-12a protons according to the NOESY cross peak, while the spatial proximity of the H-6f3 and H-8/3 protons reveals that tha A/B ring junction has a /ra t-stereochemistry. Similarly, compound 28 could be distinguished from its regioisomer 29 on the basis of the NOESY behavior of its H-13 atom <1999JST153>. 

Several basic 2D schemes have been applied in these experiments, based on (1) the use of the SQ evolutions during t and f2, in analogy to the NOESY experiment, (2) the MQ-SQ experiment, in which the evolutions of MQ coherences during t are correlated with the CTs of recoupled spins to enhance the resolution in analogy to MQMAS, and (3) the DQCT-SQ protocol similar to that used in DQMAS NMR of spin-1/2 nuclei. 

NOESY NMR spectroscopy is a homonuclear two-dimensional experiment that identifies proton nuclei that are close to each other in space. If one has already identified proton resonances in one-dimensional NMR spectroscopy or by other methods, it is then possible to determine three dimensional structure through NOESY. For instance, it is possible to determine how large molecules such as proteins fold themselves in three-dimensional space using the NOESY technique. The solution structures thus determined can be compared with solid-state information on the same protein obtained from X-ray crystallographic studies. The pulse sequence for a simple NOESY experiment is shown in Figure 3.23 as adapted from Figure 8.12 of reference 19. 

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