NOE spectra

Figure 18.17 Two-dimensional NMR spectnim of the C-terminal domain of a cellulase. The peaks along the diagonal correspond to the spectrum shown in Figure 18.16b. The off-diagonal peaks in this NOE spectrum represent interactions between hydrogen atoms that are closer than 5 A to each other in space. From such a spectrum one can obtain information on both the secondary and tertiary structures of the protein. (Courtesy of Per Kraulis, Uppsala.)...

These signals in the NOE spectra therefore in principle make it possible to determine which fingerprint in the COSY spectrum comes from a residue adjacent to the one previously identified. For example, in the case of the lac-repressor fragment the specific Ser residue that was identified from the COSY spectrum was shown in the NOE spectrum to interact with a His residue, which in turn interacted with a Val residue. Comparison with the known amino acid sequence revealed that the tripeptide Ser-His-Val occurred only once, for residues 28-30. 

Fig. 12a—c NOE experiments carried out at 200 MHz on compound 3. a Normal spectrum, with expansion of methine doublet b selective NOE spectrum, total time required 18 min c NOE difference spectrum, total time required (preparation, measurement) 42 min... 

The normal spectrum is shown below, the selective NOE spectrum, again with irradiation of the methine doublet, above. 

Fig. 13a-c Selective ID NOE spectrum of 1 a Normal spectrum b spectrum recorded with irradiation of the methine doublet (600 MHz, measurement time 4 min) c ID TOCSY spectrum of compound 1. The methyl signal was irradiated... 

FIGURE 10. Contour plot of two-dimensional nuclear Overhauser effect ll NMR (NOESY) of the protonated Schiff base of all-traos-retinal, in chloroform, with formate as the counterion. The intermolecular NOE cross-peak observed between H15 of the retinal and the counterion proton, at a mixing time of 0.4 s, is shown. Top trace f2 projection of the 2D NOE spectrum. Reproduced by permission of John Wiley Sons from Reference 36... 

Figure 2. The proton NMR spectra of a 1 mM mixture of apocarotenal and TPP in a toluene-dg/ethanol-d 1 1 solvent, (A) before the NOE experiment, (B) NOE spectrum, and (C) difference of (B) and (C) ...

In the DMSO-dg solution only one CH,-signal was found in the spectrum of atenolol, ana the solvent caused a slight downfield effect upon.the chemical shift. On the other hand, all other C chemical shifts were moved substantially upfield, the changes ranging from o.6o for C-4 to 4.27 ppm for the C=0 group (Table 4.). The NOE spectrum shows rather smeared multiplets which were not suitable for precise determination of coupling constants, except in very few cases. 

The assignment of the aromatic-olefinic part of the spectrum was not possible using only known substitution rules (8) due to simultaneous interaction of few different moieties. However, the NOE spectrum enabled a sound interpretation based on the long-range carbon-hydrogen couplings. The com-... 

There are still two remaining tasks assignment of stereochemistry of the C-2, C-3-double bond and assignment of the C-8 and C-9 methyl groups. NOE difference spectrometry is described in Section 3.16. It is a 1-D experiment that reveals H— H proximity through space because of enhancement by the nuclear Over-hauser effect. The difference spectrum is obtained by subtracting a standard H spectrum from the NOE spectrum this leaves only the enhanced peak(s). 

The traditional ID NOE experiment (Section 5.12) involves irradiating with low-power radio frequency at the resonant frequency of one peak in the H spectrum in order to equalize the populations of the two states ( saturation ). This saturated state is maintained by continued irradiation until the perturbation of populations of nearby nuclei in the molecule reaches a steady state and does not change any further. Then a 90° pulse is applied and an FID is recorded to measure the amount of perturbation on the nearby nuclei. As the enhancement of signals is quite small (a few percent), it is necessary to record a control spectrum with irradiation away from any peaks in the spectrum, and then subtract the control spectrum from the NOE spectrum. There are a number of disadvantages to this approach ... 

Comparing Figure 8.45 (TOCSY) to Figure 8.36 (NOE), we can see that TOCSY transfer is very efficient. Compared to the selected peak, the transfer peaks are of comparable intensity. NOE transfer is very inefficient and, for small molecules, of opposite sign. In the NOE spectrum, the selected peak is negative and enormous compared to the very small (around 1%) positive transfer peaks. This is an important factor in the design of NMR... 

The differential NOEs have been determined171 for [10-3H]-Org 3770 (mepirzepine) 169 with specific activities of about 1 Ci mmol-1 and for a mixture of mono/ditritiated [pyrrolidine-3H] (bepridil—170). In the case of compounds 169 and 170 the differential NOEs were found to be unimportant. The ratio of NMR signals of 169 obtained at 3.38 ppm [3H (eq)] and 4.50 ppm [3H (ax)] without decoupling was 6.87, and was changed to 6.74 only in the case of an -coupled spectrum. For 170 the ratios [mono-3-3H]/[cw-3,4-3H2] determined under NOE and NOE-suppressed conditions were 1.19 and 1.21, respectively. However, in the 3H NMR spectra of other tritiated materials substantial differential NOEs have been measured. Examples are [16-3H]-desogestrol 171 and [l,2-3H]-rimexolone 172. In the case of 172, in the NOE spectrum the 3H(1)/3H(2) ratio was 1.83, while in the NOE-suppressed spectrum a ratio of 1.56 was found171. 

Figure 12.6 shows a difference-NOE spectrum of the pictured coumarin derivative. Irradiation of the methyl signal at 5 2.23 results in NOE enhancements for only 2 of the 12 hydrogens in the molecule, the circled aromatic hydrogen at 8 6.85 (q = 4.9%) and the circled vinyl hydrogen at 8 5.85 (q = 4.8%). None of the 10 more remote methylene hydrogens (which are not explicitly shown in the structure) exhibit any enhancement. 

Figure 5-12 illustrates the difference NOE spectrum for a portion of the H spectrum of progesterone (5-30), in which the 19 methyl group has been irradiated (arrow). The unirradiated spectrum is given at the bottom, the difference spectrum at the top. Enhancements... 

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