NOE effects

The addition of paramagnetic species, such as the metal ions Cu ", Mn, or CF", can have dramatic effects on both the observed spectmm and the relaxation behavior of a molecule. The added ion reduces nuclear relaxation times, and permitting more rapid data collection. In addition, faster relaxation rates minimize NOE effects in the spectra, which can be useful in obtaining quantitative intensity data. The most widely used reagent for this purpose is chromium acetylacetonate [13681 -82-8] known as Cr(acac)2. Practically speaking, the use of such reagents requires care, because at... 

Erom a given structure, the NOE effect can be calculated more realistically by complete relaxation matrix analysis. Instead of considering only the distance between two protons, the complete network of interactions is considered (Eig. 8). Approximately, the... 

More recent studies on the folded toxin structure by Norton and colleagues have utilized h- and C-NMR techniques (19,20). By using 2D-FT-NMR, it was possible to localize a four stranded, antiparallel )5-pleated sheet "backbone structure in As II, Ax I, and Sh I (21,22), In addition, Wemmer et al. (23) have observed an identical )5-pleated structure in Hp II. No a-helix was observed in these four variants. In the near future, calculated solution conformations of these toxins, utilizing distance measurements from extracted Nuclear Overhauser Enhancement (NOE) effects should greatly stimulate structure-activity investigations. 

What is transient nOe, and why is it considered to provide a better estimate of the internuclear distance (r) than the normal nOe effect ... 

It is possible to distinguish between direct and indirect nOes from their kinetic behavior. The direct nOes grow immediately upon irradiation of the neighboring nucleus, with a first-order rate constant, and their kinetics depend initially only on the intemuclear distance r" indirect nOes are observable only after a certain time lag. We can thus suppress or enhance the indirect nOe s (e.g., at He) by short or long irradiations, respectively, of Ha- a long irradiation time of Ha allows the buildup of indirect negative nOe at He, while a short irradiation time of Ha allows only the direct positive nOe effects of Ha on He to be recorded. 

Explain what is meant by three-spin effects, or indirect nOe effects. When do such indirect effects matter ... 

If Ha, Hb, and do not lie on the same line (Fig. 4.6), then as Ha comes closer to He, the direct -fve nOe between Ha and He will increase, and a point may come when it totally cancels the larger, indirect negative nOe effect exerted by Ha on He through Hb. Thus no nOe may be observed at He upon irradiation of Ha, even though Ha and He are spatially close The absence of nOe between nuclei therefore does not necessarily mean they are far from one another. 

So far we have been concerned with homonuclear nOe effects. nOe between nuclei of different elements can also be a useful tool for structural investigations. Such heteronuclear nOe effects—for instance, between protons and carbons—can be used with advantage to locate quaternary carbon atoms. Normally, beteronuclear nOe effects are dominated by interactions between protons and directly bonded carbon atoms, and they can be recorded as either ID or 2D nOe spectra. 

As stated earlier, since tt]/ = yff2yr and since the gyromagnetic ratio of proton is about fourfold greater than that of carbon, then if C is observed and H is irradiated (expressed as C H ), at the extreme narrowing limit Ti, = 198.8% i.e., the C signal appears with a threefold enhancement of intensity due to the nOe effect. This is a very useful feature. For instance, in noise-decoupled C spectra in which C-H couplings are removed, the C signals appear with enhanced intensities due to nOe effects. 

The nOe difference spectrum has the advantage that it allows measurements of small nOe effects, even 1% or below. The experiment involves switching on the decoupler to allow the buildup of nOe. It is then switched off, and a w/2 pulse is applied before acquisition. The nOe is not affected much by the decoupler s being off during acquisition, since the nOes do not disappear instantaneously (the system takes several Ti seconds to return to its equilibrium state). 

Transient nOe represents the rate of nOe buildup. The nOe effect (so-called equilibrium value) itself depends only on the competing balance between various complex relaxation pathways. But the initial rate at which the nOe grows (so-called transient nOe) depends only on the rate of cross-relaxation t, between the relevant dipolarly coupled nuclei, which in turn depends on their internuclear distance (r). 

The magnitude of the indirect nOe effect depends on the geometry of the three-spin system. It is maximum with a linear geometry of the spin system (0 = 180°). When 0 decreases, the distance also decreases, and the direct enhancement at C becomes more and more significant. At 0 = 78°, the direct and indirect nOe effects become equal. With smaller values of 0, the direct contribution rapidly starts to dominate the nOe at C, and a strong positive enhancement results. This means that indirect nOe effects are to be expected mainly when the spins are close to having a linear geometry. 

In a system, when 0 = 180°, the distance between spins A and C (tac) will be a maximum and the nOe between A and C a mimimum. When 9 decreases from 180°, r c decreases and the direct nOe between A and C increases. As a result of the consequently more effective A-C relaxation, the B-C relaxation process becomes relatively less important, and the indirect negative A-C contribution is correspondingly decreased. When 0 = 78°, r c = 1.26, so there will be no net nOe between A and C, even though the A and C spins are very close to each other. This is because the direct (positive) and indirect (negative) nOe effects are equal and opposite. 

In the nOe difference spectrum, only the nOe effects of interest remain, while the unaffected signals are removed by subtraction. It does not therefore matter if the nOe responses are small or buried under the unaffected signals, since they show up in the difference spectrum. The main benefit of nOe difference spectroscopy is that it converts the changes in intensity into a form that is more readily recognizable. The difference spectrum is obtained by a process in which a control (normal) spectrum is subtracted from a spectrum acquired with irradiation of a particular signal. 

Runanine (17) was isolated from the roots of Stephania sinica, a species found in the Chinese provinces of Heibei, Gueizhou, and Yunnan (35). The H-NMR spectrum of runanine (17) (Table II) revealed the presence of two aromatic protons, C-5 methylene protons, one N-methyl, and four methoxyl groups. An NOE effect (10% enhancement) was observed between the protons of two methoxyl groups (53.79 and 3.80) and the aromatic protons (56.47 and 6.64), but the same phenomenon was not observed for the other methoxyl protons (53.61 and 4.05). Therefore, the former methoxyls should be situated on ring A. From the further observation of an NOE (22.6% enhancement) between the aromatic C-4 proton (56.64) and one (53.00) of the C-5 methylene protons, it was assumed that the two methoxyl groups (53.79 and 3.80) should be located at C-2 and C-3, respectively. The absence of signals for olefinic... 

The previous experiment (COSY) demonstrated the interactions (J coupling) between protons via the bonding electrons. The NOE effect which we described in Section 1.1.6 functions because of the through-space interactions between protons, and we used the NOE difference and selective NOE experiments to demonstrate it. 

NOE effects can naturally also be investigated by 2D experiments these are known as NOESY and ROESY. 

Now let us look at the NOESY spectrum (b) just as in COSY, we can identify a diagonal and a series of associated off-diagonal cross peaks. Thus the interpretation of the results is analogous to the method we have already learned for COSY. However, the cross peaks are not due to spin-spin coupling but to NOE effects between the protons concerned. However, if we look more closely we can see one big difference between the diagonal peaks, which look like irregular circles, and the cross peaks, which look just like all the peaks in the COSY spectrum. 

A minor isonitrile named acanthellin-2 with a specific rotation of — 24.1° was reported with no further details except some IR and MS data [3]. Isothiocyanate 15 and formamide 16 were also isolated from Axinella cannabina in a later study, in which isonitrile 14 and isomers 17-19 were also secured [35]. That 17-19 are C4(14)-ene isomers of 14 was concluded after extensive double resonance H NMR experiments. Coupling constants and nOe effects between the affected protons and various substituents of 17 established the relative stereochemistry of this series. 

Both 13C- and H NMR experiments using 2D NMR techniques established that 20 had a cis-eudesmane ring juncture. This was based on a 12% nOe effect between the C-10 methyl and H-5 proton. Based on this and other evidence, the relative stereochemistry as shown in 20 [6a-isocyano-5a-H, 7a-H, 10a-eudesm-4(14)ene] and its analogs (21-22) was proposed [40]. 

The coupling constants listed in Table 17 were assigned on the basis of the ID H NMR spectrum of 63, but for a few cases the analysis of phase structure of the cross-peaks in the DQF-COSY spectrum was carried out to attribute correct values to the appropriate protons. Also, the analysis of the NOE effects yields the same results (Figure 13). 

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