DPFGSE-NOE

Fig. 10.7. DPFGSE NOE of H5 of antillatoxin (3) showing good selection and clean NOE enhancements.

Fig.l a Schematic representation of Neomycin-B aiong with the numbering empioyed for the different sugar units, b Seiective 1-D NOE experiments with the ID-DPFGSE NOE puise sequence, corresponding to the inversion of Him, Hp and Hliv (from top to bottom)... 

The double pulsed field gradient spin echo (DPFGSE) pulse sequence has been used to improve the measurement of proton-proton nuclear Overhauser effect (NOE) [28]. The DPFGSE NOE does not rely on difference spectroscopy and very small NOEs can be measured. This technique has been used to determine the structure of organosilicon compounds [28]. 

Figure 6-41 The double pulse field gradient spin echo (DPFGSE) NOE experiment with llp-hydroxyproges-terone (a) the unirradiated spectrum (b)-(g) spectra with irradiation at selected frequencies. (Reproduced with permission from K. Stott,...

The apparent absence of NOE cross peaks must, therefore, be interpreted with caution. In situations in which the presence, or especially the absence, of NOE s are critical to stereochemical decisions (e.g., determining whether substituents on a carbon-carbon double bond are cis or trans to each other) and when the existence of cross peaks is uncertain, selective ID NOE experiments, such as DPFGSE-NOE, should always be performed. 

Figure 8.39. ID NOESY schemes. The basic scheme (a) requires difference spectroscopy to reveal the NOEs whereas the gradient-selected DPFGSE-NOE (b) reveals NOEs without interference from difference artefacts.

The traditional nuclear Overhauser difference spectroscopy can be replaced by DPFGSE-NOE (Double Pulsed Field Gradient Spin Echo method-NOE spectrometry) experiment. The desired NOE effects can be recorded without interference from other signals and without phase distortion [66]. 

In Chapter 5 we observed NOE interactions by ID NOE difference, measuring the steady-state NOE resulting from a long (several seconds), low-power continuous-wave irradiation of one nucleus. The modern selective (DPFGSE) ID NOE experiment... 

The most robust experiment to date [34] (Fig. 8.39b) is based upon the double pulsed field gradient spin-echo (DPFGSE) sequence as a means of selecting a target resonance from which the NOEs ultimately develop. This... 

From the point of view of the relaxation behaviour the DPFGSE experiment is essentially identical to the transient NOE experiment. The only difference is that the I spin starts out saturated rather than at equilibrium. This does not influence the build up of the NOE enhancement on I. It does, however, have the advantage of reducing the size of the I spin signal which has to be removed in the difference experiment. Further discussion of this experiment is deferred to Chapter 9. 

As for the 2D NOESY data described above, spectra are vulnerable to interference from undesirable zero-quantum contributions between coupled spins this is apparent, for example, in trace (c) of Fig. 8.42 between H4 and H5. This can be especially problematic when shorter mixing times are employed, as in the generation of NOE build-up curves [35], and again the inclusion of the swept-frequency/gradient zero-quantum filter should prove beneficial. The complete selective ID NOESY sequence incorporating this and employing the optimised DPFGSE selection procedure is presented in Fig. 8.43, and an illustration of the improvements provided by the filter may be seen in Fig. 8.44 where the removal of unwelcome anti-phase dispersive contributions between coupled spins is apparent in (b). 

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