NOESY, gradient-enhanced

Fig. 6. Selected spectral regions of a NOESY spectrum of BPTI recorded with the pulse sequence of fig, 5(A), except that the first spin-lock pulse was omitted and a Bo gradient was applied during the NOESY mixing time. Protein concentration 20 mM in 90% H2O / 10% D2O, pH 6.9, 36°C. The relaxation reagent GdDTPA-BMA was added at a concentration of 750 pM to enhance the relaxation of the water protons. Spin-lock pulse 2 ms, rm(NOE) = 50 ms, r = 190 ps. Positive and negative levels were plotted without distinction. The arrow identifies the cross section containing the intermolecular water-protein cross peaks. (Reproduced by permission of the American Chemical...

A 2002 review by Reynolds and Enriquez describes the most effective pulse sequences for natural product structure elucidation.86 For natural product chemists, the review recommends HSQC over HMQC, T-ROESY (transverse rotating-frame Overhauser enhancement) in place of NOESY (nuclear Over-hauser enhancement spectroscopy) and CIGAR (constant time inverse-detected gradient accordion rescaled) or constant time HMBC over HMBC. HSQC spectra provide better line shapes than HMQC spectra, but are more demanding on spectrometer hardware. The T-ROESY or transverse ROESY provides better signal to noise for most small molecules compared with a NOESY and limits scalar coupling artefacts. In small-molecule NMR at natural abundance, the 2D HMBC or variants experiment stands out as one of the key NMR experiments for structure elucidation. HMBC spectra provide correlations over multiple bonds and, while this is desirable, it poses the problem of distinguishing between two- and three-bond correlations. 

NMR has become a standard tool for structure determination and, in particular, for these of Strychnos alkaloids. The last general article in this field was authored by J. Sapi and G. Massiot in 1994 [65] and described the advances in spectroscopic methods applied to these molecules. More recently, strychnine (1) has even been used to illustrate newly introduced experiments [66]. We comment, here, on their advantages and sum up the principles of usual 2D experiments in Fig. (1) and Fig. (2) (COSY Correlation SpectroscopY, TOCSY TOtal Correlation SpectroscopY, NOESY Nuclear Overhauser Enhancement SpectroscopY, ROESY Rotating frame Overhauser Enhancement SpectroscopY, HMQC Heteronuclear Multiple Quantum Coherrence, HMBC Heteronuclear Multiple Bond Correlation). This section updates two areas of research in the field new H and 13C NMR experiments with gradient selection or/and selective pulses, 15N NMR, and microspectroscopy. To take these data into account, another section comments on the structure elucidation of new compounds isolated from Strychnos. It covers the literature from 1994 to early 2000. 

The NOE enhancement will depend on the degree of inversion, which can be influenced by pulse calibration. This can lead to the absolute percentage enhancement that is observed being smaller than in a NOE difference experiment, necessitating a recalibration of what the spectroscopist may consider to be a reliable response. Despite the potential shortcomings just noted, gradient ID NOESY experiments are extremely useful and will probably see more widespread use with time. 

---