TOCSY gradient-selected

Fig. 6. Gradient-selected HMQC-TOCSY spectrum of compound 11 in CDCI3.

The assignment of the carbohydrate chains of an intact glycoprotein (de Beer et al., 1994) with the help of a gradient-selected natural abundance HSQC-TOCSY spectrum is a recent example of the use of Hartmann-Hahn-type experiments in the assignment of oligosaccharides (Dabrowski, 1994). 

Figure 5.69. Gradient-selected TOCSY. Sequence (a) is suitable for absolute-value presentations with a 1 1 gradient combination selecting the N-type spectrum. Sequence (b) provides phase-sensitive data sets via the echo-antiecho method for which separate P- and N-type data are collected through inversion of the first gradient.

Further modifications (the addition of a spin-echo) allow the direct and relayed peaks to be differentiated through inversion of the direct correlations [36] and gradient-selected versions of these sequences without [37,38] and with [10,39] such editing have also been proposed. Although the addition of TOCSY... 

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 homonudear 2-D NMR experiments that use I-coupling indude the correlation spectroscopy (COSY, and variants induding gradient-selected COSY or gCOSY, double-quantum filtered COSY or DQF-COSY) experiment, the total correlation spectroscopy (TOCSY) experiment, and the incredible natural abimdance double quantum transfer experiment (INADEQUATE) [3]. 

SHAPED PULSES, PULSED FIELD GRADIENTS, AND SPIN LOCKS SELECTIVE ID NOE AND ID TOCSY... 

The ID TOCSY experiment is essentially the same as the 2D version, but with the parameters being those of a ID, rather than a 2D, experiment. The initial, hard 90° pulse is replaced by either a selective, soft 90° pulse or, still better, a DPFGSE sequence such as that shown in Figure 7-5 (the initial 90° pulse and the pair of 180° pulses with their attendant sets of gradients). 

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