NOESY Subject

Figure 3.1 The various time periods in a two-dimensional NMR experiment. Nuclei are allowed to approach a state of thermal equilibrium during the preparation period before the first pulse is applied. This pulse disturbs the equilibrium ptolariza-tion state established during the preparation period, and during the subsequent evolution period the nuclei may be subjected to the influence of other, neighboring spins. If the amplitudes of the nuclei are modulated by the chemical shifts of the nuclei to which they are coupled, 2D-shift-correlated spectra are obtained. On the other hand, if their amplitudes are modulated by the coupling frequencies, then 2D /-resolved spectra result. The evolution period may be followed by a mixing period A, as in Nuclear Overhauser Enhancement Spectroscopy (NOESY) or 2D exchange spectra. The mixing period is followed by the second evolution (detection) period) ij.

These conformations were subjected to a combined MD/EM refinement using the NOESY derived constraints as pseudoenergy functions. Families of conformations for the whole molecule were then constructed from the structures derived for each linkage. This procedure was demonstrated on a fragment from the carbohydrate-protein linkage region of connective tissue proteoglycans. 

Aryl derivatives of thieno[3,2-f]pyridines, 36 and 37, have been the subject of two-dimensional (2-D) NMR studies. Phase-sensitive nuclear Overhauser enhancement spectroscopy (NOESY) and correlation spectroscopy (COSY) experiments confirm the nonplanar conformation of the two aromatic ring systems <1999SAA1035>. 

This volume addresses in paiticular metal specific NMR methods and applications. All well known classical techniques (e.g. COSY, NOESY, etc.) can easily be integrated with considerable profit for studying organometallic compounds. These techniques are not discussed here in any detail since they are the subject of numerous textbooks in organic andbiochemistiy. 

Noteworthy results in the phytochemical studies, the N-oxides of several Amaryllidaceae alkaloids have been found for the first time in Amaryllidaceae plants, accompanied by the corresponding free bases (52) ungiminor-ine A-oxide (10) (from Pancratium maritimum), and O-methyllycorenine A-oxide (329) and homolycorine A-oxide (330) (from Lapiedra maritinezii). The stereochemistry of their nitrogen atoms was deduced based on 2D NOESY experiments. Furthermore, it has been established that these A-oxide derivatives are genuine natural products, and not artifacts, by the demonstration that the free bases are not converted into A-oxides when subjected to the same extraction procedure. It is noteworthy that the A-oxides have been isolated accompanied by the corresponding free bases. 

Fig. 8.33 Pulse sequence schematic for the gradient ID NOESY experiment. The double pulsed field gradient spin echo (DPFGSE) refocuses only for that resonance subject to the selective pulse. All other magnetization is left defocused in the xy-plane. Ultimately, a NOESY spectrum is recorded only for the selected re-...

For more detailed structural analyses it will be important to make use of new techniques that have emerged in protein NMR spectroscopy. The main breakthroughs are probably the introduction of TROSY spectroscopy [74], extending the size limit of biological macromolecules that can be subjected to NMR analyses, the creation of so called cross correlation experiments [75] that allow the direct measurement of dihedral angles, and the measurement of residual dipolar couplings [76] that complement data from NOESY spectroscopy. Unfortunately, all of these techniques require, or at least benefit from isotope enrichment which remains a difficult and expensive task for carbohydrates. Consequently, only few examples utilizing e.g. C-labelled carbohydrates have been published so far [23, 40-42]. It is... 

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