ID NOESY

Figure 7.5 Pulses sequences for ID NOESY and ID relayed NOESY experiments, (a) A ID NOESY sequence with full Gaussian pulses is inferior to the ID NOESY sequence (b) with half-Gaussian excitation, (c) A 90° Gaussian pulse in the pulse sequence of 1D relayed NOESYis appropriate for excitation, since antiphase magnetization is required for the first mixing step. (Reprinted from Mag. Reson. Chem. 29, H. Kessler el al, 527, copyright (1991), with permission from John Wiley and Sons Limited, Baffins Lane, Chichester, Sussex P019 lUD, England.)...

The difference between ID NOESY and ID TOCSY is thus the different type of interaction in ID NOESY through space, and by ID TOCSY through-bond. The analogous 2D spectra are shown in Fig. 25. 

The ID NOESY-TOCSY experiment [39] shown in fig. 1(c) is a straighffor-ward concatenation of ID NOESY and TOCSY experiments [34] (figs 1(a), (b)). Since the NOE transfer takes place along the z axis, and thus has no phase memory, no phase correction for the second selective pulse is needed to compensate for the change of the r.f. frequency during the tnoe interval. Nevertheless, any possible phase differences between the selective and consecutive nonselective pulses must be taken into account in both steps, by adjusting the phase of soft pulses. 

By applying the NOESY step first, this experiment allows us to jump from one spin system to another or to overcome a bottleneck in TOCSY transfer caused by an occurrence of a small coupling constant in the chain of J-connectivities. Both these features are illustrated in ID NOESY-TOCSY spectra of the type VI group B Streptococcus capsular polysaccharide (1). 

The proposed ID TOCSY-NOESY experiment is illustrated by the assignment of NOEs from anomeric protons H-lc and H-ld of the polysaccharide 1. Because the resonances of H-lc and H-ld overlapped, this assignment was not possible from a ID NOESY spectrum as shown in fig. 3(b). Although these protons differed in their chemical shifts, it was not possible to separate them by chemical-shift-selective filtration because of the very fast spin-spin relaxation of backbone protons (20-50 ms) in this polysaccharide. Instead, a ID TOCSY-NOESY experiment was performed in which the initial TOCSY transfer from an isolated resonance of H-2c was followed by a selective NOESY transfer from H-lc. The ID TOCSY-NOESY spectrum (fig. 3(c)) clearly separated NOE signals of the H-lc proton from those originating from the H-ld proton and established the linkage Ic —> 6a. 

ID TOCSY-NOESY-TOCSY and ID NOESY-TOCSY-NOESY... 

The NOESY and TOCSY polarization transfers can also be arranged so that two NOESY steps are interrupted by one TOCSY transfer. This is useful for situations when a proton which is intended as a starting point for a ID TOCSY-NOESY experiment cannot be selectively excited, nevertheless it has a NOE contact to an isolated proton. The ID NOESY-TOCSY-NOESY sequence [72] (fig. 4(b)) is obtained by appending another NOESY step to the ID NOESY-TOCSY pulse sequence of fig. 1(c). The last NOESY step can be either selective or nonselective depending whether a selective 180° pulse is applied after the nonselective 90° pulse at the end of the TOCSY transfer. 

ID NOESY spectra (b) and (c) were acquired using the pulse sequence of fig. 1(a). A 59.2 ms half-Gaussian pulse was applied to proton H-ld. Mixing time was 25 ms in (b) and 200 ms in (c). Water presaturation was applied during the relaxation delay and the NOE mixing... 

As illustrated on this example, interpretation of NOESY spectra of polysaccharides might be complicated by the presence of spin-diffusion and higher order elfects. ID NOESY-NOESY experiment can provide some assistance in identification and assignment of spin-diffusion signals and thus prolong networks of spins experiencing NOE contacts beyond immediate neighbours normally detected in NOESY experiments. 

As an alternative to selective pulses, chemical-shift-selective filters (CSSF) were successfully used in ID COSY, ID RELAY and ID NOESY experiments when signals partially overlapping, but different in their chemical... 

In this chapter, the discussion will be focused on the ID TOCSY (TO-tal Correlation SpectroscopY) [2] experiment, which, together with ID NOESY, is probably the most frequently and routinely used selective ID experiment for elucidating the spin-spin coupling network, and obtaining homonuclear coupling constants. We will first review the development of this technique and the essential features of the pulse sequence. In the second section, we will discuss the practical aspects of this experiment, including the choice of the selective (shaped) pulse, the phase difference of the hard and soft pulses, and the use of the z-filter. The application of the ID TOCSY pulse sequence will be illustrated by examples in oligosaccharides, peptides and mixtures in Section 3. Finally, modifications and extensions of the basic ID TOCSY experiment and their applications will be reviewed briefly in Section 4. 

While many NMR active nuclei such as JH, 13C, 31P, and 15N have been used to analyze metabolites (19-21), JH NMR analysis is the most widely used in the field because of the ubiquitous nature of JH and its high NMR sensitivity. Furthermore, ease of analysis and high-throughput capabilities make onedimensional (ID) NMR experiments, including the ID NOESY (nuclear Over-hauser enhancement spectroscopy) and CPMG (Carr-Purcell-Meiboom-Gill)... 

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