ID COSY

A 90° Gaussian pulse is employed as an excitation pulse. In the case of a simple AX spin system, the delay t between the first, soft 90° excitation pulse and the final, hard 90° detection pulse is adjusted to correspond to the coupling constant JJ x (Fig- 7.2). If the excitation frequency corresponds to the chemical shift frequency of nucleus A, then the doublet of nucleus A will disappear and the total transfer of magnetization to nucleus X will produce an antiphase doublet (Fig. 7.3). The antiphase structure of the multiplets can be removed by employing a refocused ID COSY experiment (Hore, 1983). 

Figure 7.3 One-dimensional COSYspectram for an AX system, (a) A common ID sjjectrum. (b) Selective excitation of spin A leads to a ID COSY spectrum with antiphase X lines and maximum transfer of magnetization from A to X. (Reprinted from Mag. Reson. Chem. 29, H. Kessler et at, 527, copyright (1991), with permission from John Wiley and Sons Limited, Baffins Lane, Chichester, Sussex P019 lUD, England.)...

There exists meanwhile a variety of frequency selective experiments still using the conventional CW irradiation as the ID NOE experiment, or upgraded with one or more selective pulses, as the ID TOCSY or the ID COSY experiment. These experiments and their many variants are probably the best choice in such cases as long as the response of a spin system to the perturbation of only one single spin or one single group of equivalent spins is of interest. If, however, and this is the most common situation, informations on several rather than only one spin-spin interaction is needed. 

As an example and to clarify the principle, the acquisition and the processing schemes for the modified heteronuclear inverse detected ID COSY experiment (pulse sequence IVa in fig. 1) are shown below (tables 1 and 2). 

Acquisition scheme for the multiple selective heteronuclear inverse detected ID COSY experiment. Four experiments have to be performed with 3 carbon resonances selected for selective perturbation. The frequencies are set either on-resonance (/i, fi, /j) or off-resonance to the selected resonance frequencies. 

Processing scheme for the multiple selective heteronuclear inverse detected ID COSY experiment. The final spectra A, B, C corresponding to the selective perturbation of three carbons resonating at frequencies fi, fi, and f are obtained by linear combination of the original data a, b, c and d acquired in accordance with the acquisition scheme (table 1). 

Fig. 1. Pulse sequences modified for multiple selective excitation. I ID TOCSY, II het-eronuclear ID NOE, III ID INADEQUATE, IVa heteronuclear ID COSY (optimized to detect Jch), IVb heteronuclear ID COSY (optimized to detect "Jch), V 2D TOCSY-COSY, Via 2D HMBC (designed to detect heteronuclear long-range couplings "Jch only), VIh 2D HMBC (extended pulse sequence to detect both heteronuclear long-range "Jch and...

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 a ID COSY-RELAY experiment [38] (fig. 13(a)) a multistep relay transfer is applied after the filtration. If the filtration is performed on the H-2 proton, the CSSF is incorporated into the first spin-echo. If there is not sufficient chemical shift separation between H-2 protons, the filter is shifted to the second spin-echo. The method is illustrated for the separation of the spin systems of two terminal /3-glucopyranose residues of a modified LPS (5) containing a total of nine saccharide units [76]. The anomeric proton resonances of the two /3-glycopyranoses overlapped almost completely, with a chemical shift difference of only 1.9 Hz, while the corresponding H-2 resonances were separated by 55.0 Hz. The length of the filtration interval, Ti, was adjusted to yield a maximum antiphase magnetization of H-2 pro-... 

Fig. 14. ID COSY-RELAY spectra of two terminal glucoses of oligosaccharide 5. (a) Partial H spectrum of 5 at 600 MHz and 27°C. Spectra (b) and (c) were acquired using the pulse sequence in fig. 13(a) (k = 3) with the initial polarization transfer from overlapping anomeric protons of terminal glucoses. Duration of the Gaussian pulse was 50 ms, to = 39 ms, T] = 50 ms, A = 9.09 ms, T2 = 50 ms, T3 = 40 ms, number of scans was 64, relaxation delay and acquisition times were 2 and 1.4 s, respectively. AT = 0 for the first and N = 1 for the second spectrum, (d) is the sum of (a) and (b), (e) is the difference between (a) and (b). (Reprinted with permission from ref. [38]. Copyright 1993 ESCOM Science Publisher...

Replacement of the TOCSY transfer in a ID COSY-TOCSY experiment by the NOESY step yields a ID COSY-NOESY sequence [38] (fig. 13(c)). The experiment is illustrated by the determination of NOEs from the H-7 proton of a terminal 3,6-dideoxy-4-C-(l-hydroxyethyl)-D-xylohexose (6)... 

Fig. 16. ID COSY-NOESY experiment on the polysaccharide 6 [77]. The structure of a terminal 3,6-dideoxy-4-C-(l-hydroxyethyl)-D-xylohexose is shown in the inset. The COSY transfer is depicted using the solid line, while a dotted line is used for the NOESY transfer, (a) H spectrum of 6 at 600 MHz and 50°C. (b) ID COSY-NOESY spectrum acquired using the sequence of fig. 13(c) with the initial transfer of magnetization from H-8 and the following parameters = 100 ms, to = 29 ms, Tr = 32 ms, A = 0.5 ms, N = 0, 1,...,64,...

Prior to the development of 2D homonuclear COSY experiments, the SPT experiment was used to unravel coupling networks. The use of SPT experiments to trace eoupling interactions of a spin system has two advantages whieh even today makes this technique superior to the selective ID COSY experiment. 

Fig. 5.23 Possible selective pulse schemes to generate a ID COSY experiment.

One class of selective ID COSY experiments relies upon a selective excitation pulse corresponding to the non-gradient ID selective COSY experiments (left hand scheme in Fig 5.24). 

Listed below a number of different types of ID COSY experiment with and without gradient selection are summarized. By necessity the following discussion can give only a short description of the development of the selective ID COSY experiment starting with the basic experiment and ending with the ID DPFGSE selective COSY experiment. 

The remaining Check its in this section use a test spin system based on a structural fragment of brucine which is typical of the type of molecule analyzed using the selective ID COSY experiment in a routine laboratory. The IH spin system is reduced to the protons shown in the figure below with the bold lines indicating the structural fragment of interest. The coupling constants and chemical shifts are based on the experimental data of brucine. 

The signal of proton H-1 la at 1.35 ppm is well separated and is an ideal candidate for excitation by a selective pulse. In the following Check its the selective pulse(s) is always set on-resonance to the H-1 la proton. Thus the corresponding selective ID COSY experiments must show responses to protons H-1 lb 2.30 ppm, H-6 3.05 ppm and H-10... 

In Check it 5.4.1.11 the improved selective ID COSY experiment using a selective refocusing n pulse are calculated. These category of selective COSY experiments based on a gradient flanked selective spin echo generate less artefacts and are superior to the ID COSY pulse sequences with a selective excitation pulse. Essentially the flanking gradients cancel the artefacts in a similar manner as a "perfect EXORCYCLE" scheme [5.140]. 

The z-filter In spinlock sequences and selective ID COSY experiments the z-filter improved the artefact suppression of accumulated experiments. 

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