Selective ID COSY Experiments

However some new problems arise with the implementation of selective pulses  

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). 

The second class of gradient-selected, selective ID experiments is based on a selective spin-echo-unit which consists of a 180° selective pulse flanked by two gradients with the same or of opposite sign (see Check it 5.4.1.10). 

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. 

Homonuclear correlation of signals using scalar homonuclear 3j and/or coupling. The experiment is limited to sensitive nuclei with high natural abundance. The ID analogous experiment of the 2D COSY experiment. 

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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. 

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. 

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. 

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). 

In Check it 4.2.1.1 a job file is used to simulate a typical experimental combination used for routine structure analysis a standard ID IH spectrum and a 2D gradient selected iH COSY experiment. 

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

One-dimensional double-resonance or homonuclear spin-spin decoupling experiments can be used to furnish information about the spin network. However, we have to irradiate each proton signal sequentially and to record a larger number of ID H-NMR spectra if we wish to determine all the coupling interactions. Selective irradiation (saturation) of an individual proton signal is often difficult if there are protons with close chemical shifts. Such information, however, is readily obtainable through a single COSY experiment. 

Stored data sets a, b, c and d are obtained. If for the basic inverse ID heteronuclear COSY experiment NS scans are acquired, then NS/4 scans must be used in this case for each of the subexperiments to compare the results of the two experiments on the basis of equal total measuring time. The four data sets a-d are linearly combined in three ditferent ways as outlined in the processing scheme above (processing steps 1-3) and allow to calculate the corresponding suhspectra A, B and C. These schemes may easily be expanded accordingly, if more target spins to be selectively perturbed have been selected. 

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... 

Chapter 12 Selected ID, 2D, and 3D Experiments A Further Look Table 12.1 Coherence Pathways in a COSY Experiment 1 b... 

Intensity-based methods. 1H— H RDCs have been obtained by analyzing the intensity ratios of the diagonal and cross-peaks in a series of 2D CT COSY spectra.177 This method can only be applied to resolved resonances, for example, those of anomeric protons.149 Similar limitations apply to /-modulated ID directed COSY,153,178 which uses selective 180° pulses to produce a series of ID spectra for each pair of coupled spins. This approach has recently been extended to include additional selection blocks yielding a versatile method for the measurement of coupling constants in compounds with severely overlapping proton resonances such as those found in carbohydrates.154 The problem of overlapping resonances can also be resolved by involving 13C nuclei, as demonstrated on natural abundance (13C COSMO HSQC)32 or uniformly 13C isotopically enriched carbohydrates (2D-HSQC-(sel C, sel H)-CT COSY experiment).73,158... 

In a homonuclear decoupling experiment a particular multiplet is irradiated suppressing the coupling interaction between the irradiated nucleus and its coupling partners. A comparison of the standard coupled ID spectrum and the selectively homonuclear decoupled spectrum reveals which nuclei are coupled. Whether a homonuclear decoupling experiment or a 2D homonuclear COSY experiment would be the best solution for multiplet analysis in a one-dimensional spectrum depends very much on the nature of the problem under investigation. If a large number of multiplets need to be irradiated then a two-dimension approach may be preferable. 

Further examples of the use of excitation sculpting to minimize the phase distortions introduced by a spin echo unit are discussed in section 5.2.3 on the WATERGATE sequence and in section 5.4.1.1 for a ID selective COSY experiment. 

In Check it 5.4.1.10 gradients are used in the ID selective COSY experiment to try and improve the selectivity of the selective 90° excitation pulse. 

ID selective COSY experiment with a selective excitation pulse and gradient pulses. 

A method has been presented for obtaining ID versions of 2D n.m.r. experiments, which is useful for the sequence analysis of oligosaccharides. It involves application of Gaussian-shaped pulses for selective COSY experiments, combined with one-or two-step-relayed coherence transfer. As an illustration, the method has been applied to the structural analysis of lantacoside C (41). A heptasaccharide and a decasaccharide from natural sources were chosen as examples to explain the advantage of an integrated... 

GOS) production was intensively explored. The researchers were searching for commonly available and efficient p-galactosides. Polaina and coworkers,reported efficient method for the hydrolysis of lactose and the biosynthesis of GOS by using recombinant p-galactosidase from Thermotoga maritima. The reaction mixture was veiy complex and the concentration of synthesized oligosaccharides was lower than 10%. Desired compounds were defined by means of 2D-HSQC spectra combined with selective COSY and TOCSY ID NMR experiments. 

Figure 24 Illustration of the assembly of 2D-C0SY and 2D-HMQC pulse sequences to form an HMQC-COSY 3D-NMR experiment. Hypothetical spectra from a CaH-CbH-CcH spin system the ID and spectra, (a) 2D-C0SY spectrum, (b) 2D-HMQC spectmm, and (c) the 3D-HMQC-COSY spectrum showing selected M COSY planes at the shifts of Ca, Cb, and Cc in the dimension. Reprinted with permission from Rinaldi, P. L. Monwar, M. Encyclopedia of Spectroscopy and Spectrometry, 2nd ed. Elsevier, Oxford, UK, 2010. Copyright 2010 Elsevier.

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