Phase COSY double quantum filtered

Proton NMR spectroscopy was used as a tool for profiling complex carbohydrate mixtures in non-fractionated beer by Meier et al Six samples of beer of different styles contained more than 20 small carbohydrates were tested. The authors used H1-H2 cross peaks in phase sensitive double quantum filtered COSY measurement as reporter signals to identify more than 50 structural motifs in beers. 

One problem associated with COSY spectra is the dispersive character of the diagonal peaks, which can obliterate the cross-peaks lying near the diagonal. Moreover, if the multiplets are resolved incompletely in the crosspeaks, then because of their alternating phases an overlap can weaken their intensity or even cause them to disappear. In double-quantum filtered COSY spectra, both the diagonal and the cross-peaks possess antiphase character, so they can be phased simultaneously to produce pure 2D absorption line... 

Figure 5.37 (a) Conventional phase-sensitive COSY spectrum of basic pancreatic trypsin inhibitor, (b) Double-quantum filtered (DQF) phase-sensitive COSY spectrum of the same trypsin inhibitor, in which singlet resonances and solvent signal are largely suppressed. Notice how clean the spectrum is, especially in the region near the diagonal line. (Reprinted from Biochem. Biophys. Res. Comm. 117, M. Ranee, et al., 479, copyright (1983) with permission from Academic Press, Inc.)... 

Figure 7.25 Homoniiclear double-quantum filtered COSY spectrum (400 MHz) of 8-mMangiotensin II in H,0 recorded without phase cycling. Magnetic field gradient pulses have been used to select coherence transfer pathways. (Reprinted from J. Mag. Reson. 87, R. Hurd, 422, copyright (1990), with permission from Academic Press, Inc.)...

The data from H NMR studies of 63, which included double quantum filtered phase sensitive correlated spectroscopy (DQF-COSY) and rotating frame nuclear Overhauser effect spectroscopy (ROESY) experiments (Figure 12), are collected in Table 17. 

The double quantum filter eliminates or at least suppresses the strong signals from protons that do not experience J-coupling, e.g. the solvent signal, which would otherwise dominate the spectrum and possibly be a source of troublesome tl noise. Compared to a phase-sensitive but non-DQ-filtered COSY with pure absorption lineshapes for the cross peaks but mixed lineshapes for the diagonal peaks, the phase-sensitive, DQ-filtered COSY has pure absoiption lineshapes throughout. 

H/ H-double quantum filtered COSY Note that with this experiment both the diagonal and the cross peaks may be phased to pure absorption. Therefore it is best to select diagonal peaks at the extremes and in the center of the spectrum for phase adjustment. The cros.s peaks when correctly phased consist of positive and negative peaks, which are anti-phase with respect to the active, and are in-phase with respect to the passive coupling(s). 

The MQC intermediate state in coherence ( INEPT ) transfer can also be used to clean up the spectrum. In this case, we can apply a double-quantum filter (using either gradients or a phase cycle) to kill all coherences at the intermediate step that are not DQC. We will see the usefulness of this technique in the DQF (double-quantum filtered) COSY experiment (Chapter 10). As with the spoiler gradient applied to the 2IZSZ intermediate state, a doublequantum filter destroys any unwanted magnetization, leaving only DQC that can then be carried on to observable antiphase magnetization in the second step of INEPT transfer. 

A simple variant of the COSY experiment is COSY-35 (sometimes called COSY-45), in which the second 90° pulse is reduced from a 90° pulse to a 35° or 45° pulse (Fig. B.3). The result is that the fine structure of crosspeaks is simplified, with half the number of peaks within the crosspeak. This makes it much easier to sort out the coupling patterns in both dimensions and to measure couplings (active and passive) from the crosspeak fine structure. A more important variant of the COSY experiment is the DQF (double-quantum filtered)— COSY (Fig. B.4), which adds a short delay and a third 90° pulse. The INEPT transfer is divided into two steps antiphase I spin SQC to I,S DQC, and I,S DQC to antiphase S spin SQC. The filter enforces the DQC state during the short delay between the second and third pulses either by phase cycling or with gradients. DQF-COSY spectra have better phase characteristics and weaker diagonal peaks than a simple COSY, so this has become the standard COSY experiment. 

By allowing multiple-quantum coherence to process during the evolution period of a two-dimensional experiment, Drobny et al. were able to detect its effects indirectly. This idea subsequently blossomed into the new technique of filtration through double-quantum coherence. Multiple-quantum coherence of order n possesses an n-fold sensitivity to radiofrequency phase shifts, which permits separation from the normal single-quantum coherence. This concept inspired the popular new techniques of double-quantum filtered correlation spectroscopy (DQ-COSY) and the carbon-carbon backbone experiment (INADEQUATE), both designed to extract useful connectivity information from undesirable interfering signals. 

Double Quantum Filtered COSY The double quantum filtered COSY experiment (DQF-COSY, Section 6-1 and Figure 6-16) is similar to COSY, with three 90° pulses in the sequence 90°-/i-90°-T-90°-f2 (acquire). The DQF-COSY experiment is performed in the phase-sensitive mode, but, unlike the situation in the phase-sensitive COSY experiment, in the DQF-COSY both diagonal and cross peaks can be phased as absorptive signals. This difference not... 

Previous sections have already made the case for acquiring COSY data such that it may be presented in the phase-sensitive mode. The pure-absorption lineshapes associated with this provide the highest possible resolution and allow one to extract information from the fine-structure within crosspeak multiplets. However, it was also pointed out that the basic COSY-90 sequence suffers from one serious drawback in that diagonal peaks possess dispersion-mode lineshapes when crosspeaks are phased into pure absorption-mode. The broad tails associated with these can mask crosspeaks that fall close to the diagonal, so there is potential for useful information to be lost. The presence of dispersive contributions to the diagonal may be (largely) overcome by the use of the double-quantum filtered variant of COSY [37], and for this reason DQF-COSY is the experiment of choice for recording phase-sensitive COSY data. 

The DQF-COSY sequence (Fig. 5.40) differs from the basic COSY experiment by the addition of a third pulse and the use of a modified phase-cycle or gradient sequence to provide the desired selection. Thus, following tj frequency labelling, the second 90° pulse generates multiple-quantum coherence which is not observed in the COSY-90 sequence since it remains invisible to the detector. This may, however, be reconverted into single-quantum coherence by the application of the third pulse, and hence subsequently detected. The required phase-cycle or gradient combination selects only signals that existed as double-quantum coherence between the last two pulses, whilst all other routes are cancelled, hence the term double-quantum filtered COSY. 

Figure 5.41. The gradient-selected DQF-COSY experiment and coherence transfer pathway. No phase-cycling is needed as the required pathway is selected with gradient ratios of 1 2. Both gradient pulses are applied within spin-echoes for phase-sensitive presentations. Note only one pathway is retained from the double-quantum filter.

Figure 5>t2. The double-quantum filtered COSY spectrum (right) provides greater clarity close to the diagonal peaks than the basic phase-sensitive COSY (left) as it does not suffer from broad, disposive diagonal peaks. 

Check it 3.3.2.2 illustrates the phase correction of a phase sensitive IR COSY TPPI spectrum whilst in Check it 3.3.2.3 a phase sensitive COSY spectrum with double quantum filter is simulated. The double quantum filter effects the lineshape in the spectrum such that the diagonal and cross peaks have a narrower absorptive lineshape. 

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