Double quantum-filtered COSY spectra

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... 

The DQF (double-quantum filtered)-COSY spectrum of an isoprenyl coumarin along with H-NMR data are shown. Determine the H/ H homonuclear interactions in the DQF-COSY spectrum. 

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.)...

For example, from protein basic pancreatic trypsin inhibitor (BPTI), four experimental ISNets are observed from a double quantum filtered COSY spectrum as shown at the bottom of Fig. 6. No one of them is completely matched onto its ISNet cluster center. 

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. 

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. 

In some instances, we may find that examination of a cross peak close to the diagonal (5j is almost equal to 82) is difficult because of distortion of the cross peak due to intensity on the diagonal of the spectrum. The double-quantum filtered COSY (DQF-COSY) experiment fortunately provides a method for suppression of most of the signal intensity found on the diagonal. 

Double-quantum filtered COSY is now the most widely used version of the COSY experiment. It has two main advantages over its precursor diagonal and off-diagonal peaks can be phased to be absorptive simultaneously, and singlets are removed from the spectrum. The pulse sequence is... 

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.)... 

Table 25 summarizes the XH and 13C NMR spectra of 75. The proton resonances were analyzed by double quantum filtered H—H shift-correlated COSY spectrum and... 

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. 

Record the 2-D H- H double quantum filtered correlation spectroscopy (DQF-COSY) spectrum (Braun et al., 1998, pp. 481-484). 

The DQF H— H COSY spectrum of ipsenol can be found at the bottom of Figure 5.9. Note that the spectrum seems cleaner, especially along the diagonal, making the task of interpretation significantly easier. Because of the greatly improved appearance of DQF-COSY, all COSYs in this book are double quantum filtered. 

Figure 13.12. Double quantum filtered hydrogen COSY spectrum of menthol. (Contributed by David Lankin.)...

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