Quantum Filters

Double quantum coherence (DQC) can be produced in a pair of coupled spins by a 90°, 2r, 90° sequence, where the pulses are nonselective, and r = 1/4/. The coherence pathway is  

As we saw in Eq. 11.82b, the last expression represents DQC. Chemical shifts are often refocused by inserting 180° I and S pulses at time r, the midpoint in the evolution period, so the DQC-generating sequence may appear as 90°, t, 180°, r, 90°. In a 2D experiment, the DQC thus produced is allowed to evolve and is eventually converted to observable magnetization whereas in a ID experiment the DQC is often converted almost immediately to observable magnetization. 

This procedure can be used as a filter if there is another pathway by which unwanted observable magnetization is produced but never goes through the DQC step. By phase cycling and/or by imposition of suitable magnetic field gradients (as discussed in Section 11.7), it is usually feasible to discriminate very effectively between the coherence pathways and to eliminate virtually all traces of unwanted signal. We turn now to one important example. 

In practice, additional phase cycling is needed in order to generate quadrature signals that can be processed as discussed in Section 10.3 to produce properly phased line shapes. Because of the DQC precession frequency, phase increments of tt/4 are needed to provide the same sort of data usually obtained with tt/2 phase shifts. Hence, a second set of phase-cycled repetitions is needed, with data stored in a separate location. Overall, the following phase cycling can be 

Pulses 1,2,3 Pulse 4 Receiver Pulses 1,2,3 Pulse 4 Receiver 

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Suppression of the tme diagonal peaks by double-quantum filtering (DQF-COSY) may resolve such problems. Finally, quantitative measurements of the magnitude of the coupling constants is possible using the Z-COSY modification, These experiments ate restricted to systems of abundant spins such as H, and which have reasonably narrow linewidths. 

Figure 1. Pulse sequences of some typical 2D-NMR experiments. COSY = correlation SpectroscopY, DQFCOSY = Double Quantum Filtered COSY, RELAY = RELAYed Magnetization Spectroscopy, and NOESY = Nuclear Overhauser Effect SpectroscopY.

Figure 1.45 Coherence transfer pathways in 2D NMR experiments. (A) Pathways in homonuclear 2D correlation spectroscopy. The first 90° pulse excites singlequantum coherence of order p= . The second mixing pulse of angle /3 converts the coherence into detectable magnetization (p= —1). (Bra) Coherence transfer pathways in NOESY/2D exchange spectroscopy (B b) relayed COSY (B c) doublequantum spectroscopy (B d) 2D COSY with double-quantum filter (t = 0). The pathways shown in (B a,b, and d) involve a fixed mixing interval (t ). (Reprinted from G. Bodenhausen et al, J. Magn. Resonance, 58, 370, copyright 1984, Rights and Permission Department, Academic Press Inc., 6277 Sea Harbor Drive, Orlando, Florida 32887.)...

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.)... 

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 illustrates the power of magnetic field gradient pulses to eliminate unwanted coherences. The double-quantum filtered COSY spec-... 

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.)...

Spin-Spin Couplings. - 31P Relayed and double quantum filtered... 

Zheng et al. [1] postulated that the driving force for placing Zr and B on the same carbon might stem from interactions between the zirconium and oxygen or boron and chlorine atoms. However, an X-ray analysis of 22 revealed that there are no intra- or intermo-lecular interactions between any of these atoms [35]. Compound 22 was also unambiguously characterized by 1H-1H double quantum filtered COSY [36] and 13C-1H heteronuc-lear chemical shift correlation NMR spectroscopy [37,38]. Considerable differences in the chemical shifts of the diastereotopic Cp groups were found in both the XH and 13C NMR spectra. The NMR study unequivocally showed that the methine proton was at-... 

Sowinski and coworkers40 reported a structure of vacidin A (63), an aromatic hep-taene macrolide antibiotic. The constitution of vacidin A, a representative of the aromatic heptaene macrolide antibiotics, was established on the basis of 13C and H- H double quantum filtered correlated spectroscopy, rotating frame nuclear Overhauser effect spectroscopy, 7-resolved 11 as well as H-13C correlation NMR spectra. The geometry of the polyene chromophore was determined as 22E, 24E, 26E, 28Z, 30Z, 32E, 34E. 

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. 

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... 

These results suggest that the signals arise from dipole-dipole coupled protons. Kreis et al. confirmed this finding by measurements using one-dimensional zero- and double-quantum filtering, two-dimensional J-resolved spectroscopy, two-dimensional constant time COSY and longitudinal order separation... 

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). 

Load the raw data of the gradient enhanced, double quantum filtered 2D... 

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

NMR spectroscopy was carried out using a Varian Unity 300MHz spectrometer. Peptides were dissolved in 500 pL of 90% H,O/10% D20 (or 100% D20) giving a sample concentration of 1-2 mM and the pH adjusted to 5.5. H DQF-COSY (double quantum filtered two-dimensional correlated spectroscopy), ROESY, and TOCSY spectra were collected at 25 °C and processed as described.1 6-281... 

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