The HSQC Experiment

As in HMQC, a delay time A governs both the initial defocusing of the C-bonded H vectors and the final refocusing of these vectors and is, likewise, selected in a compromise manner. Carbon decoupling also can be performed with the GARP or WURST sequences as 

A decision as to whether to employ HSQC or gHSQC may be reached in the following manner. For a relatively concentrated sample that displays strong signals in its H NMR spectrum, where t noise ridges could be a problem, gHSQC is preferred, because t ridges are 

steady-state scans = 32 (gradient and nongradient versions) 

LP = 768 (recommended minimum), but can be up to 1,792 (eightfold) with gradients and 3,840 (sixteenfold) for nongradient versions 

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Nolls, P., Espinosa, J. F., Parella, T. Optimum spin-state selection for all multiplicities in the acquisition dimension of the HSQC experiment. 

The HSQC experiment is based on single rather than multiple quantum coherence during the evolution time, t. The contemporary multiplicity-edited gradient HSQC pulse sequence is shown in Fig. 10.15. Relative to the much simpler HMQC pulse sequence, the HSQC... 

Figure 16 shows the 13C spectra of N-acetylglycine obtained from the traces of the indirectly detected dimension of the HSQC experiment. The spectrum... 

Traces through the spectrum along co2 are shown in Fig. 7.10 together with the fitting of the coupled to the decoupled spectrum after convolution by an in-phase stick doublet. The fit delivers the coupling constant with high precision. The sensitivity of this experiment is practically identical to that of the HSQC experiment since the splitting is normally smaller or in the order of the line widths. 

In the following, three different experiments are discussed, where short, high-power spin-lock pulses are used to purge the spectrum from undesired resonances. The experiments are (i) the HSQC experiment [5], (ii) experiments with C half-filter elements [6], and (iii) NOESY and ROESY experiments for the observation of water-protein NOEs [7]. In the first two experiments, spin-lock purge pulses are used to suppress the signals from... 

Because of the favorable cross-peak multiplet fine-structure, the HSQC experiment offers superior spectral resolution over the HMQC (heteronuclear multiple quantum coherence) experiment [13, 14], On the other hand, the HMQC experiment works with fewer pulses and is thus less prone to pulse imperfections. The real advantage of the HSQC experiment is for measurements of samples at natural isotopic abundance and without the use of pulsed field gradients, since the HSQC experiment lends itself to purging with a spin-lock pulse. Spin-lock purging in the HMQC experiment... 

In contrast to the basic "C detected experiment, and as a consequence of the final H detection, the 2D spectra obtained with HMQC or HSQC have a projection onto the F2 axis which corresponds to the normal H spectrum and includes all chemical shifts and all Jfi, couplings. The latter may give rise to rather broad cross peaks for extensively coupled protons. The projection onto the Fl axis corresponds to a normal C spectrum but with the quaternary carbons missing. With HMQC, but not with HSQC, cross peaks are additionally split in Fl by "J couplings. The HMQC and the HSQC experiment are usually performed in phase-sensitive mode, which, after proper phasing in both dimensions, allow peaks to be displayed in pure absorption. 

In every 2D experiment we have looked at, the chemical-shift evolution during the h delay produces two terms—sine and cosine—and in each case only one of them survives the mixing step to reach the FID as observable magnetization. The HSQC experiment is no exception ... 

The second 90° 13C pulse converts the ZQC and DQC back into antiphase H SQC, which is refocused by the final 1/(2J) delay just as it is in the HSQC experiment. This is an important theme that occurs in many of the more sophisticated NMR experiments multiple-quantum coherences (DQC and ZQC) cannot be directly observed, but they can be created from SQC, allowed to precess, and converted back to measurable SQC. The multiple-quantum coherences can be detected then, but only indirectly. Multiple quantum coherence is essential to the DQF-COSY and DEPT experiments as well, so even though it is difficult to understand it is very important in modern NMR and cannot be ignored. 

Similar to the HSQC experiment, multiple quantum coherences can be used to correlate protons with Q-coupled heteronuclei. The information content of the Heteronuclear Multiple Quantum Correlation (HMQC) experiment (56) is equivalent to the HSQC, but the sensitivity can be improved in certain cases. Additionally, by proper tuning of delays and phase cycling, it can be transformed into the heteronuclear multiple bond correlation experiment (57-59), which results in correlations between J- and J-coupled nuclei. 

Figure 12.30. Summary of the SAR by NMR drug discovery methodology. A protein target is screened against a library consisting of small organic molecules by use of the HSQC experi-...

In contrast to the HMQC and the HSQC experiment, transversal magnetization of the nucleus 2, which is not detected, will be created in the beginning of the polarization transfer experiment (Figure Ic). In the following evolution time this magnetization is modulated with the chemical shift of nucleus 2. Since nucleus 1 is detected, the overall sensitivity in this experiment is related to the factors therefore, the choice of the detected nucleus is... 

Figure 6.6. The HSQC experiment and asscx iated coherence transfer pathway. The experiment uses the INEE T sequence to generate transverse X magnetisation which evolves and is then transferred back to the proton by an INEPT step in reverse. Notice that, in contrast to HMQC, only single-quantum X coherence evolves during ti (see text).

A similar logic to that above applies to gradient selection in the HSQC experiment, for which a variety of different approaches are also possible [7]. A suitable sequence employing the echo-antiecho approach is illustrated in Fig. 6.10, and requires only two gradients in proportion to the magnetogyric ratios of the X and H spins since each acts on single-quantum X and H magnetisation. Thus, for a correlation experiment, ratios of 4 1 and... 

The key to obtaining the best suppression of the uncoupled magnetization is to apply a gradient when transverse magnetization is present on the S spin. An example of the HSQC experiment utilising such a principle is shown below... 

Particular improvements to the HSQC experiment has been the implementation of phase sensitive echo/antiecho-detection in combination with gradient coherence selection. Further variants include the sensitivity-enhanced HSQC experiment [5.194] and experiment developed for long-range coupling detection and J-scaled experiments (for references see sections 5.2.5 and 5.7.3). 

Of course, the HSQC experiment is not the perfect solution for the detection of one-bond correlation spectra, since artefacts can also be observed. Some studies have determined the cause of these artefacts which can be assigned to off-resonance effects of the 180° pulses and H, H coupling [5.195, 5.196]. 

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