HSQC experiment

HC HMQC (heteronuclear multiple quantum coherence) and HC HSQC (heteronuclear single quantum coherence) are the acronyms of the pulse sequences used for inverse carbon-proton shift correlations. These sensitive inverse experiments detect one-bond carbon-proton connectivities within some minutes instead of some hours as required for CH COSY as demonstrated by an HC HSQC experiment with a-pinene in Fig. 2.15. 

Figure 2.15. HC HSQC experiment (contour plot) of a-pinene [ CDCI3, 5 % v/v, 25 °C, 125 MHz for C, 500 MHz for h, 4 scans, 256 experiments]. This experiment gives the same information as Fig. 2.14 within 8 minutes instead of two hours required for the CH-COSY in Fig. 2.14 due to higher sensitivity because of proton detection and stronger magnetic field. Deviations of proton shifts from those in Fig. 2.14 arise from the change of the solvent. The methylene protons collapsing in Fig. 2.14 at Sh = 2.19 (200 MHz) display in this experiment an AB system with = 2.17 and Sg = 2.21 (500 MHz)...

These two-dimensional CH shift correlations indicate CH relationships through two and more bonds (predominantly Jch and Jch connectivities) in addition to more or less suppressed Jch relationships which are in any case established from the CH COSY contour diagram. Format and analysis of the CH COLOC or HMBC plots correspond to those of a C//COSY or HSQC experiment, as is shown for a-pinene (1) in Figs. 2.14 - 2.17. 

Additionally, all carbon-proton bonds can be assigned by means of the HC HSQC experiment b ... 

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

HSQC is based on proteins containing C-labelled methyl groups and is increasingly used to complement HSQC experiments [37]. 

NMR methods of diastereomeric excess determination were used in the separation of 2-methyl-l-boraadamantane into optical antipodes (see Section 12.13.2.5.1) <2003MC121>. Total assignment of H (Figure 4) and 13C NMR spectra (Table 3) of THF-2-methyl-l-boraadamantane 15 was based upon COSY and HSQC experiments <2003MC121>. 

Finally, Deschamps et al. reported for the first time a homonuclear connectivity map between the quadrupolar nuclei based on J couplings [284], The experiment uses a relayed heteronuclear transfer via spin-1/2 nuclei to a second quadrupolar spin via four chemical bonds (in this case / -mediated Al-O- P-O- Al transfer), in a scheme equivalent to the heteronuclear SQ correlation (HSQC) experiment. 

No general studies have been carried out for these compounds, but there are several reports in which the stereochemistry of the final product has been elucidated by NOESY, correlation spectroscopy (COSY), or heteronuclear single quantum correlation (HSQC) experiments. For example, intensive NOESY experiments were used to establish the exact nature of each of the three cycloadducts 151a-c generated by the cycloaddition of a substituted nitrone to dimethyl (Z)-diethylenedicarboxylate <2000EJ03633>. 

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... 

Fig. 10.15. Pulse sequence for the multiplicity-edited gradient HSQC experiment. Heteronuclear single quantum coherence is created by the first INEPT step within the pulse sequence, followed by the evolution period, t . Following evolution, the heteronuclear single quantum coherence is reconverted to observable proton magnetization by the reverse INEPT step. The simultaneous 180° XH and 13C pulses flanked by the delays, A = l/2( 1 edits magnetization inverting signals for methylene resonances, while leaving methine and methyl signals with positive phase (Fig. 16A). Eliminating this pulse sequence element affords a heteronuclear shift correlation experiment in which all resonances have the same phase (Fig. 16B).

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

Fig. 3.1 Comparison of resolution of a [15N,1H]-HSQC experiment performed on ubiquitin at 800 MHz (left) and 600 MHz (right) recorded at 295...

The first experiment to be recorded on isotope-labeled proteins is the [ N/HJ-HSQC experiment. Inspection of the [ N. HJ-correlalion map and simple counting of cross peaks reveals whether multiple conformers exist, whether some parts of the backbone signals are broadened, possibly because of slow conformational exchange, or whether parts of the sequence are not visible at all. As mentioned above, the spectrum will also show whether the protein is well folded or not. 

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. 

Conceptually similar to FIDS is the so-called /-modulated CT-HSQC experiment [14] (Fig. 7.8b). The coupling evolves in one experiment during x and in a second experiment not at all. The intensity ratio between first and second experiment is cos (njx). As an example we show the measurement of an NH dipolar coupling by this method. The pulse sequence of the constant time HSQC and the oscillatory behavior of the cross-peak intensities are shown in Fig. 7.11. 

Fig. 7.26 J-resolved constant r C,H-HSQC experiment (a) to measure the cross-correlated relaxation rates in RNA with a geometry given in b.

The two-bond HNC dipolar coupling is observable in a 15N-HSQC experiment in which the J coupling between the carbonyl atom C and the 15N amide is active. The doublet components in the 15 N dimension that represent the C N coupling are displaced with respect to one other in the H dimension as in an E.COSY [39] because of this two-bond coupling. 

When the protein rotational correlation time exceeds 15 ns, the upfield component of the JNH doublet broadens. In such cases the amide JNH coupling can be obtained from the displacement of the 15N chemical shift of the TROSY component (see Chapt. 10) relative to the one from a XH-decoupled HSQC experiment. 

This relaxation pathways cannot be influenced by TROSY. Only with the replacement of nonlabile protons with deuterons is the transverse relaxation significantly reduced further, as can be inferred from Tab. 10.1. In Tab. 10.1 the transverse relaxation rates of XH and 15N are predicted for a 23 kDa protein. In a conventional [15N,1H]-HSQC experi-... 

Fig. 14.3 Comparison of [ N/ HJ-HSQC experiments before (dark) and after (light) addition of a ligand.

A less obvious dipole-dipole interaction is the /hnnhnh CCR. This CCR-rate is conveniently measured in a HSQC experiment [47] for which... 

Fig. 6 HSQC experiment of the NBD peptide bound to NEMO for the measurement of HNN-HNHof dipole-dipole CCR. The resonance assignment is indicated...

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... 

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