Heteronuclear gradient HMQC

The first of the proton-detected experiments is the Heteronuclear Multiple Quantum Correlation HMQC experiment of Bax, Griffey and Hawkins reported in 1983, which was first demonstrated using 1H-15N heteronuclear shift correlation [42]. The version that has come into wide-spread usage, particularly among the natural products community, is that of Bax and Subramanian reported in 1986 [43]. A more contemporary gradient-enhanced version of the experiment is shown in Fig. 10.14 [44],... 

Fig. 10.14. Gradient-enhanced HMQC pulse sequence described in 1991 by Hurd and John derived from the earlier non-gradient experiment of Bax and Subramanian. For 1H-13C heteronuclear shift correlation, the gradient ratio, G1 G2 G3 should be 2 2 1 or a comparable ratio. The pulses sequence creates heteronuclear multiple quantum of orders zero and two with the application of the 90° 13C pulse. The multiple quantum coherence evolves during the first half of ti. The 180° proton pulse midway through the evolution period decouples proton chemical shift evolution and interchanges the zero and double quantum coherence terms. Antiphase proton magnetization is created by the second 90° 13C pulse that is refocused during the interval A prior to detection and the application of broadband X-decoupling.

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... 

HPLC analysis was carried out using a 250 x 4.6 mm id Spherisorb ODS-2 column at 35°C with elution using isocratic D20/phosphate buffer at pH 2.5 for 5 min, followed by a linear gradient of acetonitrile to 50% after 50 min with UV monitoring of the eluent peaks at 225 nm. The NMR measurements were carried out with solvent suppression at a H NMR frequency of 600 MHz. The assignment of the resonances were based on those of standard compounds such as A-A-A-OH and Y-Y-Y-OH using standard 1-dimensional (ID) spectroscopy and 2-dimensional (2D) heteronuclear correlation methods such as and H-15N HMQC spectra. 

If the intensity of the desired signal is the limiting factor, then phase cycle selection is preferred. A phase cycle selection version of HMQC that uses a spin-lock to purge signals from molecules containing NMR silent 5 nuclei has been developed—poor man s gradient-heteronuclear... 

NMR has become a standard tool for structure determination and, in particular, for these of Strychnos alkaloids. The last general article in this field was authored by J. Sapi and G. Massiot in 1994 [65] and described the advances in spectroscopic methods applied to these molecules. More recently, strychnine (1) has even been used to illustrate newly introduced experiments [66]. We comment, here, on their advantages and sum up the principles of usual 2D experiments in Fig. (1) and Fig. (2) (COSY Correlation SpectroscopY, TOCSY TOtal Correlation SpectroscopY, NOESY Nuclear Overhauser Enhancement SpectroscopY, ROESY Rotating frame Overhauser Enhancement SpectroscopY, HMQC Heteronuclear Multiple Quantum Coherrence, HMBC Heteronuclear Multiple Bond Correlation). This section updates two areas of research in the field new H and 13C NMR experiments with gradient selection or/and selective pulses, 15N NMR, and microspectroscopy. To take these data into account, another section comments on the structure elucidation of new compounds isolated from Strychnos. It covers the literature from 1994 to early 2000. 

In the heteronuclear experiment category, the experiments of interest are the heteronuclear multiple quantum correlation (HMQC) experiment, the heteronuclear single quantum correlation (HSQC) experiment, and the heteronuclear multiple bond correlation (HMBC, including the gradient-selected version gHMBC) experiment. Both the HMQC and HSQC produce similar results, but each has its own unique advantages and disadvantages. 

Beginning with the report of the HMQC experiment in 1986 by Bax and Subramanian- the utilization of proton- or inverse-detected heteronuclear shift correlation experiment was essentially ushered in. Despite reports that have demonstrated the superior resolution of single quantum-based heteronuclear shift correlation methods,-" the HMQC experiment still remains the most widely employed, proton-detected heteronuclear shift correlation method. Gradients were incorporated into the HMQC experiment in 1991 by Hurd and John- and are discussed in the excellent contribution of Ruiz-Cabello and co-workers- and... 

The HMQC or HSQC sequences may be transformed into their ID equivalents by simply removing the incremental t time period (Fig. 6.22) so that the experiment becomes just a heteronuclear filter. Only magnetisation that has passed via the X spin will be observed in the final spectrum and again the suppression of all unwanted signals is greatly improved by the use of pulsed field gradients. The selective observation of C-labelled glycine in an aqueous mixture is illustrated in Fig. 6.23. 

Figure 8.19 shows the gradient version of the HMQC experiment since in most cases users will want to opt for the improved performance of the gradient experiment. Following a preparation period, heteronuclear multiple quantum coherence (zero and double) is created by the 90° X-nucleus pulse applied at the initiation of the evolution period, ti. Evolution occurs and the 180° pulse serves to refocus... 

Heteronuclear Correlation Experiments A comparison of HMQC, HSQC, and the analogous gradient experiments used to correlate one bond proton and carbon chemical shifts ( Jch) showed little differences. In all cases the signal/noise for the backbone CH2 of a methacrylate copolymer was identical within experimental error. As expected, the C detected heteronuclear detection experiment (HETCOR) showed significantly (-- 4 fold) lower s/n. 

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