Gradient heteronuclear multiple quantum coherence

Fig. 8.19 Schematic representation of the gradient heteronuclear multiple quantum coherence or GHMQC pulse sequence. The gradient version of this experiment now in use [114] is derived from the earlier non-gradient experiment described by Bax and Subramanian [113]. Coherence pathway selection is obtained by the application of gradients in a ratio of 2 2 1 as shown. Other ratios are also possible, as considered in the reports of Ruiz-Cabello et al. [115] and Parella [116]. The experiment creates heteronuclear multiple quantum coherence with the 90° C pulse that precedes evolution. Both zero and double quantum coherences are created and begin to evolve through the first half...

COR, H- F COSY, and H- F gradient heteronuclear multiple quantum coherence (gHMQC). Since the abundance of F and H nuclei are both 100%, het-... 

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... 

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... 

Eye. -detected gradient-enhanced heteronuclear multiple-quantum coherence spectroscopy has been used to determine the distribution of [l- C]glucose and [l- C]sorbitol in the bovine lens. Following incubation of lenses in 50 mM [l- C]glucose for 1, 2 and 4 days, [l- C]glucose and [l- C]sorbitol were mainly detected in the periphery lens. ... 

Hurd, Ralph E. and John, B. K. Gradient-enhanced proton-detected heteronuclear multiple-quantum coherence spectroscopy. J. Magn. Reson. 91 648, 1991. 

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.

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).

The inverse-detected 2D NMR experiments that have been discussed to this point have all been discrete, single-purpose experiments, e.g. correlating protons with their directly bound heteronucHde (typically or N). There are another class of inverse-detected 2D NMR experiments that are generally referred to as hyphenated 2D experiments. These are experiments that first establish one type of correlation, followed by an additional experiment segment that then pursues a further spectroscopic task. Predecessors of the inverse-detected variants of these experiments were the HC-RELAY (proton—carbon heteronuclear relayed coherence transfer) experiments pioneered by Bolton [151—155]. Examples of these include, but are by no means hmited to HXQC-COSY and -TOCSY [156—158], -NOESY [159], -ROESY [160], and more recent gradient variants [161] etc., where X = S (single) or M (multiple) quantum variants of the experiments. 

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