Gradient-enhanced experiments

A variety of sequences exist, which differ with respect to the detected interaction ( J, or Jx ) and the mode of detection ( C or H detected, magnitude or phased mode, phase cycling or gradients for coherence selection). In view of the reduced sensitivity of heteronudear experiments with respect to homonuclear COSY experiments and the steadily decreasing sample amounts submitted for NMR experiments, there is no doubt that the inverse ( H) detected, gradient enhanced experiments are currently the best methods to apply. However on older type spectrometers, not equipped for inverse detection the old-fashioned direct C detected experiments are still in use. 

Muhandiram DR, Kay LE. Gradient-enhanced triple resonance three-dimensional NMR experiments with improved sensitivity. J Magn Reson 1994 103 203-216. 

Kay LE, Xu G-Y, Singer AU, Muhandiram DR, Forman-Kay JD. A gradient-enhanced HCCH-TOCSY experiment for recording side-chain 3H and 13C correlations in H20 samples of proteins. J Magn Reson 1993 101B 333-337. 

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.

The development of gradient-enhanced heteronuclear shift correlation experiments in the early 1990s heralded a major improvement in the applicability of these experiments for H- N direct and long-range heteronuclear shift correlation... 

Figure 2. Heteronuclear Single Quantum Correlation Spectra of I N-rCRALBP with and without 11-c/s-retinaldehyde. Results are shown from a gradient-enhanced sensitivity-enhanced heteronuclear single quantum correlation (GESE-HSQC) NMR experiment. The experiment correlates directly bonded pairs in rCRALBP. The correlation map for rCRALBP (29 mg/ml) with bound 11-c/s-retinaidehyde (heavy lines)...

In a recent article by Verotta et al. [286] the structures of new cycloartane saponins from A. sieberi have been elucidated by the combination of ID- and 2D- gradient-enhanced NMR techniques H- C GHSQC, 1H-13C GHMBC, besides the DQC-COSY, E-COSY, ROESY, ID- and 2D- TOCSY experiments performed at 600 MHz. 

More recently, a gradient-enhanced heteronuclear single quantum multiple bond correlation (HSQMBC) experiment and its variants34,47,60-62 have been proposed for the measurement of heteronuclear coupling... 

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