Gradient enhanced

Flurd R E 1990 Gradient enhanced spectroscopy J. Magn. Reson. 87 422-8... 

Figure 7.26 Gradient-enhanced TOCSY spectrum of 10 mM sucrose in D. O recorded at 400 MHz by using modified MLKV-16 pulse sequence.

Gradient-enhanced 2D TOCSY spectrum of 10 mMof sucrose in D..,0 is shown in figure 7.26. The clean spectrum obtainable without any noise and without the nece.ssity of any phase cycling illustrates the power of this new technique in modern NMR spectroscopy. 

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.

Almost all spectra were acquired on a AMX-600 Bruker NMR spectrometer equipped with a 5 mm inverse broad-band probe. The only exception were the gradient-enhanced spectra acquired on an INOVA-600 Varian NMR spectrometer using a 5 mm triple-resonance probe with z gradients. The experimental details are given for each spectrum in the figure captions. 

As is the case in most gradient-enhanced pulse sequences, GROESY spectra should preferably be obtained with non-spinning samples. In our spectrometer, a Bruker ARX-400 equipped with an inverse broadband probehead incorporating a shielded Z-gradient coil, we have used the following experimental parameters ... 

Gradient enhanced versions of the ID HMQC-TOCSY and ID HSQC-TOCSY have also been published recently [31, 55]. 

Fig. 3. Sections of two-dimensional 31P/15N H correlation spectra of the azido-substituted monophosphazene derivative shown. The 2D spectra were recorded by using a conventional 31P/15N HMQC pulse scheme with phase-cycling (left), and the gradient-enhanced enhanced sensitivity HSQC pulse sequence of Fig. 2 (right). The onedimensional spectra on top of the 2D-maps were acquired with the lD-versions of both pulse sequences. The right spectrum is distinguished by a substantially lower artefact level and displays an additional clearly visible correlation of the 31P with nitrogen atom N3. Reproduced from Ref. 25 by permission of Elsevier Ltd.

Even though the NMR receptivity of 31P is substantially lower than that of 19F, 31P-detected 31P/15N HMQC79 and gradient enhanced HSQC correlation... 

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. 

Load the raw data of the gradient enhanced inverse detected "C/ H COSY... 

Hurd RE. Gradient-enhanced spectroscopy. J. Magn. Reson. 1990 87 422-428. 

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