Gradient-enhanced spectroscopy

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

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

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. 

A 2002 review by Reynolds and Enriquez describes the most effective pulse sequences for natural product structure elucidation.86 For natural product chemists, the review recommends HSQC over HMQC, T-ROESY (transverse rotating-frame Overhauser enhancement) in place of NOESY (nuclear Over-hauser enhancement spectroscopy) and CIGAR (constant time inverse-detected gradient accordion rescaled) or constant time HMBC over HMBC. HSQC spectra provide better line shapes than HMQC spectra, but are more demanding on spectrometer hardware. The T-ROESY or transverse ROESY provides better signal to noise for most small molecules compared with a NOESY and limits scalar coupling artefacts. In small-molecule NMR at natural abundance, the 2D HMBC or variants experiment stands out as one of the key NMR experiments for structure elucidation. HMBC spectra provide correlations over multiple bonds and, while this is desirable, it poses the problem of distinguishing between two- and three-bond correlations. 

Kay, L. E., Keifer, R, and Saarinen, T. (1992) Pnre absorption gradient enhanced het-eronnclear single qnantum correlation spectroscopy with improved sensitivity. J. Am. Chem. Soc. 114(26), 10663-10665. 

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. 

Eye. - H-detected gradient-enhanced hetronuclear multiple quantum coherence spectroscopy has been used to map the distribution of [l- C]glucose and [l- C]sorbitol in the bovine lens that have been incubated with 50mM [1- C]glucose for 1, 2 or 4 days. The majority of the [l- C]glucose and [l- C] sorbitol was found in the cortex of the lens. ... 

The increasing analytic ability of modern NMR spectroscopy will make use of long-range coupling constants "/( Si,X) (n> 3) for assignment purposes, in particular for X = H, when gradient-enhanced 2D HMBC experiments can be set up to detect Si resonances as a consequence of very small scalar Si- H spin-spin coupling. 

The relaxation interference between the H- N dipolar interaction and the nitrogen-15 CSA is the basis of the transverse relaxation optimized spectroscopy (TROSY), nowadays a standard tool in NMR of larger proteins. Zuiderweg and Rousaki reviewed the field of gradient-enhanced TROSY and described the experiments of this kind in terms of the cartesian product operators. Other midifications of TROSY have also been reported, but are judged to be beyond the scope of this review. 

Tyburn, J. M., Brereton, I. M. and Doddrell, D. M. Coherence selection in gradient-enhanced, heteronuclear correlation spectroscopy.. Magn. Reson. 97 305-312, 1992. 

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

Ruiz-Cabello, J., Vuister, G. W., Moonen, C. T., van Gelderen, P., Cohen, J. S. and van Zijl, P. C. M. Gradient-enhanced heteronuclear correlation spectroscopy. Theory and experimental aspects. /. Magn. Reson. 100 282-303, 1992. 

Judeinstein et al have conducted direct measurement of through-space NMR interactions that provide definitive evidence for spatial proximity of different species. Dipole-dipole interactions can be measured in principle between any NMR active nuclei with heteronuclear correlation experiments in the liquid or solid state." The dipole-dipole interactions decay quickly with the internuclear distances (r ), and are difficult to evaluate for long-range distances and even more difficult when exchange, conformation, or motion phenomena are present. However, the measurement of the nuclear Overhauser method" based on the dipole-dipole-induced crossrelaxation, was proposed to successfully measure intermolecular interactions" and the formation of ion pairs." " In agreement with recent studies, the pulsed field gradient enhanced inverse HOESY (heteronuclear Overhauser enhancement spectroscopy) sequence is usually preferred because it is more sensitive for isotope pairs H- Li and also improves the digital resolution in the H crowded spectrum." ... 

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