Three-dimensional NMR

Spectrometric Analysis. Remarkable developments ia mass spectrometry (ms) and nuclear magnetic resonance methods (nmr), eg, secondary ion mass spectrometry (sims), plasma desorption (pd), thermospray (tsp), two or three dimensional nmr, high resolution nmr of soHds, give useful stmcture analysis information (131). Because nmr analysis of or N-labeled amino acids enables determiaation of amino acids without isolation from organic samples, and without destroyiag the sample, amino acid metaboHsm can be dynamically analy2ed (132). Proteia metaboHsm and biosynthesis of many important metaboUtes have been studied by this method. Preparative methods for labeled compounds have been reviewed (133). 

Three-dimensional NMR spectra, like 2D NMR spectra, may be broadly classified into three mtyor types (a) 3D J-resolved spectra (in which the... 

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

Nuclear magnetic resonance (NMR) spectroscopy is a most effective and significant method for observing the structure and dynamics of polymer chains both in solution and in the solid state [1]. Undoubtedly the widest application of NMR spectroscopy is in the field of structure determination. The identification of certain atoms or groups in a molecule as well as their position relative to each other can be obtained by one-, two-, and three-dimensional NMR. Of importance to polymerization of vinyl monomers is the orientation of each vinyl monomer unit to the growing chain tacticity. The time scale involved in NMR measurements makes it possible to study certain rate processes, including chemical reaction rates. Other applications are isomerism, internal relaxation, conformational analysis, and tautomerism. 

The idea of back transformation of a three-dimensional NMR experiment involving heteronuclear 3H/X/Y out-and-back coherence transfer can in principle be carried to the extreme by fixing the mixing time in both indirect domains. Even if one-dimensional experiments of this kind fall short of providing any information on heteronuclear chemical shifts, they may still serve to obtain isotope-filtered 3H NMR spectra. A potential application of this technique is the detection of appropriately labelled metabolites in metabolism studies, and a one dimensional variant of the double INEPT 111/X/Y sequence has in fact been applied to pharmacokinetics studies of doubly 13C, 15N labelled metabolites.46 Even if the pulse scheme relied exclusively on phase-cycling for coherence selection, a suppression of matrix signals by a factor of 104 proved feasible, and it is easily conceivable that the performance can still be improved by the application of pulsed field gradients. 

Figure 15.5. Two-dimensional spectrum produced from an F1-F2 slice through the 3-D HMQC-TOCSY spectrum of a pine forest soil fulvic acid at 1.3 ppm on the F3 (proton) axis (Figure 15.2). Labels on cross-peaks correspond to the C-H structures in the aliphatic structures shown. The full 3D cube is superimposed onto the example slice. Reprinted from Simpson, A. I, Kingery, W. L., and Hatcher, R G. (2003a). The identification of plant derived structures in humic materials using three-dimensional NMR spectroscopy. Environ. Sci. Technol. 37,337-342, with permission from the American Chemical Society.

Simpson, A. J., Kingery, W. L., and Hatcher, P. G. (2003a). The identification of plant derived structures in humic materials using three-dimensional NMR spectroscopy. Environ. Sci. Technol. 37,337-342. 

Basic NMR Experiments—A Practical Course by S. Braun et al,41 describes the operation of an NMR spectrometer and, as its title implies, gives guidance, with specific experimental parameters, for carrying out a variety of NMR procedures—from measuring the width of a 90° pulse to complex pulse sequences in two- and three-dimensional NMR. 

As illustrated in Fig. 4.4, nitrogen chemical shifts cover a range of about 1000 ppm and make 14N and 1SN very useful nuclides for distinguishing structural features. Both nuclides have very low inherent sensitivity, about 10-3 as great as that for H. 14N is over 99% naturally abundant, but it has large quadrupole moment, which often leads to rapid relaxation and very broad lines, as we shall see in Chapter 8. Nevertheless, in many compounds line widths are narrow enough to allow discrimination between chemically shifted environments. 15N has a spin of V2, hence no quadrupole moment, but its natural abundance of less than 0.4% makes direct observation difficult at natural abundance. However, isotopic enrichment and/or the use of indirect detection methods (discussed in Chapter 10) permits relatively facile study of 15N, particularly in two- and three-dimensional NMR. 

FIGURE 12.14 Schematic representation of the data from a three-dimensional NMR experiment shown as a set of planes, as compared with data from a 2D experiment in a single plane. 

Direct and indirect methods are used to make sequence specific assignments. The direct method is to use a number of larger than two-dimensional hetero NMR spectra to generate the heteronuclei-based interresidue backbone atom correlations. One of the schemes of the direct method is shown in Fig. 12. These three-dimensional NMR experiments need not only C- and N-enriched samples, but also very expen-... 

Griesinger, C. Sorensen, O.W. Ernst, R.R. A practical approach to three-dimensional NMR spectroscopy. J. Magn. Reson. 1987, 73, 574-579. 

Eesik, S.W. Zuiderweg, E.R.P. Heteronuclear three-dimensional NMR spectroscopy. A strategy for the simplification of homonuclear two-dimensional NMR spectra. J. Magn. Reson. 1988, 78, 588-593. 

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