Spin-1 nuclei improving resolution

New techniques for data analysis and improvements in instrumentation have now made it possible to carry out stmctural and conformational studies of biopolymers including proteins, polysaccharides, and nucleic acids. NMR, which may be done on noncrystalline materials in solution, provides a technique complementary to X-ray diffraction, which requires crystals for analysis. One-dimensional NMR, as described to this point, can offer structural data for smaller molecules. But proteins and other biopolymers with large numbers of protons will yield a very crowded spectrum with many overlapping lines. In multidimensional NMR (2-D, 3-D, 4-D), peaks are spread out through two or more axes to improve resolution. The techniques of correlation spectroscopy (COSY), nuclear Overhausser effect spectroscopy (NOESY), and transverse relaxation-optimized spectroscopy (TROSY) depend on the observation that nonequivalent protons interact with each other. By using multiple-pulse techniques, it is possible to perturb one nucleus and observe the effect on the spin states of other nuclei. The availability of powerful computers and Fourier transform (FT) calculations makes it possible to elucidate structures of proteins up to 40,000 daltons in molecular mass and there is future promise for studies on proteins over 100,000... 

Two-dimensional (2D) spectroscopy is used to obtain some kind of correlation between two nuclear spins 7 and J, for instance through scalar or dipolar connectivities, or to improve resolution in crowded regions of spectra. The parameters to obtain 2D spectra are nowadays well optimized for paramagnetic molecules, and useful information is obtained as long as the conditions dictated by the correlation time for the electron-nucleus interaction are not too severe. Sometimes care has to be taken to avoid that the fast return to thermal equilibrium of nuclei wipes out the effects of the intemuclear interactions that are sought through 2D spectroscopy. 

Although is 99.63% abundant, by contrast with the spin- /2 isotope N (0.37 % abundance), most of the nitrogen studies reported to date have used N even though the latter usually involves the preparation of isotopically enriched samples. Nevertheless, the prospect of making natural abundance studies is attractive, and several ingenious methods have been developed to improve the resolution of N NMR and overcome some of the drawbacks with this nucleus. These are ... 

Initially, NMR spectroscopy studies were difficult to perform as the nuclide is inherently less sensitive ( 100-fold) than (Table 1). Fm hermore, with a nuclear spin of unity the signals were broader and resolution problems were common. In addition, the dynamic range (100% natural abimdance) is much smaller than is the case for H. Howevei by now, the tremendous improvements that have been achieved in spectrometer performance are such that is now an attractive nucleus for NMR studies. Thus, spectra can be obtained in a short time interval at the natural abimdance level so that for many applications it is not necessary to spend time synthesizing deuteriated compounds. Second, the small differences that exist in the natural abundance levels of different compounds, and of even different hydrogens within the same compound, can be exploited. Finally, the development of magic angle spinning (MAS), which reduces dipolar and other anisotropic interactions, makes it possible to produce NMR... 

The element aluminum-27 occurs at 100% natural abundance and its nucleus has a good receptivity, though its quadrupole moment means that its resonances are broadened relative to those of spin-1/2 nuclei. However, this amounts to only a few hertz or tens of hertz in many cases, with the result that Al has been considered worthy of study from the early days of NMR spectroscopy and has already been the subject of three reviews " and figures in a fourth, and another is to appear. Historically, the first observations were carried out in the derivative mode and with poor quality magnets, which were deemed sufficient for a quadrupolar nucleus but as increasingly improved instrumentation became available, high-resolution absorption... 

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