One-dimensional NMR method

In many cases, the analytical tasks are simply to detect and quantify a specific known analyte. Examples include the detection and quantification of commonly used buffer components (e.g., Tris, acetate, citrate, MES, propylene glycol, etc.). These simple tasks can readily be accomplished by using a standard one-dimensional NMR method. In other situations, the analytical tasks may involve identifying unknown compounds. This type of task usually requires homonuclear and heteronuclear two-dimensional NMR experiments, such as COSY, TOCSY, NOESY, HSQC, HMBC, etc. The identification of unknown molecules may also require additional information from other analytical methods, such as mass spectrometry, UV-Vis spectroscopy, and IR spectroscopy.14... 

Numerous organisms, both marine and terrestrial, produce protein toxins. These are typically relatively small, and rich in disulfide crosslinks. Since they are often difficult to crystallize, relatively few structures from this class of proteins are known. In the past five years two dimensional NMR methods have developed to the point where they can be used to determine the solution structures of small proteins and nucleic acids. We have analyzed the structures of toxins II and III of RadiarUhus paumotensis using this approach. We find that the dominant structure is )9-sheet, with the strands connected by loops of irregular structure. Most of the residues which have been determined to be important for toxicity are contained in one of the loops. The general methods used for structure analysis will be described, and the structures of the toxins RpII and RpIII will be discussed and compared with homologous toxins from other anemone species. 

Two-dimensional NMR spectroscopy may be defined as a spectral method in which the data are collected in two different time domains acquisition of the FID tz), and a successively incremented delay (tj). The resulting FID (data matrix) is accordingly subjected to two successive sets of Fourier transformations to furnish a two-dimensional NMR spectrum in the two frequency axes. The time sequence of a typical 2D NMR experiment is given in Fig. 3.1. The major difference between one- and two-dimensional NMR methods is therefore the insertion of an evolution time, t, that is systematically incremented within a sequence of pulse cycles. Many experiments are generally performed with variable /], which is incremented by a constant Atj. The resulting signals (FIDs) from this experiment depend... 

NOESY NMR spectroscopy is a homonuclear two-dimensional experiment that identifies proton nuclei that are close to each other in space. If one has already identified proton resonances in one-dimensional NMR spectroscopy or by other methods, it is then possible to determine three dimensional structure through NOESY. For instance, it is possible to determine how large molecules such as proteins fold themselves in three-dimensional space using the NOESY technique. The solution structures thus determined can be compared with solid-state information on the same protein obtained from X-ray crystallographic studies. The pulse sequence for a simple NOESY experiment is shown in Figure 3.23 as adapted from Figure 8.12 of reference 19. 

H is particularly important in NMR experiments because of its high sensitivity and natural abundance. For macromolecules, 1H NMR spectra can become quite complicated. Even a small protein has hundreds of 1H atoms, typically resulting in a one-dimensional NMR spectrum too complex for analysis. Structural analysis of proteins became possible with the advent of two-dimensional NMR techniques (Fig. 3). These methods allow measurement of distance-dependent coupling of nuclear spins in nearby atoms through space (the nuclear Overhauser effect (NOE), in a method dubbed NOESY) or the coupling of nuclear spins in atoms connected by covalent bonds (total correlation spectroscopy, or TOCSY). 

It is interesting that, in analytical chemistry, besides the efforts to increase the sample throughput and to decrease the detection limits, another trend can be observed which is directed to the analysis of more and more complex mixtures without laborious sample preparation and separation steps. This development was triggered by the requirements of bio- and environmental analysis and is closely connected to the development of multidimensional analytical methods, as well as hyphenated techniques which provide much more selectivity than one-dimensional analytical methods. Among the range of hyphenated techniques, those which combine a high separation efficiency with a maximum of structural information are of particular importance. These are hyphenated techniques such as GC-MS, LC-MS, LC-NMR and LC-NMR-MS. 

In most cases, structural characterization of carbosilane dendrimers is accomplished by multinuclear one-dimensional NMR spectroscopy (1H, 13C and 29Si). However, as larger dendrimers are characterized standard spectroscopic methods become less useful due to the overlap of signals. This problem has been elegantly circumvented as described in a recent paper by Tessier, Rinaldi and coworkers56. In this paper the researchers described the use of 1 H/13C/29Si triple resonance, 3D and pulse field gradient NMR techniques to... 

Note that there are two conformers, one with the two H8 atoms on the same side of the coordination plane (syn isomer) and the other with one H8 atom on each side of the coordination plane (anti isomer). Refine and save both using the MOMEC97 force field. Section 17.14 describes how to enforce planarity of a coordination compound. Two dimensional NMR methods can be used to determine which isomer dominates - as long as interconversion of the isomers is not rapid on the NMR time scale. The data used here are hypothetical and we have assumed that one isomer dominates to the exclusion of the other and that there is no interconversion, i. e., the observed NMR spectrum is that of an isomerically pure compound. 

Satellites are not restricted to 13C, but may be seen with other magnetic nuclei that are present at low abundance when the principal isotope has 1=0. Among the best known are 29Si (8.5%), nlCd and l13Cd (each about 9%), 199Hg (7.6%), and 207Pb (8.9%). Other nuclides, such as 15N, have a natural abundance so low that satellite signals are rarely observed in normal one-dimensional NMR spectra, but with polarization transfer methods described in Chapters 9 and 10, the existence of these weak satellites often permits observation of the less sensitive, low abundance nuclide by indirect detection. 

An illustration of CP-MAS is given in Fig. 7.9. The sharp lines that are obtained permit CP-MAS to be used for structure elucidation of organic and inorganic compounds, in much the same way as liquid state spectra are used. As we see in Chapter 10, two-dimensional NMR methods are applicable to solids and form part of the analytical capability of CP-MAS and CRAMPS. One important application is in combinatorial chemistry, where molecules are synthesized on resin beads. It is feasible to obtain good H and 13C spectra from a single bead. 

The development of new and improved NMR techniques is continual and fast-paced, with the range of systems that are suitable for study and the information that can be elucidated from them increasing aU the time. Recent years have seen a shift away from one-dimensional NMR techniques towards two-dimensional techniques, such as MQMAS, which are often capable of providing higher resolution. In particular, the recent development of the SATRAS technique opens up new possibilities. In addition to the applications outlined above, Antonijevic and coworkers [28] have reported on the use of SATRAS to study host-guest interactions and guest dynamics in framework solids. Nuclei that could be studied by this method include 0, Na, Al and Ga. 

Although solution-state NMR has revealed much regarding the chemical structural composition of humic substances and dissolved organic matter (5), the development of more sophisticated software, pulse sequences, probes in recent years have not been exploited much in the field of soil science. Most of the novel applications have strictly relied on conventional one-dimensional spectroscopy. Two-dimensional NMR methods have become the mainstay of the chemical research field, especially for the structural elucidation of complex soluble biopolymers. The complexity of humic substances along with other inherent physical properties such as limited solubility and the presence of paramagnetics... 

Steady-state, or dummy, scans are used to allow a sample to come to equilibrium before data collection begins. As in a regular experiment, a number of scans are taken, but data are not collected during what would be the normal acquisition time. Steady-state scans are usually performed before the start of an experiment, but, for certain experiments on older instruments, may be acquired before the start of each incremented time value. This technique is not necessary in typical one-dimensional NMR experiments, but is employed in onedimensional methods that involve spectral subtraction (e.g., DEPT Section 7-2b) and virtually all two-dimensional experiments. 

J. K. M. Sandersand B. K. Hunter, Modern NMR Spectroscopy. A Guide for Chemists, Oxford University Press, Oxford, 1987. The authors give a clear introduction to experimental techniques which use the most important one-and two-dimensional NMR methods. 

Two- and three-dimensional or even higher-dimensional NMR spectroscopy is changing from specialised techniques to more commonly used ones. As the complexity of the acquired NMR data increases, the task of analysing these data constantly becomes more and more demanding and new methods are required to facilitate the analysis. With one-dimensional NMR data multivariate data analysis has proven to be a strong tool, but how should one analyse higher-dimensional NMR data in order to extract as much relevant information as possible without having to break data down into smaller dimensions and thus lose the inherent structure A class of... 

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