3D-NMR

Gaussian pulses are frequently applied as soft pulses in modern ID, 2D, and 3D NMR experiments. The power in such pulses is adjusted in milliwatts. Hard" pulses, on the other hand, are short-duration pulses (duration in microseconds), with their power adjusted in the 1-100 W range. Figures 1.15 and 1.16 illustrate schematically the excitation profiles of hard and soft pulses, respectively. Readers wishing to know more about the use of shaped pulses for frequency-selective excitation in modern NMR experiments are referred to an excellent review on the subject (Kessler et ai, 1991). 

Figure 6.3 Schematic representation of the resolution advantages of 3D NMR spectroscopy, (a) Both pairs of protons have the same resonance frequency, (b) Due to the same resonance frequency, both pairs exhibit overlapping crosspeaks in the 2D NOESY spectrum, (c) When the frequency of the carbon atoms is plotted as the third dimension, the problem of overlapping is solved, since their resonance frequencies are different. The NOESY cross-peaks are thus distributed in different planes.

Archer SJ, Ikura M, Torchia DA, Bax A. An alternative 3D NMR technique for correlating backbone 15N with side chain H 3 resonances in larger proteins. J Magn Reson 1991 95 636-641. 

The 140-residue protein AS is able to form amyloid fibrils and as such is the main component of protein inclusions involved in Parkinson s disease. Full-length 13C/15N-labelled AS fibrils and AS reverse-labelled for two of the most abundant amino acids, K and V, were examined by homonuclear and heteronuclear 2D and 3D NMR.147 Two different types of fibrils display chemical shift differences of up to 13 ppm in the l5N dimension and up to 5 ppm for the backbone and side-chain 13C chemical shifts. Selection of regions with different mobility indicates the existence of monomers in the sample and allows the identification of mobile segments of the protein within the fibril in the presence of monomeric protein. At least 35 C-terminal residues are mobile and lack a defined secondary structure, whereas the N terminus is rigid starting from residue 22. In addition, temperature-dependent sensitivity enhancement is also noted for the AS fibrils due to both the CP efficiency and motional interference with proton decoupling.148... 

The use of NMR continues to improve existing methods, and to develop new concepts. By cleverly combining existing pulse-sequences, new sequences are formed with improved properties. An example is the combination of the COSY and DOSY sequence to a new 3D-NMR COSY-IDOSY sequence with improved sensitivity, a 32-fold decrease in experiment time, and an improved resolution resulting in better data analysis [34]. 

Users of any NMR instrument are well aware of the extensive employment of what is known as pulse sequences. The roots of the term go back to the early days of pulsed NMR when multiple, precisely spaced RF excitation pulses had been invented (17,98-110) and employed to overcome instrumental imperfections such as magnetic field inhomogeneity (Hahn echo) or receiver dead time (solid echo), monitor relaxation phenomena (saturationrrecovery, inversion recovery, CPMG), excite and/or isolate specific components of NMR signals (stimulated echo, quadrupole echo), etc. Later on, employment of pulse sequences of increasing complexity, combined with the so-called phase-cycling technique, has revolutionized FT-NMR spectroscopy, a field where hundreds of useful excitation and detection sequences (111,112) are at present routinely used to acquire qualitatively distinct ID, 2D, and 3D NMR... 

The most frequently used NMR solvents for flavonoid analyses are hexadeuterodimethylsulf-oxide (DMSO-J6) and tetradeuteromethanol (CD3OD). Anthocyanins require the addition of an acid to ensure conversion to the flavylium form. For the analysis of relatively nonpolar flavonoids, solvents such as hexadeuteroacetone (acetone-J6), deuterochloroform (CDCI3), carbontetrachloride (CCI4), and pentadeuteropyridine have found some application. The choice of NMR solvent may depend on the solubility of the analyte, the temperature of the NMR experiments, solvent viscosity, and how easily the flavonoid can be recovered from the solvent after analysis. In recent years, the problem of overlap of solvent signals with key portions of the NMR spectrum has been reduced by solvent suppression and the application of improved 2D and 3D NMR techniques. 

In a chapter regarding 2D NMR spectroscopy of paramagnetic molecules, the obvious perspective is that of using three-dimensional (3D) NMR for paramagnetic molecules. The demand for 3D spectroscopy is based on a need of increased resolution when macromolecules are concerned. It is possible that for small complexes 3D spectroscopy will never be necessary. However, every time something new has appeared in science, the majority has reacted by saying that the utility was scarce in their own field, and the majority has not always been right. Therefore, we do not commit ourselves. 

The analytical NMR flow-cell (see Figure 1.8) was originally developed for continuous-flow NMR acquisition, but the need for full structural assignment of unknown compounds led to major applications in the stopped-flow mode. Here, the benefits of the closed-loop separation-identification circuit, together with the possibilities to use all types of present available 2D and 3D NMR techniques in a fully automated way, has convinced a lot of application chemists [17-70], A detailed description of the different modes for stopped-flow acquisition (e.g. time-slice mode) is found in Chapters 2 und 3. 

One-dimensional 119Sn NMR spectroscopy in solution was used to characterise soluble polymer-supported organotin compounds181,199. To get a more detailed understanding of the microstructure of tin-containing polymers, 1H/13C/119Sn triple resonance 3D NMR... 

Both NMR and molecular computations are greatly enhanced by interactive scientific visualization. High speed 2D and 3D color graphics can provide the investigator with nearly instantaneous feedback. (No table of numbers can show us molecular shapes, or allow us to get perspective on a complex 2D or 3D NMR spectrum). 

The method today is mostly applicable to relatively small molecules of up to 15,000 Daltons. With higher field NMR spectrometers and with practical 3D NMR, 25-30,000 Daltons will generally approach the limit (early- to mid-1990s). Figure 10.2 shows the 2D NMR spectrum of a double helical nucleic acid (MW 7000) while a stereo pair of the structure is shown in Figure 10.3. 

Key Words Field-gradient NMR, Diffusion coefficient, Polymer network, Spatial inhomogeneity of cavities, 3D NMR imaging. 

The bicontinuous structure of the PMMA-rich and PS-Br-rich domains can be non-destructively observed in both the 3D NMR and X-ray CT images as shown in Figure 16. 

The animation video clip for the surface of the phase-separated blend samples and another of the inner cavity of the blend sample can analyse the branch structure for all of the inner cavities. The scale width increases as the thermal treatment time increases. The micrometre-scale phase-separated structure of the PS-Br/PMMA blends is shown in Figure 23. It is seen that phase separation advances with the thermal treatment time. The distribution of the branch structure for a bicontinuous structure obtained from 3D NMR images is shown in Figure 23, where the thermal treatment times for the samples are 6 h for A, 8 h for B and 10 h for C at 180 °C. As seen from this figure, the fraction that the bicontinuous structure takes of the three branches, at each junction point, is more than 50%. The average distribution of the branching number at the junction points is almost independent of the thermal treatment time in the present experiments. 

The log-log plots of the integrated intensity (s(q)) versus (/-space calculated by FT of both the 3D NMR and X-ray CT images of PS-Br/PMMA blend samples are shown in Figure 24A-C, where the thermal treatment times for the samples are 6 h... 

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