High-resolution solid-state NMR methods

The sensitivity of the - C NMR experiment is limited by the often very long spin-lattice relaxation times of the - C nuclei. Values of T of over 1000 s have been measured in solid poly(ethylene). It is likely that - C nuclei involved in rigid cross-link structures also possess very long C T relaxation times. As a result pulse repetition times, and hence total scan times are often prohibitively long, especially when the aim is to observe peaks due to low concentrations of products of irradiation. The technique of cross-polarization reduces this problem since the spin temperature of the protons and not the - C nuclei has to reequilibrate before pulse repetition. The H Ti in proton-rich condensed systems is usually much shorter than T. In addition the signal-to-noise in the CPMAS spectrum is increased by a factor of up to four compared with spectra obtained by direct excitation of the spins, due to the larger Boltzmann population of the proton nuclei. 

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J.P. Amoureux, C. Morais, J. Trebosc, J. Rocha, C. Fernandez, I-STMAS, a new high-resolution solid-state NMR method for half-integer quadrupolar nuclei, SoHd State Nucl. Magn. Reson. 23 (2003) 213-223. 

Site-specific longitudinal relaxation rates have been measured for the microcrystalline dimeric form of the protein Crh using multidimensional high-resolution solid-state NMR methods. " The measured rates were used to provide a qualitative description of the site-specific internal mobility of the protein present in the solid state. 

While single crystals of natural minerals readily lend themselves to conventional methods of structural investigation, synthetic zeolites, which are almost always microcrystalline, must of necessity be studied by the less powerful powder X-ray diffractometry. The development of high-resolution solid-state NMR techniques was therefore very timely. 

The resurgence of interest in the field of zeolite chemistry, which has been stimulated by the appreciation of their enormous potential as catalysts, has led to the application of several sophisticated physical methods in the study of their structural properties. Important advances have already been made using high resolution, solid state NMR (1,2) and electron microscopy (3), and in this paper we discuss the scope and limitations of neutron diffraction studies with powder samples, with some specific applications to zeolite-A and synthetic faujasite. 

The MQMAS has now become the most widespread method for the acquisition of high-resolution solid-state NMR spectra of quadmpolar nuclei. The MQMAS step has been incorporated into the other pulse sequences focusing on achieving the correlation between the quadmpolar and spin-1/2 nuclei (i.e., HETCOR-MQMAS). ... 

The recently developed high-resolution solid-state NMR technique proton CRAMPS NMR has become a very useful research tool, corresponding to X-ray crystallography, and enabling the study of crystal structure polymorphs of amino acids. In this chapter, we first discuss a recent research example application to crystal structure analysis of polymorphic forms of some typical a-amino acids in order to test the power of H CRAMPS NMR, compared with C and N NMR methods. 

On the other hand, high resolution solid-state NMR has been available to analyze solid structures for approximately 20 years. This is a very powerful method to analyze the structures of solids, and can reveal conformation, crystallographic forms and the morphological character of solid without special sample preparations. It is expected that the information obtained from solid-state NMR will show how the mechanical properties of the solid arise. 

High resolution solid-state NMR spectroscopy is also a very powerful method for characterizing the solid structure and the local motion of different solid polymers. We recently characterized the crystalline-noncrystalline structure for different crystalline and liquid crystalline polymers, such as polyolefins [7-12], polyesters [13-15], polyether [16], polyurethanes [17, 18] and polysaccharides, including cellulose [19-29], amylose [30, 31] and dextran [32]. On the basis of these analytical methods, we also investigated the intra- and intermolecular hydrogen bonds of PVA in both crystalline and noncrystalline regions as well as in the frozen solution state. In this chapter. 

Kobayashi et al. [16] studied the PVA gels by making high resolution solid-state NMR experiments with the CP/MAS and PST/MAS methods. The degree of polymerization and the degree of saponification of the PVA they employed were 1700 and 99.9%, respectively. The PVA gel was prepared from PVA/water solution (9% wt/wt) by four repeating freeze-thaw cycles (frozen at -20°C for 20 h, and then melted and sustained at 25°C for 4h). The PVA gel samples with different polymer concentrations were prepared by evaporation of water from them. 

For this purpose, it has been demonstrated that the high resolution solid-state NMR approach provides one with an alternative and convenient means to distinguish a variety of crystalline polymorphs and to reveal the secondary structures of biological macromolecules, because the chemical shifts of backbone carbons are displaced (up to 8 ppm) [1, 2] depending on their respective conformations. In addition, it is emphasized that this type of empirical approach can be used as a very valuable constraint to construct the three-dimensional structure of biological molecules, such as peptides and proteins, based on a set of accurately determined interatomic distances measured by a partial dipolar recoupling method, such as REDOR (rotational echo double resonance) [3-6]. 

An overview of high-resolution solid state NMR applications to polypeptides and membrane proteins has been presented by Luca et a/. The importance of the MAS based techniques at ultrahigh magnetic fields for the studies of insoluble or noncrystalline molecules at the atomic level is highlighted. Recently developed NMR methods suitable for the study of multiply or uniformly [ C, N]-labelled polypeptides and proteins are discussed. In addition, latest biophysical applications are reviewed. 

Fig. 38. High-resolution solid-state NMR spectra of the [3- C]Ala-labeled PLC-51 PH domains forming complex with PC/PIP2 vesicles obtained by (A) the CPMAS and (B) the DDMAS method. Assignments of the individual signals are shown at the top of the spectra. The vertical bars at the bottom of the spectra indicate the chemical shifts of the [3- C]Ala signals for the PLC-(51 PH domain-IPs complex in solution.

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