A comparison of glycans and polyglycans using solid-state NMR and X-ray powder diffraction

Individual polyglycans and their corresponding monomers have been studied separately for several decades. Attention has focused primarily on the modifications of these polyglycans instead of the simple relationship between the polyglycans themselves and their corresponding monomers. Two polyglycans, chitin and chitosan, were examined along with their respective monomeric units, N-acetyl-D-glucosamine (GlcNAc) and (+)D-glucosamine (GlcN) using solid-state proton decoupling Magic Angle Turning (MAT) techniques and X-Ray Powder Diffraction (XRPD). A down-field shift in isotropic (13)C chemical shifts was observed for both polymers in Cross Polarization/Magic Angle Spinning (CP/MAS) spectra. An explanation of misleading peak assignments in previous NMR studies for these polyglycans was determined by comparing sideband patterns of the polymers with their corresponding monomers generated in a 2D FIve pi REplicated Magic Angle Turning (FIREMAT) experiment processed by Technique for Importing Greater Evolution Resolution (TIGER). Structural changes in the crystalline framework were supported by XRPD diffraction data.     

Analysis of ball-milled Mo powder using X-ray diffraction

X-ray diffraction measurements and analysis were carried out on ball-milled Mo powder. During the ball milling of Mo powder, several stages of deformation could be identified. After short durations of ball milling, still undeformed starting powder was present and the volume fraction of this was determined. The initial aggregates of deformed powder particles exhibited a deformation texture. On prolonged ball milling, the particle size decreased, the deformation texture disappeared and internal strains built up. By simulation and matching of the corresponding line profiles using a Monte Carlo type of line-profile simulation based on a simple three-dimensional model of the distribution of straight dislocations, an estimate of the dislocation density in the ball-milled particles was obtained.     

X-ray resonant powder diffraction

X-ray resonant diffraction can be applied in structural chemistry studies on powder samples. It enables an important limitation of powder diffraction to be overcome. This limitation is related to the low ability of powder diffraction to differentiate elements with close atomic numbers when they occupy the same or close crystallographic sites (mixed occupancy case) and also to discriminate cations with different valence states in different sites. However the resonant effect usually has a second order influence on the measured intensity. As a consequence, the efficiency of this method directly implies the need for excellent quality data collection and has generally been better assessed on elements present in single phase powder samples. In recent years, instrumental developments have been made in synchrotron radiation facilities which allow easier use of resonant powder diffraction for site-specific contrast and valence i.e. oxidation state analyses. Moreover, resonant contrast diffraction tools also have been proposed for better visualization of the anomalous effect both in direct and reciprocal space by using differences between electron density maps or diffraction patterns. Finally the potentialities of this technique for de novo structure solution on macromolecular systems are mentioned.     

Fractal analysis of powder X-ray diffraction patterns

X-ray diffraction (XRD) patterns with broad background are commonly found in the characterization of materials with a certain degree of amorphicity, so the sharp intensity peaks associated with material phases are not well defined. This work used rescaled range (denoted by R/S) analysis, a method intended for fractal analysis of noisy signals, to characterize XRD patterns with broad background. It is found that XRD patterns with broad background are not random at all, but contain information on regularities expressed as autocorrelations of the intensity signal. Sol-gel alumina fired at different temperatures was used as an example to illustrate the applicability of the method. It is shown that fractal R/S analysis is able to locate angular regions that can be associated to ideal International Centre for Diffraction Data Powder Diffraction File (ICDD PDF) lines of diverse alumina phases.     

粉晶X射线衍射在矿物岩石学研究中的应用

文章简单介绍了粉晶X射线衍射在岩石矿物学研究中的主要应用.指出粉晶X射线衍射不仅在矿物的定性/定量分析、晶胞参数测定、类质同像研究、多型研究、有序/无序结构研究、岩组学研究等传统领域继续发挥着重要作用,在矿物结晶过程和相转变研究、矿物表面物相研究、矿物缺陷研究和矿物晶体结构测定等新领域也展现出广阔的应用前景.     

粉晶X射线衍射在矿物岩石学研究中的应用

文章简单介绍了粉晶Ｘ射线衍射在岩石矿物学研究中的主要应用.指出粉晶Ｘ射线衍射不仅在矿物的定性/定量分析、晶胞参数测定、类质同像研究、多型研究、有序/无序结构研究、岩组学研究等传统领域继续发挥着重要作用,在矿物结晶过程和相转变研究、矿物表面物相研究、矿物缺陷研究和矿物晶体结构测定等新领域也展现出广阔的应用前景.     

A comparison of NCO and NCA transfer methods for biological solid-state NMR spectroscopy

Three different techniques (adiabatic passage Hartman-Hahn cross-polarization, optimal control designed pulses, and EXPORT) are compared for transferring (15)N magnetization to (13)C in solid-state NMR experiments under magic-angle-spinning conditions. We demonstrate that, in comparison to adiabatic passage Hartman-Hahn cross-polarization, optimal control transfer pulses achieve similar or better transfer efficiencies for uniformly-(13)C,(15)N labeled samples and are generally superior for samples with non-uniform labeling schemes (such as 1,3- and 2-(13)C glycerol labeling). In addition, the optimal control pulses typically use substantially lower average RF field strengths and are more robust with respect to experimental variation and RF inhomogeneity. Consequently, they are better suited for demanding samples.     

Design and fabrication of a high temperature X-ray powder diffraction camera

A high temperature X-ray powder diffraction camera has been designed and fabricated with indigenous material. The camera design is similar to the Unicam model but overcomes some inconvenient features of the latter. The camera incorporates a miniheater which is inexpensive and easily replaceable. The camera enables determination of lattice parameters with an accuracy of ±0.0002 Å up to 800°C.     

Liquid water structure from X-ray absorption and emission,NMR shielding and X-ray diffraction

Here we investigate to what extent X-ray absorption(XAS) and emission(XES) spectroscopy, the oxygen-oxygen radial distribution function and σ(~1H) and σ(~(17)O) NMR shielding can be represented by a common set of model structures of liquid water. This is done by using a Monte Carlo-based fitting technique which fits the spectra based on a library of ～1400 precomputed spectra and assigns weights to contributions from different model structures. These are then used to reweight the contributions from the structures in the library to reveal classes of structures that are over-or under-represented in the library. The goal is to include different experimental data sets which are sensitive to different aspects of liquid water structure and thus narrow down which types of structures must exist in the real liquid.     

Ab initio X-ray powder diffraction characterisation of molecule-based magnetic materials

Use of ab initio X-ray powder diffraction (XRPD) methods in the study of magnetically active species of covalent nature is presented. Selected cases are chosen in order to underline the power of XRPD methodologies, highlighting the importance of independent physico-chemical information from ancillary techniques.     

Calculation of solid-state NMR lineshapes using contour analysis

Two new methods for calculating lineshapes in solid-state NMR spectra are described. The first method, which we refer to as semi-analytical, allows the rapid calculation of quadrupolar central-transition lineshapes in both static and magic-angle spinning cases. The second method, which is fully numerical, allows the calculation of lineshapes resulting from any combination of interactions, including quadrupolar, dipolar and chemical shift anisotropy, and is not restricted to cases in which the principal axis systems for the different interactions are aligned. Both methods are derived from consideration of the contour lines on a plot of the resonance frequency against the Euler angles, allowing the intensity of the lineshape to be calculated at each frequency. Consequently, highly accurate lineshapes can be calculated more rapidly than previously possible, since only orientations contributing to each specific frequency are considered. For our semi-analytical method, the intensity of each point in the lineshape can be directly calculated in tens of milliseconds on a standard PC. In contrast, established methods can take several hours to calculate the same lineshape.     

Atomic refinement using orientational restraints from solid-state NMR

We describe a procedure for using orientational restraints from solid-state NMR in the atomic refinement of molecular structures. Minimization of an energy function can be performed through either (or both) least-squares minimization or molecular dynamics employing simulated annealing. The energy, or penalty, function consists of terms penalizing deviation from "ideal" parameters such as covalent bond lengths and terms penalizing deviation from orientational data. Thus, the refinement strives to produce a good fit to orientational data while maintaining good stereochemistry. The software is in the form of a module for the popular refinement package CNS and is several orders of magnitude faster than previous software for refinement with orientational data. The short computer time required for refinement removes one of the difficulties in protein structure determination with solid-state NMR.     

Determination of absolute chirality using resonant X-ray diffraction

We have demonstrated that resonant diffraction experiments using the circularly polarized X-ray beam absolutely determines the crystal chirality. Both berlinite and quartz crystals belonging to space group P3₂21 show higher azimuth-constant intensity for the negative (?) helicity beam than that for the the positive (+) helicity beam for space-group forbidden reflection 001. The relation is opposite for quartz crystal belonging to space group P3₁21. Theoretical calculation shows that this relation completely agrees with the experimental findings for the enantiomorphic space-group pair P3₁21 and P3₂21. This method is applicable to chiral motifs that occur in biomolecules, liquid crystals, ferroelectrics, and multiferroics, etc.     

X射线粉晶衍射在新功能材料探索中的一些应用

1912年发现的晶体X射线衍射现象标志着现代晶体学的诞生,对近代物理学乃至整个科学领域的发展都产生了极为深刻的影响。X射线粉晶衍射将结构分析拓展到多晶材料,成为物理学、化学和材料科学重要的研究手段。文章结合作者在硼酸盐及铁基超导体研究领域的典型工作,介绍了X射线粉晶衍射在新型功能材料探索中的一些应用。随着新方法和新技术的不断出现和完善,相信未来X射线粉晶衍射在新材料探索和其他应用领域将起到越来越重要的作用。     

A thermometer for nonspinning solid-state NMR spectroscopy

We discuss a method to determine temperature in a static NMR experiment from the temperature variation of the lead nitrate peak shift.     

用X射线衍射仪验证康普顿效应的实验研究

充分利用多晶X射线衍射仪的衍射、单转、定时计数、叠扫等功能测量数据,采用3种实验测量方法验证了康普顿散射波长的位移公式,分析了不同测量方法对实验结果的影响.