Synchrotron X-ray data

Fig. 13 (a) Schematic of a lamellar (LasiRNA) DOTAP/DOPC-siRNA complex. Partial bilayers have been removed, exposing 19 bp siRNAs in the isotropic phase, (b, c) Synchrotron X-ray data of CL-siRNA complexes reveal lamellar (LasiRNA) patterns for DOTAP/DOPC-siRNA complexes (b) and MVL5 DOPC complexes (c). Note the broad siRNA-siRNA correlation peak in (c), between y002 and. Reprinted with permission from [79]. Copyright 2007 American Chemical Society... 

A 0.5 mm diameter Lindemann glass capillary of sample I was mounted on the powder diffractometer at station 2.3 at CCLRC Daresbury Laboratory, Synchrotron Radiation Source and room temperature synchrotron X-ray data were collected. The mean wavelength used was 1.2999 A and data were collected from 6 to 80° 20. The diffractometer operated with a Si(lll) monochromator, parallel foils prior to the detector and a scintillation detector. The sample was spun during data collection to minimise preferred orientation and sampling effects. Data were collected on sample II in a similar manner but at a wavelength of 1.3000 A. 

P. Thompson, D.E. Cox, and J.B. Hastings. Rietveld refinement of Debye-Scherrer synchrotron x-ray data from AI2O3, J. Appl. Cryst. 20, 79 (1987). 

The NMR data may now be used to construct a three-dimensional phase diagram, that is, a plot of the amount of orthorhombic form present as a function of both sorbate and temperature (Fig. 27) [56]. This in turn determines the limiting conditions under which the different structures exist in phase-pure forms. Synchrotron x-ray data can now be collected on exactly these characterized structures. 

Only recently, with the publication by Maron et al. [307], has the x-ray RDF approach to local order in PANI seen its continuation. TTiese authors present RDFs derived from synchrotron x-ray data (2 = 0.502 A) out tos=16 A ( = 47t sin0/A), where the experimental data are gradually matched with theoretical curves from a linked-atom modelling... 

Symmetry-lowering deviations, 19 Synchronization, 751 Synchrotron diffraction, 114 Synchrotron radiation, 12 Synchrotron X-ray data, 62 Synchrotron X-rays, 114 Synergic effect, 814... 

The use of synchrotron X-ray data collected from ramie fibers after ad hoc treatment in NaOH provided a revised crystal-structure determination of mercerized cellulose II at 1 A resolution." The unit-cell dimensions of the P2i monochnic space group are a = 8.10 A, h = 9.04 k,c= 10.36 k,y= 117.1°. As with the regenerated cellulose, the chains are located on the 2i axes of the cell. This indicates that the different ways of preparing cellulose II result in similar crystal and molecular structures. The crystal structure consists of antiparallel chains having different conformations, but with the... 

La and Y are both trivalent elements, but there was uncertainty whether Sc in Sc Cg2 was in the divalent or trivalent state. Synchrotron X-ray data was used to determine tiiat the electron count for the Sc atom is 18.8, which confirmed tiiat Sc is a divalent species donating both s electrons to the fullerene cage. ... 

Fig. 7. A typical X-ray diffraction pattern of the Fepr protein fromZJ. vulgaris (Hil-denborough). The pattern was recorded on station 9.6 at the Synchrotron Radiation Source at the CCLRC Daresbury Laboratory using a wavelength 0.87 A and a MAR-Research image-plate detector system with a crystal-to-detector distance of 220 nun. X-ray data clearly extend to a resolution of 1.5 A, or even higher. The crystal system is orthorhombic, spacegroup P2i2i2i with unit cell dimensions, a = 63.87, b = 65.01, c = 153.49 A. The unit cell contains four molecules of 60 kDa moleculEu- weight with a corresponding solvent content of approximately 48%.

Soft X-ray absorption measurements are done at low-energy synchrotron X-ray facilities such as the UV ring at NSLS or the Advanced Photon Source (APS) at Lawrence Berkeley National Laboratory (LBNL). The beam size is typically 1 mm in diameter. The electron yield data are usually obtained in the total electron yield (EY) mode, measuring the current from a channel electron multiplier (Channeltron). Sometimes a voltage bias is applied to increase surface sensitivity. This is referred to as the partial electron yield (PEY) mode. Huorescence yield (EY) data are recorded using a windowless energy dispersive Si (Li) detector. The experiments are conducted in vacuum at a pressure of 2 X 10 torr. 

Unsubstituted, unsolvated complexes have been observed for all metals, but their low solubility prevents the growth of quality single crystals. This problem was recently overcome with data collected on a powder sample on a synchrotron X-ray source followed by Rietveld refinement.17-19 Not surprisingly, the structures display polymeric chains with alternating metal and Cp units. 

Time-Resolved Crystallography. Time-resolved crystallography (TC) uses an intense synchrotron X-ray source and Laue data collection techniques to greatly reduce crystallographic exposure times. Normal time resolution for X-ray... 

The notion of a common core structure has been further supported by synchrotron X-ray fiber diffraction patterns of several amyloid fibrils the patterns show common reflections in addition to those at 4.7 and 10 A (Sunde et al., 1997). Although these data give some insight into the arrangement of the amyloid fibril core, the exact molecular structure and organization of the proteins making up this common core have yet to be uniquely defined. The inherently noncrystalline, insoluble nature of the fibrils makes their structures difficult to study via traditional techniques of X-ray crystallography and solution NMR. An impressive breadth of biochemical and biophysical techniques has therefore been employed to illuminate additional features of amyloid fibril structure. 

The complexity of the solvable structure strongly depends on the spectral resolution of the diffraction method in use. Structures with about 60 atoms in the asymmetric unit were solved from powder data combining synchrotron X-ray diffraction with refinement from neutron diffraction data from the same material (Morris et al. 1994 Admans 2000). About half of that complexity can be achieved with good laboratory X-ray diffractometers (Masciocchi et al. 1996 Kariuki et al. 1999). Neutron diffraction data can better be used for structure refinement than for structure determination, for the same reason. 

The eomparison of our results (a = 16.70 A, b = 16.92(1) A, c = 5.22 A, a-P=90°, y=119.7°) with the former cell determination of Rius and a recent one refined by him from synchrotron x-ray powder diffraction (a = b = 16.8820(9) A, c = 5.2251(3) A, a=P=90°, y=120°) [7] is quite satisfying. Due to the spike function and some possible misalignment the uncertainty in tilt angle determination is at least 0.5°. Therefore, electron diffraction data will never be as accurate as x-ray powder data. Assuming a correct alignment of the eucentric height we still find an inaccuracy of minimum 0.2 A for direct measured values and of at least 0.5 A for calculated cell distances. 

KcsA crystals suitable for X-ray crystallographic analysis using synchrotron radiation were obtained and the data collected and analyzed for multiple crystals and six different data sets as described in the 1998 Science publication (reference 15). The final KcsA pore structure, including amino acid residues 23 to 119 of the K+ channel, refined to 3.2 A. The X-ray data were deposited in the Protein Data Bank with the accession number 1BL8. 

Table 2.1 shows the crystal structure data of the phases existing in the Mg-H system. Pnre Mg has a hexagonal crystal structure and its hydride has a tetragonal lattice nnit cell (rutile type). The low-pressure MgH is commonly designated as P-MgH in order to differentiate it from its high-pressure polymorph, which will be discussed later. Figure 2.2 shows the crystal structure of p-MgH where the positions of Mg and H atoms are clearly discerned. Precise measurements of the lattice parameters of p-MgH by synchrotron X-ray diffraction yielded a = 0.45180(6) mn and c = 0.30211(4) nm [2]. The powder diffraction file JCPDS 12-0697 lists a = 0.4517 nm and c = 0.30205 nm. The density of MgH is 1.45 g/cm [3]. 

Wikoff, W. R., Schildkamp, W. and Johnson, J. E. (1997). Increased resolution data from a large unit ceU crystal collected at a third-generation synchrotron X-ray source. Acta Crystallogr. D 56, 890-893. 

The five room-temperature and two liquid-nitrogen temperature data sets obtained in these experiments were reanalyzed by Cummings and Hart (1988). Where necessary, they introduced improved corrections for residual strain, resonance, and nuclear scattering to arrive at a set of mean structure-factor values with typical errors of 3-5 millielectrons. This is extremely good, but as noted, for some reflections the errors are significantly larger than the 0.1% claimed earlier. Additional accurate values for individual reflections are available for y-ray (Alkire et al. 1982), X-ray (Roberto and Batterman 1970, Trucano and Batterman 1972), and synchrotron X-ray (Tischler 1983, Tischler and Batterman 1984), measurements. Combined, these measurements provide a superbly accurate set for analysis of the charge density in silicon. 

---