scholarly journals The influence of π-bonding and steric factors on helahalogeno-metallates

1974 ◽  
Vol 27 (3) ◽  
pp. 661 ◽  
Author(s):  
JE Fergusson ◽  
PF Heveldt
Keyword(s):  
Unit Cell ◽  
Cell Length ◽  
Tin Compounds ◽  
Opposite Trend ◽  
Steric Factors ◽  
Unit Cell Length

A comparison of the asymmetric stretching frequency v3 and unit cell length a0 for a series of hexahalogeno-metallates A2MX6 [A = K+, Rb+, Cs+ and NH4+, M = Ru, Os, Sn and Te and X = Cl,Br] has been made. Trends observed for the tellurium and tin compounds may be related to stericfactors whereas possible M → X dπ-pπ bonding may explain the opposite trend found for ruthenium and osmium compounds.

scholarly journals Powder X-ray diffraction of trimethoprim Form I, C14H18N4O3

2020 ◽  
Vol 35 (1) ◽  
pp. 69-70
Author(s):  
Jerry Hong ◽  
Joseph T. Golab ◽  
James A. Kaduk ◽  
Amy M. Gindhart ◽  
Thomas N. Blanton
Keyword(s):  
Powder Diffraction ◽  
Diffraction Data ◽  
Room Temperature ◽  
The United States ◽  
Unit Cell ◽  
Cell Length ◽  
X Ray Diffraction ◽  
X Ray ◽  
Structure Determinations ◽  
Unit Cell Length

Trimethoprim crystallizes in the triclinic space group P-1 (#2) with a = 10.5085(3), b = 10.5417(2), c = 8.05869(13) Å, α = 101.23371(21), β = 112.1787(3), γ = 112.6321(4)°, V = 743.729 Å3, and Z = 2. A reduced cell search in the Cambridge Structural Database yielded three previous structure determinations, using data collected at 100 K, 173 K, and room temperature. In this work, the sample was ordered from the United States Pharmacopeial Convention (USP) and analyzed as-received. The room temperature (295 K) crystal structure was refined using synchrotron (λ = 0.412826 Å) powder diffraction data and optimized using density functional theory techniques. We found similar hydrogen bonding patterns with the previous determinations. In addition, we identified two C–H⋯O hydrogen bonds, which also contribute to the crystal energy. When comparing the previously reported trimethoprim structure determinations, the unit cell length lattice parameters were found to contract at lower temperatures, particularly 100 K. All structures show reasonable agreement, with unit cell length differences ranging between 0.05 and 0.15 Å. The diffraction data for this study were collected on beamline 11-BM at the Advanced Photon Source, and the powder X-ray diffraction pattern of the compound has been submitted to ICDD® for inclusion in the Powder Diffraction File™ (PDF®).

Microhardness, reflectance and unit cell length of pyrites from Swedish base metal ores

1985 ◽  
Vol 107 (1) ◽  
pp. 45-52 ◽  
Author(s):  
Bengt E. H. Loberg ◽  
Milan Háber ◽  
Stig-Björn Westberg
Keyword(s):  
Base Metal ◽  
Unit Cell ◽  
Cell Length ◽  
Unit Cell Length

Analysis of cryo-electron micrographs of HbS fibers

1993 ◽  
Vol 51 ◽  
pp. 556-557
Author(s):  
Michael R. Lewis ◽  
Leon J. Gross ◽  
Robert Josephs
Keyword(s):  
Electron Scattering ◽  
Signal To Noise Ratio ◽  
Unit Cell ◽  
Signal To Noise ◽  
Angular Rotation ◽  
Sickle Hemoglobin ◽  
Negative Stain ◽  
Cryo Electron Microscopy ◽  
Constant Pitch ◽  
Unit Cell Length

We are studying the structure of deoxy-sickle hemoglobin (HbS) fibers by cryo-electron microscopy (cryo-EM). Cryo-EM affords potentially higher resolution than negative staining in part because the electron scattering by unstained protein includes internal structure which may be poorly depicted by an envelope of negative stain. This advantage, however, is partially compromised by a low signal to noise ratio (snr) and low image contrast. Our cryo-EM images were recorded with specimen doses of 7 e-/Å2 at 100 keV, ca. 100 nm defocus, and 39,000 magnification on Kodak SO-163 film.Helical particles usually have a constant pitch. Thus the rotation about the axis is coupled to translation along the length of the particle. HbS fibers, however, have a variable pitch which ranges from 6° to 12° rotation per unit cell length (63 Å). Consequently, the rotation of HbS fibers is partially decoupled from translation and the twist of each unit cell and its relative angular rotation about fiber the axis cannot be determined by particle symmetry.

Robust topological edge states from one-dimensional diatomic chain photonic crystals

2021 ◽  
pp. 2150146
Author(s):  
Yun-Tuan fang ◽  
Xiao-Xue Li ◽  
Li-Xia Yang
Keyword(s):  
Photonic Crystal ◽  
Photonic Crystals ◽  
Edge State ◽  
Unit Cell ◽  
Topological Phase ◽  
Edge States ◽  
One Dimensional ◽  
Diatomic Chain ◽  
Ssh Model ◽  
Unit Cell Length

The Su–Schrieffer–Heeger (SSH) model can occur in a one-dimensional (1D) diatomic chain photonic crystal (PC) in which a unit cell includes two same slabs (atoms). With different intervals of the two slabs, the two combined 1D PCs can support topological edge states in all photonic boundary bandgaps. These topological edge states come from the inversion of topological phase of the bands through the band folding effect. When the sum of the two atom intervals in the two different 1D PCs equals to the unit cell length, these edge state frequencies keep invariant.

High-resolution structure determination by ALCHEMI

1983 ◽  
Vol 41 ◽  
pp. 178-181 ◽  
Author(s):  
Peter G. Self ◽  
Peter R. Buseck
Keyword(s):  
Site Occupancy ◽  
Real Space ◽  
Unit Cell ◽  
Bragg Angle ◽  
Electron Current ◽  
High Resolution Structure ◽  
Beam Incident ◽  
Crystallographic Planes ◽  
Electron Beam Incident ◽  
Resolution Structure

ALCHEMI (Atom Location by CHanneling Enhanced Microanalysis) enables the site occupancy of atoms in single crystals to be determined. In this article the fundamentals of the method for both EDS and EELS will be discussed. Unlike HRTEM, ALCHEMI does not place stringent resolution requirements on the microscope and, because EDS clearly distinguishes between elements of similar atomic number, it can offer some advantages over HRTEM. It does however, place certain constraints on the crystal. These constraints are: a) the sites of interest must lie on alternate crystallographic planes, b) the projected charge density on the alternate planes must be significantly different, and c) there must be at least one atomic species that lies solely on one of the planes.An electron beam incident on a crystal undergoes elastic scattering; in reciprocal space this is seen as a diffraction pattern and in real space this is a modulation of the electron current across the unit cell. When diffraction is strong (i.e., when the crystal is oriented near to the Bragg angle of a low-order reflection) the electron current at one point in the unit cell will differ significantly from that at another point.

The External Shape of Human γ GI Immunoglobulin from Crystal Sections

1971 ◽  
Vol 29 ◽  
pp. 428-429
Author(s):  
L. W. Labaw
Keyword(s):  
Molecular Weight ◽  
Sodium Phosphate ◽  
Osmium Tetroxide ◽  
Unit Cell ◽  
Monoclinic Space Group ◽  
X Ray ◽  
Large Molecular Weight ◽  
Cell Dimensions ◽  
Unit Cell Dimensions ◽  
Crystallographic Axes

Crystals of a human γGl immunoglobulin have the external morphology of diamond shaped prisms. X-ray studies have shown them to be monoclinic, space group C2, with 2 molecules per unit cell. The unit cell dimensions are a = 194.1, b = 91.7, c = 51.6Å, 8 = 102°. The relatively large molecular weight of 151,000 and these unit cell dimensions made this a promising crystal to study in the EM.Crystals similar to those used in the x-ray studies were fixed at 5°C for three weeks in a solution of mother liquor containing 5 x 10-5M sodium phosphate, pH 7.0, and 0.03% glutaraldehyde. They were postfixed with 1% osmium tetroxide for 15 min. and embedded in Maraglas the usual way. Sections were cut perpendicular to the three crystallographic axes. Such a section cut with its plane perpendicular to the z direction is shown in Fig. 1.This projection of the crystal in the z direction shows periodicities in at least four different directions but these are only seen clearly by sighting obliquely along the micrograph.

The orientation of sickle hemoglobin fibers within fascicles

1988 ◽  
Vol 46 ◽  
pp. 400-401
Author(s):  
Christopher A. Miller ◽  
Bridget Carragher ◽  
William A. McDade ◽  
Robert Josephs
Keyword(s):  
Sickle Cell ◽  
Sickle Cell Anemia ◽  
Relative Orientation ◽  
Unit Cell ◽  
Red Cell ◽  
High Ph ◽  
Sickle Hemoglobin ◽  
Polar Crystal ◽  
Helical Configuration

Highly ordered bundles of deoxyhemoglobin S (HbS) fibers, termed fascicles, are intermediates in the high pH crystallization pathway of HbS. These fibers consist of 7 Wishner-Love double strands in a helical configuration. Since each double strand has a polarity, the odd number of double strands in the fiber imparts a net polarity to the structure. HbS crystals have a unit cell containing two double strands, one of each polarity, resulting in a net polarity of zero. Therefore a rearrangement of the double strands must occur to form a non-polar crystal from the polar fibers. To determine the role of fascicles as an intermediate in the crystallization pathway it is important to understand the relative orientation of fibers within fascicles. Furthermore, an understanding of fascicle structure may have implications for the design of potential sickling inhibitors, since it is bundles of fibers which cause the red cell distortion responsible for the vaso-occlusive complications characteristic of sickle cell anemia.

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