Zigzag Configuration Research Articles

Morphogenesis of the giant sperm axoneme in Asphondylia ruebsaameni Kertesz (Diptera, Cecidomyiidae)

The formation of the sperm giant axoneme of the gall-midge fly Asphondylia ruebsaameni is described here. The axoneme consists of a great number of microtubular doublets (up to 2500) arranged in a double spiral wrapping around an axial cluster of mitochondria. Each microtubular doublet is provided with an outer arm only. In the early spermatid the occurrence of a large system of curved multi-layered filamentous material associated with membranous cisternae has been observed in the perinuclear region. Such a system extends throughout the cytoplasm to contact the plasma membrane. The filamentous material appears to act as a nucleating centre for the assembly of the microtubular doublets, which initially have a submembranous location and later are distributed in the interior of the cell. After their assembly, microtubular doublets are associated pairwise and are arranged in a single microtubular row with a zig-zag configuration. This configuration changes during spermiogenesis as a consequence both of a rotation of the microtubular doublet pairs and a compaction of the axonemal complex due to the elimination of the excess cytoplasm. As a result of this process, a double parallel spiral of microtubular doublets is formed.

Assessment of a Polyester-covered Nitinol Stent in an Atherosclerotic Swine Model

To evaluate the short-term patency and healing characteristics of the Cragg EndoPro covered stent in an atherosclerotic model as one of the Food and Drug Administration requirements before possible approval of the device for human use in the United States. Seventeen self-expanding stent-grafts were placed in the aorta and right and left iliac arteries of six Yucatan microswine exposed to a regimen of accelerated atherosclerosis. The stent-grafts were constructed from nitinol wire formed into a tubular zigzag configuration. The stent frame was covered by a thin woven polyester fabric tube, with medium permeability, available in multiple diameters and lengths. The animals were killed at 24 hours, 3 months, and 6 months. Assessment was done by angiography and histology. All stents were patent immediately after deployment. Two stents were occluded at follow-up, indicating an 88% patency rate. Minor lumen narrowing was found at the follow-up intervals. Histologic examination revealed a mixture of mature and immature endothelial cells lining both the native and stented regions in all vessels examined. The new endoluminal surface was composed primarily of fibrocollagen and elastic fibers and smooth muscle cells. Intimal thickness was inversely correlated to medial thickness. Medial compression with atrophy was observed routinely with rare necrosis. Complete tissue ingrowth was seen by 3 months. The degree of vascular inflammation increased over time, as well as the foreign body giant cell reaction to the polyester fabric. The patency rate does not appear to be better than that with angioplasty or noncovered stent placement in the studied time frame. The progressive vascular inflammatory changes noted should be further investigated in longer-term animal trials to ensure its safety in humans because this device is meant to be permanent.

The consequences of a non-uniform tension across kinetochores: lessons from segregation of chromosomes in the permanent translocation heterozygote Oenothera.

The alternate (zigzag) configuration of the chromosome ring in oenotheras fulfills the requirement of high tension across kinetochores for stability of the configuration and the progression to anaphase. However, also semialternate configurations (two pairs of adjacent kinetochores interspaced among the zigzag) fulfill the requirement of high tension across kinetochores. If only the magnitude of tensile force acting on a kinetochore pair governs the stability of microtubule attachments, the probability of occurrence of the semialternate configurations would be higher than that of fully alternate configurations. Yet the percentage of irregularity in the zigzag configuration is surprisingly low, which means that the semialternate configurations are corrected. The only difference which distinguishes the fully alternate and the semialternate configurations with respect to the tension across kinetochores is that the tension across a kinetochore alternating with its neighbors is rather uniformly distributed over the kinetochore, while there is a gradient of the tension in the kinetochore having a non-alternating neighbor, with low tension on the side of this neighbor. Apparently, a low tension across a part of a kinetochore brings about correction of its attachment to microtubules. This hypothesis fits with the repeat subunit model of the kinetochore; apparently, each subunit can function autonomously in the tension-governed mechanisms, stabilizing its attachment and controlling the metaphase-to-anaphase transition.

REMARKS ON THE ZIG-ZAG THEOREM

The classical Zig-zag Theorem [1] says that if an equilateral closed 2m-gon shuttles between two given circles of the Euclidean 3-space, then the vertices of the polygon can be moved smoothly along the circles without changing the lengths of the sides of the polygon. First we prove that the Zig-zag Theorem holds also in the hyperbolic, Euclidean and spherical n-spaces, and in fact the circles can be replaced by straight lines or any kind of cycles. In the second part of the paper we restrict our attention to planar zig-zag configurations. With the help of an alternative formulation of the Zig-zag Theorem, we establish two duality theorems for periodic zig-zags between two circles.

Control of Hierarchical Order in Crystalline Composites of Diblock Copolymers and a Molecular Chromophore

Addition of 2-chloro-4-nitroaniline (CNA) to diblock copolymers of poly(ethylene oxide) (PEO) and polystyrene (PS), poly(ethylethylene) (PEE), or poly(ethylenepropylene) (PEP) results in selective partitioning of CNA into the polar PEO domains. Calorimetry, infrared spectrosocpy, density measurements, and wide-angle X-ray diffraction support the formation of a crystalline molecular complex, comprising two ethylene oxide repeat units per one CNA. The structure of the complex is the same for PEO homopolymer and PEO-based diblocks. Wide-angle X-ray diffraction from uniaxially aligned samples of the PEO:CNA suggests a triclinic unit cell for the complex with a = 9.08 Å, b = 10.48 Å, c = 7.01 Å; and α = 90.98°, β = 88.38°, and γ = 116.72°. The data are consistent with a structural model in which the PEO chains adopt a nominally all-trans zigzag configuration, the chains organized as (100) layers separated by layers of one-dimensional stacks of CNA molecules. Polarized infrared measurements indicate that the molecular planes of the CNA molecules are nominally perpendicular to the PEO chains. The metrics associated with the zigzag PEO configuration appear to allow for optimal hydrogen bonding between the PEO oxygen atoms and the amine protons of the CNA chromophores as well as hydrogen bonding between CNA molecules in adjacent stacks. The arrangement of the CNA molecules in the crystalline PEO:CNA complex differs from the structure of bulk CNA, clearly indicating that host−guest interactions play a major role in chromophore alignment. Small-angle X-ray scattering from a series of samples reveal changes in the block copolymer microstructure as the effective volume of the PEO block is altered by the inclusion of CNA. Whereas the SAXS data for PS-PEO and PEP−PEO copolymers used here reveal hexagonally packed cylinder microstructures in which cylinders of the minority PEO block are surrounded by PS or PEP, the lamellar microstructure is observed for all three block copolymers at the composition of the 2:1 complex. Consequently, these materials can be described as rigid crystalline molecular complexes embedded in robust, ordered polymer microstructures. This control of hierarchical order over length scales, spanning several orders of magnitude, suggests a route to permanent macroscopic ordering of functional molecules, a desirable feature for applications such as optoelectronics. The conformational rigidity associated with these systems offers considerable advantages for the design of SHG materials as entropically driven disordering is inhibited compared to noncrystalline polymer−chromophore materials.

Lateral growth of focused ion beam deposited platinum for stencil mask repair

Focused ion beams can repair conventional lithography masks through milling and deposition. Repairs to stencil masks for ion projection lithography present a greater challenge than conventional mask repair, as the standard model of ion induced deposition requires a surface for deposition to take place. In a stencil mask, a bridge must be formed across the open area, anchored to the mask on either side of a slot. Focused ion beam deposition can induce such horizontal growth of material. Ion-induced growth of platinum over slots up to 3 μm wide in a 2.75 μm silicon membrane is demonstrated for an average beam current density of 7 mA/cm2. The primary method used to accomplish this is by scanning the focused ion beam in a rectangular pattern that extends over the slot. Material is built up from the edges, and grows inwards to span the slot and form a bridge. The direction of gas flow is an important consideration. When gas flow is perpendicular to the slot, the downstream side of the bridge will grow much faster than the upstream side. Gas flow parallel to the slot results in uniform growth from both sides. Platinum deposition was not found to be gas flow limited within the deposition parameters used. The horizontal growth as a function of time was measured, and found linear. The slope of this line is the horizontal growth rate, 23±0.05 nm/s. The x intercept of this line yields the minimum time for deposition to begin, which is 5.96±0.11 s. Sample bridges were deposited, and trimmed of excess material and redeposited platinum. These are presented as examples that uniformly wide bridges may be obtained at submicron dimensions, both in simple spanning and zig-zag configurations.

Syntheses and crystal structures of cdta ( trans-cyclohexane-1,2-diamine– N, N, N′, N′-tetra-acetate)-based copper(II) dimers and polymers

Four cdta ( trans-cyclohexane-1,2-diamine– N, N, N′, N′-tetra-acetate)-based copper(II) compounds, [(H 2cdta)Cu(4,4′-bpy)Cu(H 2cdta)]·4,4′-bpy·2H 2O ( 1), [(H 2cdta)Cu(pyz)–Cu(H 2cdta)]·14H 2O ( 2), [Cu(cdta)Cu(H 2O) 3(4,4′-bpy)] n ·8 nH 2O ( 3) and [{Cu(cdta)(H 2O) 2} 2–(pyz)] n ·4 nH 2O ( 4), were obtained from a Cu 2+–diimine–cdta system (4,4′-bpy=4,4′-bipyridine and pyz=pyrazine). The X-ray crystallography analysis shows that 1 and 2 are diimine-bridged dimers, 3 has an infinite zigzag configuration and 4 contains a `step ladder'-like structure.

Ordering of Octahedral Vacancies in Transition Aluminas

The microstructure of transition aluminas obtained via the dehydration of boehmite has been characterized by using transmission electron microscopy (TEM). The presence of γ‐, δ‐, and θ‐aluminas was identified by using selected‐area electron diffraction. Modifications that resulted from the reordering of aluminum vacancies on octahedral sites in a cubic close‐packed oxygen network have been detected and analyzed by using high‐resolution transmission electron microscopy (HRTEM) combined with image simulations. A good correspondence of the observed and calculated images confirmed the ordering of vacant octahedral sites located on {011} and {011} planes that formed a zigzag configuration along the <010> direction. Two more arrangements of empty octahedral sites, but now concentrated on {001} planes, have been determined in the sintered powder‐gel agglomerates. Structure analysis suggested that the modifications are all associated with the rearrangement of vacant sites during the phase transformation from gamma‐alumina to δ‐alumina, and further to theta‐alumina, and may be driven by configuration entropy minimization.

Observations of grain boundary structure in submicrometer-grained Cu and Ni using high-resolution electron microscopy

Submicrometer-grained (SMG) structures were produced in Cu and Ni using an intense plastic straining technique, and the grain boundaries and their vicinities were observed by high-resolution electron microscopy. The grain boundaries exhibited zigzag configurations with irregular arrangements of facets and steps, and thus they were found to be in a high-energy nonequilibrium state. A similar conclusion was reached earlier for SMG Al–Mg solid solution alloys which have much lower melting points than Cu and Ni, suggesting that nonequilibrium grain boundaries are a typical feature of metals processed by intense plastic straining.

Boron nitride nanotubes

Pure boron nitride (BN) nanotubes have been synthesised by arc discharge between HfB 2 electrodes in a nitrogen atmosphere. The high resolution electron microscopy (HREM) observations reveal that this route leads to the formation of highly crystalline tubes with reduced numbers of layers, including tubes with only one or two layers. These nanotubes are found to be chiral or non-chiral, however, a preference towards the armchair and zig-zag configurations is suggested. Electron energy loss spectroscopy yields a B:N ratio of approximately one and a perfect chemical homogeneity. Tubes are empty and closed at their ends by flat layers perpendicular to the tube axis. A tip model consisting of a triangular facet based on three 120 ° disclinations and preserving BN bonds in the honeycomb network accounts for the observations. Finally, preliminary electron irradiation experiments on these tubes reveal a specific behaviour. BN tubes transform into aggregates of small cages with diameter between 0.5 and 0.8 nm. These diameters suggest that these shells might be B 12N 12, B 16N 16 and B 28N 28 fullerenes which were predicted to be magic clusters.

Benign and malignant esophageal strictures: treatment with a polyurethane-covered retrievable expandable metallic stent.

To evaluate the clinical effectiveness of a polyurethane-covered, retrievable, self-expandable metallic stent and hook catheter in the treatment of esophageal strictures. Stents were constructed of 0.4-mm stainless steel wire in a cylindric zig-zag configuration of six to nine bends. Four to eight stents were connected in tandem by dipping in a polyurethane solution. A nylon loop was hooked inside to each bend of the proximal portion of the stent and strung with a thread. Under fluoroscopic guidance, 22 stents were placed in 16 patients with a malignant stricture and five patients with a benign stricture. The stent was removed with a hook catheter 2 months after placement in patients with a benign stricture and when complications occurred in patients with a malignant stricture. All patients had dysphagia with ingestion of soft foods or liquids. Stent placement was technically successful and well tolerated in 20 patients. In one patient, the stent was misplaced but relocated successfully. After stent placement, all patients were able to ingest solid and/or soft foods without dysphagia. After stent removal, strictures showed improvement but recurred in two patients. Use of polyurethane-covered, retrievable expandable stents seems to be a feasible and effective method of treatment of benign and malignant esophageal strictures.

Polyoxyethylene intercalation in lamellar copper iron sulfide

The insertion of polyethyleneoxide in a lamellar copper iron sulfide of formula CuFeS 2 by an exchange reaction of potassium from KCuFeS 2 with an aqueous solution of the polymer was performed. The characterization of the new compound by X-ray diffraction and IR absorption spectroscopy shows that the layer structure of the precursor has been retained and that the polymer adopts a zigzag configuration between the layers.

Structural and electronic properties of pentagon-heptagon pair defects in carbon nanotubes.

General features of the energetics and electronic states of carbon nanotubes, containing pentagon-heptagon pair (5/7) topological defects in the hexagonal network of the zigzag configuration, are investigated within simple tight-binding models. This 5/7 pair defect on the nanotube structure is not only responsible for a change in nanotube diameter, but also governs the electronic behavior around the Fermi level. Furthermore, nanotubes having different helicities and different electronic properties can be connected using such a pair. Various configurations including one, two, and three 5/7 pairs are explored in order to understand the influence of several defects on the electronic properties of the system. Topological aspects associated with the presence of 5/7 pair defects in armchair and chiral nanotubes are also discussed. \textcopyright{} 1996 The American Physical Society.

Microscope studies of the morphology and structure of carbon nanotubes

We have studied the morphologies and structures of carbon nanotubes (bucky-tubes) and carbon nanoparticles (buckyonions) using scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and scanning tunnelling microscopy (STM). By SEM the carbon nanotubes are observed with features similar to those of some fibrous whiskers grown from pyrolytic graphite. This growth feature is supported by transmission electron microscopy (TEM) observations. The TEM results show also that the graphitic sheets can be bent into curved shapes to cap the nanotubes or form the onions. In the curved graphitic sheets elastic strains induced by layer mismatches and dislocations are revealed. The STM observations on the nanotubes show a bundle-like morphology of the carbon nanotubes, and by atomic resolution images the zigzag and armchair atomic configurations may be identified. The results also show structural distortions which may be produced by folding the graphite sheets to create the nanotubes and are responsible for the lattice mismatch.

Complete obstruction of the nasolacrimal system. Part II. Treatment with expandable metallic stents.

To evaluate the clinical usefulness of expandable metallic stents for maintenance of the dilated nasolacrimal sac and duct, double stents were implanted under fluoroscopic guidance in seven patients with severe epiphora after failed balloon dacryocystoplasty. Stents were constructed of 0.25-mm stainless steel wire in a cylindrical zigzag configuration of 10 bends. A single stent was 4 mm in diameter when fully expanded and 10 mm long. The stent was placed accurately in all but one patient, in whom a second stent was placed accurately after removal of the dislodged stent with a hemostat under endoscopic guidance. After stent placement, all patients demonstrated complete resolution of epiphora. During follow-up of 4-20 weeks, complete blockage of the stent occurred in one patient. This was treated with intrastent balloon dacryocystoplasty. No stents migrated. Although the long-term validity of this treatment has not been proved, expandable metallic stents seem to be of value in the treatment of complete obstruction of the lacrimal sac and nasolacrimal duct.

Neutron powder diffraction study of the crystal structure of YSr 2CoCu 2O 7 and Y 1− xCa xSr 2CoCu 2O 7

The structures of YSr 2CoCu 2O 7 and Y 1− x Ca x Sr 2CoCu 2O 7 have been analyzed by neutron powder diffraction techniques. Both materials crystallize with the symmetry of space group Ima2. The lattice parameters are a=22.7900(5), b=5.4516(1), c=5.4097(1)Å for YSr 2CoCu 2O 7, and a=22.8266(7), b=5.4312(2), c=5.4048(2) Å for Y 1− x Ca x Sr 2CoCu 2O 7. In the undoped compound, the Co ions exclusively substitute for the copper ions located on the chain sites of the 123 parent structure (YBa 2Cu 3O 6+ x ). The coordination of Co is tetrahedral. The CoO 4 tetrahedra form chains running with a zig-zag configuration along the c-axis of the structure. The oxygen atoms of the CoO layers were found to be disordered over two positions. A re-analysis of the compound YSr 2GaCu 2O 7 showed that this type of disordering is much less pronounced in this material. The disorder of the CoO 4 tetrahedra results in the coexistence of two types of chains. In the structure of the doped compound Y 1− x Ca x Sr 2CoCu 2O 7, calcium substitutes for yttrium, but cobalt replaces copper both on the chain sites and in the CuO 2 planar sites, thus explaining the lack of superconductivity in this material. The oxygen atoms on the Co layers are more disordered that in the undoped material.

Structure of 1,3-propanediammonium tetrachlorocobaltate(II)

[CoCl4(C3H12N2)], M(r) = 276.87, monoclinic, P2(1)/n, a = 10.703 (2), b = 10.653 (1), c = 10.852 (2) angstrom, beta = 118.46 (1)-degrees, V = 1087.8 angstrom 3, Z = 4, D(x) = 1.69 g cm-3, lambda(Mo K-alpha) = 0.71073 angstrom, mu = 22.60 cm-1, F(000) = 556, T = 298 K, final R = 0.059 for 1068 unique reflections [I > 3-sigma(I)]. The Co(II) ion is coordinated by four Cl atoms in a tetrahedral geometry. The paraffinic chains which bridge the tetrahedra have a nearly planar zigzag configuration.

Force constant calculations of the LAM-1 modes ofn-alkanes and their substituted ?-Monohalo- and ?,?-dihalo- species in their extended zig-zag planar configurations as found in urea inclusion compounds

Force constants were determined for the C8, C10, C12 and C14 series ofn-alkanes C n H2n + 2 using an approximate SVFF calculation and observed LAM = 1 wave numbers. In this calculation the hydrogen atoms were neglected and only the carbon backbone chain and terminal atoms were considered; this was valid since only low-frequency vibrations were under consideration. Using force constant transfer, the wavenumbers of the LAM = 1 accordion modes for the analogous α-Cn H2n + 1 X and α,ω-Cn H2nX2 species, where X = C1, Br or I were calculated. For α-chloroalkanes and α,ω-dichloroalkanes, them = 1 accordion modes are calculated to be in the 220–130 cm−1 and 200–120 cm−1 regions, respectively. For the bromo- and iodo-analogues them = 1 accordion modes are calculated to be in the 200–100 cm−1, 150–90 cm−1 and in the 170–100, 135–80 cm−1 regions, respectively.

The crystal structure of dimethylphenyltin(IV) acetate

Dimethylphenyltin(IV) acetate, (CH 3) 2(C 6H 5)SnOC(O)CH 3, crystallizes in the orthorhombic space group Pbca with a 10.068(3), b 15.065(7), c 14.976(5) Å; Z = 8; V 2271(2) Å 3. The structure was determined from 1793 observed out of 2348 unique reflections and refined on an R factor of 0.034. The geometry at tin is trigonal bipyramidal, with the oxygen atom of the acetate and the carbonyl oxygen of the adjacent symmetry-related ( 1 2 + x, 1 2 − y, 1 − z) acetate occupying the axial positions of the trans-C 3SnO 2 polyhedron to result in a polymer whose backbone, formed by the repetition of SnOC(O) fragment, adopts a conformation that is closer to the zig-zag configuration of trimethyltin acetate than to the helical form of triphenyltin acetate.

Buckling of Three‐Dimensional Rigid‐Link Models

Two‐dimensional rigid‐link models have been used often in the study of elastic stability to provide insight into the behavior of engineering structures; the buckling modes of such models take only planar shapes. Many engineering structures, however, possess three‐dimensional buckling characteristics, so it is important to have models with three‐dimensional buckling characteristics. A three‐dimensional three‐link model that initially assumes either a straight‐line or a planar zigzag configuration is studied and both in‐plane and out‐of‐plane buckling modes are found in each case. The model is formulated in terms of the total potential energy and differentiation is carried out using the MACSYMA program. This structure may deflect continuously with increasing load, buckle in a plane, or buckle out of plane, depending on the angle between the adjoining links and the relative stiffness between the bending springs and the internal rotation springs. We find that in the case of in‐plane buckling, the buckling load is greater than the lowest buckling load of the perfect structure.