High Twist Angle Research Articles

Single cell-gap transflective liquid crystal device utilizing continuous director rotation with different twist angle in the fringe-field switching mode

In this paper, a single cell-gap and single gamma transflective liquid crystal displays (LCDs) associated with fringe-field switching (FFS) device is proposed. The device utilizes unique characteristic of the FFS device in which the twist angle of LC director is dependent on electrode position in on state. With optimization of pixel electrode width and distance between them, the LC director can be rotated about 22.5° and 45° of twist angles depending on electrode positions. Consequently, the parts with low and high twist angle could be played for the reflective ( R) and transmissive ( T) regions respectively, realizing a high image quality transflective display.

The Effects of Fluorine and Chlorine Substituents across the Fjords of Bifluorenylidenes: Overcrowding and Stereochemistry

AbstractThe bistricyclic aromatic enes (BAEs) (E)‐ and (Z)‐1,1′‐difluorobifluorenylidene, 1,8,1′,8′‐tetrafluorobifluorenylidene, (E)‐ and (Z)‐3,3′‐difluorobifluorenylidene, 3,6,3′,6′‐tetrafluorobifluorenylidene, and their chlorinated analogues were subjected to a DFT study of overcrowding in their fjord regions. The B3LYP hybrid functional was employed to calculate energies and geometries of the twisted conformations of these BAEs. The diastereomers E11′F2 and Z11′F2 have identical twist angles (ω = 37.1°) and similar degrees of overcrowding, but differ in the degree and mode of pyramidalization, χ. In E11′F2, χ(C9) = +χ(C9′) = 7.0° (syn‐pyramidalization), while in Z11′F2, χ(C9) = –χ(C9′) = 1.0° (anti‐pyramidalization). By contrast, in E11′Cl2 and Z11′Cl2, ω = 40.6° and 42.7°, respectively. Introducing four halogen substituents results in higher twist angles: ω = 40.3° in 181′8′F4 and 52.6° in 181′8′Cl4. Surprisingly, Z11′F2 is more stable than E11′F2 (ΔH298 = –1.9 kJ/mol), whereas Z11′Cl2 is less stable than E11′Cl2 (ΔH298 = 2.2 kJ/mol). Both results are consistent with the experimental relative stabilities of these diastereomers. The unexpected stability of Z11′F2 is explained by a combination of steric and electronic effects. Calculations of Coulomb energies for point charge systems of atoms C, F, and H in the fjord regions shows stabilization of the (Z) diastereomer by –45.5 kJ/mol. The dipole–dipole interactions in the fjord region destabilize Z11′F2 by 6.4 kJ/mol relative to E11′F2. Careful examination of the NMR spectra of E11′F2 and Z11′F2 shows, in the latter, evidence of long‐range fluorine–fluorine coupling over seven bonds (11.4 Hz) and carbon–fluorine coupling over six bonds (4.8 Hz).(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Ordered growth of germanium dots induced by the strain field of tilt dislocations in molecular bonded silicon (0 0 1) thin films

Germanium dots have been grown on high twist angle (twist angle as high as 20°) molecular bonded silicon (0 0 1) substrates. We show that, depending on the thickness of the silicon film, the strain field generated by an ordered array of mixed edge interfacial tilt (miss-cut) dislocations may induce an ordered growth of germanium dots. We also show that in order to observe an influence of the mixed edge interfacial dislocations on the growth of germanium dots, the thickness of the film has to be much lower that the period of the mixed edge dislocations array. Germanium dots grown by molecular beam epitaxy on 10–15 nm thick silicon films with the period of tilt dislocation array of 43 nm show a high degree of self-ordering.

Structure of an RNA duplex r(GGCGBrUGCGCU)2with terminal and internal tandem G·U base pairs

The crystal structure of a self-complementary RNA duplex r(GGCG(Br)UGCGCU)(2) with terminal G.U and internal tandem G.U base pairs has been determined at 2.1 Angstroms resolution. The crystals belong to the tetragonal space group P4(3), with unit-cell parameters a = b = 37.69, c = 96.28 Angstroms and two duplexes in the asymmetric unit. The two strands of each duplex are related by a pseudodyad axis. The structure was refined to final R(work) and R(free) values of 20.9 and 25.3%, respectively. The duplexes stack in an end-to-end manner, forming infinite columns along the c axis. This is the first structural study of an RNA duplex containing G.U pairs at the termini. The stacking overlaps of the terminal G.U base pairs with their adjacent Watson-Crick base pairs are larger than those of Watson-Crick base pairs of the 5'-YR-3'/3'-RY-5' type. The terminal G.U base pairs of neighbouring duplexes are also stacked with each other. An alternating underwound-overwound pattern of the twist angles is seen at each step along the duplex. This observation is typical for internal tandem G.U pairs, while the terminal G.U base pairs exhibit high twist angles with the adjacent Watson-Crick pairs. The 3'-side of U of the internal G.U base pair, which is unstacked, appears to be stabilized by pi-cation interaction with an Mg(2+) ion.

Ordered growth of germanium hut islands on Si (001) molecular bonded substrates

Ordered germanium hut islands are grown by molecular-beam epitaxy on high twist angle molecular bonded silicon (001) substrates (twist angle higher than 20°). We show that the growth organization is induced by an array of interfacial tilt dislocations. Plan-view transmission electron microscopy and atomic force microscopy observations show that the orientation and period of the tilt dislocation array determine the orientation, period, and length of elongated germanium hut islands. The strain field generated by an array of tilt dislocations is proposed as the driving force of the reported organization.

The relationship of tilt and twist of fringe cracks in granite plutons

Abstract Joint fractography in European plutons frequently shows large fringe-tilt angles connected to small fringe-crack twist angles (type A). In contrast to this first type, fringes of joints that tilt at small angles out of the parent joint plane are often associated with high twist angles of en echelon fringe cracks (type B). The interaction of tilt and twist angles gives evidence for the mode (I, II and III) acting at the advancing crack front during fracture propagation and the formation of fringes. The different fringe types depend on the varying influence of mode II or mode III, which establishes the degree of tilt or twist, in addition to opening-mode I that governs the crack propagation. Fringe types A and B are not randomly distributed. Within several plutons the first joints are characterized by a frequency of type A, and in other plutons by the dominance of type B. A third group of plutons is characterized by low tilt and low angle of twist for early joint fringes. The range of tilt/twist ratios of the earliest joints decreases with increasing depth of pluton emplacement and joint formation. The trend of the ratio approaches a value of between 0.5 and 1.0 at greater depth ( c. 15 km). The ratio seems to be suitable for a prognosis of the possible depth of first joint formation.

Conformational Characteristics and Correlations in Crystal Structures of Nucleic Acid Oligonucleotides: Evidence for Sub-states

Sugar phosphate backbone conformations are a structural element inextricably involved in a complete understanding of specific recognition nucleic acid ligand interactions, from early stage discrimination of the correct target to complexation per se, including any structural adaptation on binding. The collective results of high resolution DNA, RNA and protein/DNA crystal structures provide an opportunity for an improved and enhanced statistical analysis of standard and unusual sugar-phosphate backbone conformations together with corresponding dinucleotide sequence effects as a basis for further exploration of conformational effects on binding. In this study, we have analyzed the conformations of all relevant crystal structures in the nucleic acids data base, determined the frequency distribution of all possible ϵ, ζ, α, β and γ backbone angle arrangements within four nucleic acid categories (A-RNA and A-DNA, free and bound B-DNA) and explored the relationships between backbone angles, sugar puckers and selected helical parameters. The trends in the correlations are found to be similar regardless of the nucleic acid category. It is interesting that specific structural effects exhibited by the different unusual backbone sub-states are in some cases contravariant. Certain α/γ changes are accompanied by C3′ endo (north) sugars, small twist angles and positive values of base pair roll, and favor a displacement of nucleotide bases towards the minor groove compared to that of canonical B form structures. Unusual ϵ/ζ combinations occur with C2′ (south) sugars, high twist angles, negative values of base pair roll, and base displacements towards the major groove. Furthermore, any unusual backbone correlates with a reduced dispersion of equilibrium structural parameters of the whole double helix, as evidenced by the reduced standard deviations of almost all con- formational parameters. Finally, a strong sequence effect is displayed in the free oligomers, but reduced somewhat in the ligand bound forms. The most variable steps are GpA and CpA, and, to a lesser extent, their partners TpC and TpG. The results provide a basis for considering if the variable and non-variable steps within a biological active sequence precisely determine morphological structural features as the curvature direction, the groove depth, and the accessibility of base pair for non covalent associations.

The 1.19 A X-ray structure of 2'-O-Me(CGCGCG)(2) duplex shows dehydrated RNA with 2-methyl-2,4-pentanediol in the minor groove.

The crystal and molecular structure of 2'-O-Me(CGCGCG)(2) has been determined at 1.19 A resolution, at 100 K, using synchrotron radiation. The structure in space group P3(2)12 is a half-turn right-handed helix that includes two 2-methyl-2,4-pentanediol (MPD) molecules bound in the minor groove. The structure deviates from A-form RNA. The duplex is overwound with an average value of 9.7 bp per turn, characterised as having a C3'-endo sugar pucker, very low base pair rise and high helical twist and inclination angles. The structure includes 65 ordered water molecules. Only a single row of water molecules is observed in the minor groove due to the presence of hydrophobic 2'-O-methyl groups. As many as five magnesium ions are located in the structure. Two are in the major groove and interact with O(6) and N(7) of guanosine and N(4) of cytidine residues through their hydration spheres. This work provides the first example of molecular interactions of nucleic acids with MPD, which was used as a precipitant, cryo-solvent and resolution enhancing agent. The two MPD molecules intrude into the hydration network in the minor groove, each forming hydrogen bonds between their secondary hydroxyl group and exo-amino functions of guanosine residues. Comparison of the 2'-O-Me(CGCGCG)(2) structure in the P3(2)12 and P6(1)22 crystals delineates stability of the water network within the minor groove to dehydration by MPD and is of interest for evaluating factors governing small molecule binding to RNA. Intrusion of MPD into the minor groove of 2'-O-Me(CGCGCG)(2) is discussed with respect to RNA dehydration, a prerequisite of Z-RNA formation.

Structural characterization of ultra-thin (0 0 1) silicon films bonded onto (0 0 1) silicon wafers:: a transmission electron microscopy study

Ultra-thin (001) silicon films (thickness less than 25nm) directly bonded onto (001) silicon wafers have been investigated by transmission electron microscopy. Twist interfacial dislocations accommodate the twist between the two crystals, whereas tilt interfacial dislocations accommodate the tilt resulting from the residual vicinality of the initial surfaces. In low-twist angle grain boundaries, twist interfacial dislocations are dissociated and no precipitates are detected. In high-twist angle grain boundaries, there is no dissociation and a high density of silicon oxide precipitates is observed at the interface. Tilt interfacial dislocations are pinned by these precipitates, they are more mobile than precipitates. Without precipitates, their lines are straighter than those with precipitates, and this is especially when the bonded wafers are annealed at a high temperature. When no precipitates are present, tilt interfacial dislocations are associated by pairs, and we demonstrate that each tilt interfacial dislocations introduce a diatomic interfacial step at the interface.

Huge differences between low- and high-angle twist grain boundaries: The case of ultrathin (001) Si films bonded to (001) Si wafers

Ultrathin (001) Si films bonded onto (001) Si wafers, inducing grain boundaries with twist angles varying from 0.5° to 12°, were studied by transmission electron microscopy. A great structural difference between low (ψ<5°) and high (ψ>6°) twist angles was observed. In low twist angle grain boundaries, “twist interfacial dislocations” are dissociated and produce rough interfaces with no oxide precipitates. It is the opposite in high-angle grain boundaries: there is no dissociation, the interfaces are smoother but contain oxide precipitates. These differences are not attributed to the thin thickness of one grain, but to the large atomic differences between high- and low-angle twist grain boundaries, which is not the case for tilt grain boundaries.

Structure determination and analysis of helix parameters in the DNA decamer d(CATGGCCATG)2 comparison of results from NMR and crystallography

The solution structure of the DNA decamer (CATGGCCATG)2 has been determined by NMR spectroscopy and restrained molecular dynamic and distance geometry calculations. The restrainted data set includes interproton distances and torsion angles for the deoxyribose sugar ring which were obtained by nuclear Overhauser enhancement intensities and quantitative simulation of cross-peaks from double quantum filtered correlation spectroscopy. The backbone torsion angles were constrained using experimental data from NOE cross-peaks, 1H-1H and 1H-31P-coupling constants. The NMR structure and the crystal structure of the DNA decamer deviates from the structure of the canonical form of B-DNA in a number of observable characteristics. Particularly, both structures display a specific pattern of stacking interaction in the central GGC base triplet. Furthermore, a specific local conformation of the TG/CA base-pair step is present in NMR and crystal structure, highlighting the unusually high flexibility of this DNA duplex part. The solution structure of the TG/CA base-pair step obtained by our high resolution NMR study is characterized by a positive roll angle, whereas in crystal this base-pair step tends to adopt remarkably high twist angles.

Solution structure of RNA duplexes containing alternating CG base pairs: NMR study of r(CGCGCG)2 and 2'-O-Me(CGCGCG)2 under low salt conditions.

Structures of r(CGCGCG)2 and 2'-O-Me(CGCGCG)2 have been determined by NMR spectroscopy under low salt conditions. All protons and phosphorus nuclei resonances have been assigned. Signals of H5'/5" have been assigned stereospecifically. All 3JH,H and 3JP,H coupling constants have been measured. The structures were determined and refined using an iterative relaxation matrix procedure (IRMA) and the restrained MD simulation. Both duplexes form half-turn, right-handed helices with several conformational features which deviate significantly from a canonical A-RNA structure. Duplexes are characterised as having C3'-endo sugar pucker, very low base-pair rise and high helical twist and inclination angles. Helices are overwound with <10 bp per turn. There is limited inter-strand guanine stacking for CG steps. Within CG steps of both duplexes, the planes of the inter-strand cytosines are not parallel while guanines are almost parallel. For the GC steps this pattern is reversed. The 2'-O-methyl groups are spatially close to the 5'-hydrogens of neighbouring residues from the 3'-side and are directed towards the minor groove of 2'-O-Me(CGCGCG)2 forming a hydrophobic layer. Solution structures of both duplexes are similar; the effect of 2'-O-methylation on the parent RNA structure is small. This suggests that intrinsic properties imposed by alternating CG base pairs govern the overall conformation of both duplexes.

Structural Comparison of the B-DNA Dodecamers d(CGCGTTAACGCG) and d(CGCGAATTCGCG) with T2A2and A2T2Tracts

The x-ray structure of the deoxy oligonucleotide dodecamer d(CGCGTTAACGCG) recently determined in our laboratory shows that the helical parameters of the central TTAA segment are significantly different compared to the central AATT in d(CGCGAATTCGCG). The roll in the central TA step of the T2A2 dodecamer opens towards the minor groove while the AT step of the A2T2 dodecamer opens towards the major groove. Also, the roll angles at the steps 4 and 8 (GT and AC in T2A2) and (GA and TC in A2T2) are in opposite directions. The high cup and helical twist angles at the central base-pair of T2A2 decreases the base stacking interactions compared to A2T2. Tilt angles within the tetranucleotide segments TTAA and AATT have opposite signs. In spite of the local differences caused by the sequence inversion (TTAA----AATT), the two dodecamers exhibit similar overall bending. The top third is more bent than the bottom third relative to the central segment. This asymmetric bending in the two dodecamers is mainly due to crystal packing interactions.

A highly propeller-twisted adenine–adenine base pair in 8-tert-butyladenine

C9H13N5, Mr = 191.24, monoclinic, P2(1)/c, a = 7.562 (1), b = 6.825 (1), c = 20.905 (1) A, beta = 104.84 (1) degree, V = 1042.9 A3, Z = 4, room temperature, Dx = 1.218 g cm-3, lambda (Cu K alpha) = 1.5418 A, mu = 6.6 cm-1, F(000) = 408. The structure was solved by the multisolution technique and refined by the block-diagonal least-squares method to a final R index of 0.045 using 1970 intensities. The adenine bases form three pairs of hydrogen bonds to symmetry-related molecules in the crystal lattice. Two distinct modes of hydrogen bonding are observed. One mode involves a planar interaction between adjacent adenine bases while the second mode is characterized by an unusually high propeller twist angle of 79.3 degrees between the planes through the two participating adenine bases.

Effects of torsion on domains of coated Si-Fe sheets

The paper discusses consequences of twisting a grain oriented transformer sheet of conventional type and HI-B type, respectively, around an axis parallel to the rolling direction; a situation which (in a less defined way) will occur in regions close to core bands, for example. Mechanically, apart from shear stress, there results tension at the sheet's peripheral regions which is counterbalanced by compression in the central region. Here, torsion leads to domain widening and, in the case of higher twist angles, to extensive domain reconstructions connected with increases of iron losses. Quantitatively, the effectiveness of a given twist angle increases strongly with increasing sheet width.

Structure of Twisted Nylon 6 and Poly(ethylene terephthalate) Fibers

Nylon 6 and poly(ethylene terephthalate) monofilaments were subjected to various levels of twist inserted under constant load. X-ray diffraction patterns of the twisted filaments were obtained and the axial dispersion of the crystalline regions was estimated from the azimuthal spreading of the equatorial diffraction arcs. The experimentally obtained values were compared with those calculated by assuming that the fiber under goes a uniform deformation. The data show that the crystalline orientation does not follow the macroscopic twist imparted to the filament. The discrepancy is largest at small twist angles, indicating that at this stage the twist is taken up primarily by regions that do not give rise to coherent x-ray scattering. At higher twist angles, there is indi cation that a tilting of crystallographic planes accompanies the reorientation of the crystallites. The effect of crystallite disorientation on the properties of the fibers is discussed.