Twofold Rotation Symmetry Research Articles

Crystal structures of Na2SeO4·1.5H2O and Na2SeO4·10H2O.

The crystal structures of Na2SeO4·1.5H2O (sodium selenate sesquihydrate) and Na2SeO4·10H2O (sodium selenate deca-hydrate) are isotypic with those of Na2CrO4·1.5H2O and Na2 XSeO4·10H2O (X = S, Cr), respectively. The asymmetric unit of the sesquihydrate contains two Na(+) cations, one SeO4 tetra-hedron and one and a half water mol-ecules, the other half being generated by twofold rotation symmetry. The coordination polyhedra of the cations are a distorted monocapped octa-hedron and a square pyramid; these [NaO x ] polyhedra are linked through common edges and corners into a three-dimensional framework structure, the voids of which are filled with the Se atoms of the SeO4 tetra-hedra. The structure is consolidated by O-H⋯O hydrogen bonds between coordinating water mol-ecules and framework O atoms. The asymmetric unit of the deca-hydrate consists of two Na(+) cations, one SeO4 tetra-hedron and ten water mol-ecules. Both Na(+) cations are octa-hedrally surrounded by water mol-ecules and by edge-sharing condensed into zigzag chains extending parallel to [001]. The SeO4 tetra-hedra and two uncoordinating water mol-ecules are situated between the chains and are connected to the chains through an intricate network of medium-strength O-H⋯O hydrogen bonds.

Crystal structure of a tetra-kis-substituted pyrazine compound: 2,3,5,6-tetra-kis-(bromo-meth-yl)pyrazine.

The title compound, C8H8Br4N2, crystallizes in the enanti-omorphic-defining space group P41212 and has a refined Flack x parameter of 0.04 (4). In the asymmetric unit, there are two half-mol-ecules; the whole mol-ecules (A and B) are generated by twofold rotation symmetry. In mol-ecule A, the twofold axis is normal to the pyrazine ring passing through the centre of the ring, while in mol-ecule B, the twofold rotation axis lies in the plane of the pyrazine ring bis-ecting the C-C aromatic bonds. The two mol-ecules are pseudo-mirror images of one another, and the best fit of the two mol-ecules was obtained for inverted mol-ecule B on mol-ecule A, with an r.m.s. deviation of 0.1048 Å and a maximum deviation of any two equivalent atoms of 0.2246 Å. In the crystal, the A mol-ecules are linked by weak C-H⋯Br hydrogen bonds and Br⋯Br inter-actions [3.524 (3) Å], forming a three-dimensional framework. The B mol-ecules are also linked by weak C-H⋯Br hydrogen bonds and Br⋯Br inter-actions [3.548 (3) Å], forming a three-dimensional network that inter-penetrates the network of A mol-ecules.

Bonds that belong in the Hall of fame.

Life could not exist without water and its unusual properties. These properties prevent lakes from freezing from the bottom and allow plants to defy gravity and raise it to their leaves. Water’s special properties are also critical to the function of the eye. They are conferred by the hydrogen bond. Water is a funny little molecule possessing twofold rotational symmetry in both its physical structure and electrical dipole. It comprises an oxygen atom that has two unpaired electrons covalently bonded to two hydrogen atoms that are located 105 degrees from each other. Electrons tend to be closer to the oxygen atom than the hydrogen atom, giving water its polarized charge. The attraction of the electropositive hydrogen ends of one water molecule to the electronegative oxygen ends of another water molecule produce hydrogen bonds. Hydrogen bonds are weak, 1/10 as strong as covalent bonds. Hydrogen bonds provide water with a structure and substance as the molecules form a loose crystal structure, or quasi-polymeric system. This creates high surface tension allowing water to bead up when placed on a glass slide or slither into a canaliculus by capillary action. This property allows the aqueous layer of the tear film to have a smooth uniform thickness, and for the compressibility of the tear film so the aqueous layer remains stable after a blink. It also allows for an adequate volume of tears to contain necessary vital gases (oxygen and carbon dioxide exchange), buffering salts, enzymes, and protective proteins. Its polarization allows for these substances to go into solution. To produce free water molecules that can evaporate, the hydrogen bonds of liquid water must first be broken. This takes a significant energy input (high specific heat). This requirement fortunately prevents the rapid evaporation of tears. The aqueous layer is sandwiched between the mucin and oily layers of the tear film. Membrane-bound mucins form the immediate protective coat of the cornea. The aqueous layer then creates a smooth corneal surface by filling in the gaps and adhering to the hydrophilic mucins with hydrogen bonds strong enough to allow for stability but weak enough to allow for fluid exchange. The outer oily layer of the tear film is non-polarized and therefore hydrophobic, allowing it to remain discrete and float on, rather than mix with, the aqueous layer. This enhances the refractive power of the eyes because oil has a higher index of refraction. The oil coating helps prevent aqueous evaporation. The Meibomian glands secrete the oil at the periphery of the tear film and it is these oily reservoirs that are picked up by the eyelid and spread over the tear film with a blink. The reservoirs themselves produce the hydrophobic barrier that limits the aqueous layer from running off. The hydrogen bond makes water a magical substance, allowing us to see and life to exist.

Single and multiple doping effects on charge transport in zigzag silicene nanoribbons.

A non-equilibrium Green's function technique combined with density functional theory is used to study the spin-dependent electronic band structure and transport properties of zigzag silicene nanoribbons (ZSiNRs) doped with aluminum (Al) or phosphorus (P) atoms. The presence of a single Al or P atom induces quasibound states in ZSiNRs that can be observed as new dips in the electron conductance. The Al atom acts as an acceptor whereas the P atom acts as a donor if it is placed at the center of the ribbon. This behavior is reversed if the dopant is placed on the edges. Accordingly, an acceptor-donor transition is observed in ZSiNRs upon changing the dopant's position. Similar results are obtained if two silicon atoms are replaced by two impurities (Al or P atoms) but the conductance is generally modified due to the impurity-impurity interaction. If the doping breaks the twofold rotational symmetry about the central line, the transport becomes spin-dependent.

(2E,7E)-2,7-Bis[(thio-phen-2-yl)methyl-idene]cyclo-hepta-none.

The whole molecule of the title compound, C17H16OS2, is generated by two-fold rotational symmetry. The carbonyl C and O atoms of the cycloheptanone ring lie on the twofold rotation axis which bisects the opposite –CH2–CH2– bond of the ring. The mol­ecule exists in an E,E conformation with respect to the C=C double bond. The cyclo­hepta­none ring exhibits a twisted chair conformation and its mean plane makes a dihedral angle of 50.12 (19)° with the planes of the thio­phene rings. The two S atoms are in an anti arrangement with respect the carbonyl O atom and the dihedral angle between the two thio­phene ring planes is 69.38 (7)°. In the molecule, there are two intramolecular C—H⋯S hydrogen bond, forming S(6) ring motifs. In the crystal, inversion dimers are generated via pairs of C—H⋯O hydrogen bonds. These dimers are inter­connected by another inter­action of the same kind with a neighbouring mol­ecule, forming a mol­ecular chain along the c-axis direction.

4-Methyl-N-(4-methyl-phenyl-sulfon-yl)-N-phenyl-benzene-sulfonamide.

The whole mol­ecule of the title compound, C20H19NO4S2, is generated by twofold rotational symmetry. The N atom is located on the twofold rotation axis and has a trigonal-planar geometry. It is bonded by two S atoms of two symmetry-related 4-methyl­phenyl­sulfonyl groups and by the C atom of the phenyl ring, which is bis­ected by the twofold rotation axis. The benzene and phenyl rings are oriented at a dihedral angle of 51.48 (5)° while the pendant benzene rings are inclined to one another by 87.76 (9)°. In the crystal, weak C—H⋯O hydrogen bonds link the mol­ecules, forming a three-dimensional network.

Bis(dicyclohexylammonium) sulfate dihydrate

In the title dihydrate salt, 2C12H24N+·SO4 2−·2H2O, the cation possesses twofold rotational symmetry, with the N atom situated on the twofold axis. The sulfate anion has fourfold roto-inversion symmetry, with the S atom located on the -4 axis. In the crystal, the components are linked via ammonium–sulfate N—H⋯O and water–sulfate O—H⋯O hydrogen bonds, forming a three-dimensional network.

2,6-Bis(4-meth-oxy-phen-yl)-1,4-dithiine.

The title mol­ecule, C18H16O2S2, reveals crystallographic twofold rotation symmetry (with both S atoms lying on the axis) and one half-mol­ecule defines an asymmetric unit. The dithiine ring is in a boat conformation. The aromatic ring and the C=C bond are nearly coplanar, with small torsion angles of −171.26 (19) and 8.5 (3)°. The two S—C bond lengths [1.7391 (19) and 1.7795 (18) Å] are shorter than single C—S bonds and longer than analogous C=S double bonds, which indicates a certain degree of conjugation between the lone pair on the S atom and π electrons of the C=C bond. The crystal packing only features van der Waals inter­actions.

Drug Extrusion Process of Mate Multidrug Efflux Transporter Revealed by Molecular Dynamics Simulations

MATE (the Multidrug And Toxic compound Extrusion) is one of the five multidrug efflux transporter super-families. It is featured as primary secondary active transporter and comprises of 12 trans-membrane helices. The first and last six transmembrane helices (TMs) form N-lobe and C-lobe domains, showing a two-fold rotational symmetry. It is considered that the drug release/capture related motions are characterized by switching of inward/outward-facing conformations (rocker-switch model).Recently, multiple structures of the MATE from P.furiosus (PfMATE), has been determined by X-ray crystallography in outward-facing conformation. Two distinct structures ‘Straight’ and ‘Bent’ are characterized by conformations of TM1. Furthermore, PfMATE is shown to be H+ driven antiporter, suggesting that D41 and D184 in N-lobe are involved as important proton pathway. It is also interesting that MATE has the hydrophobic surface of inner cavity, since other multidrug transporters exhibit hydrophilic. The sharply opened interface to the lipid molecules suggests the possibilities that the lipid molecules intensively interact with the inside of the cavity.To further elucidate the drug release mechanism in outward facing stage, we adopted several promising simulation methods, continuum electrostatic analysis to predict protonation states, a number of independent all-atom Molecular Dynamics (MD) simulations by changing conditions (initial conformations, protonations, lipid positions, etc.) for dynamics and interactions, and quantum mechanics calculation for drug forcefield development. Our results suggest that D41 is protonated in Straight, while D41 and D184 are both protonated in Bent. Furthermore, we successfully observed multiple drug release events in some conditions of MD simulations. In the poster presentation, we will report and discuss the dynamics and interactions of PfMATE revealed by different conditions of MDs. We will also discuss the insights of the drug release mechanism of PfMATE obtained by the simulations.

Poly[μ-aqua-bis-(μ5-2,4-di-chloro-benzoato)dipotassium

In the title compound, [K2(C7H3Cl2O2)2(H2O)]n, the potassium salt of 2,4-di­chloro­benzoic acid, the repeating unit in the polymeric structure consists of two identical irregular KO6Cl units related by twofold rotational symmetry, linked by a bridging water mol­ecule lying on the twofold axis. The coordination polyhedron about the K+ ion comprises a carboxyl­ate O atom and a Cl-atom donor from a bidentate chelate ligand inter­action, four O-atom donors from a doubly bridging bidentate carboxyl­ate O,O′-chelate inter­action and the water mol­ecule. A two-dimensional polymeric structure lying parallel to (100) is generated through a series of conjoined cyclic bridges between K+ ions and is stabilized by water–carboxyl­ate O—H⋯O hydrogen-bonding inter­actions.

A Structural Analysis of DNA Binding by Myelin Transcription Factor 1 Double Zinc Fingers

Myelin transcription factor 1 (MyT1/NZF2), a member of the neural zinc-finger (NZF) protein family, is a transcription factor that plays a central role in the developing central nervous system. It has also recently been shown that, in combination with two other transcription factors, the highly similar paralog MyT1L is able to direct the differentiation of murine and human stem cells into functional neurons. MyT1 contains seven zinc fingers (ZFs) that are highly conserved throughout the protein and throughout the NZF family. We recently presented a model for the interaction of the fifth ZF of MyT1 with a DNA sequence derived from the promoter of the retinoic acid receptor (RARE) gene. Here, we have used NMR spectroscopy, in combination with surface plasmon resonance and data-driven molecular docking, to delineate the mechanism of DNA binding for double ZF polypeptides derived from MyT1. Our data indicate that a two-ZF unit interacts with the major groove of the entire RARE motif and that both fingers bind in an identical manner and with overall two-fold rotational symmetry, consistent with the palindromic nature of the target DNA. Several key residues located in one of the irregular loops of the ZFs are utilized to achieve specific binding. Analysis of the human and mouse genomes based on our structural data reveals three putative MyT1 target genes involved in neuronal development.

Structure of the twofold surface of the icosahedral Ag–In–Yb quasicrystal

The structure of the twofold surface of the icosahedral (i-)Ag–In–Yb quasicrystal has been investigated using low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). The LEED confirms that the surface exhibits quasicrystalline long range order with the twofold rotational symmetry expected from the bulk. STM images reveal a step–terrace structure with terrace size comparable to that of the other high symmetry surfaces of the same quasicrystal. The distribution of step heights and high resolution STM images of terraces suggest that the surface terminates at bulk planes that intersect the center of rhombic triacontahedral clusters, the building blocks of the system, as in the case of the threefold and fivefold surfaces of the system. These planes are rich in Yb and In. No facets are observed on the surface, suggesting that the twofold surface is as stable as the other high symmetry surfaces.

1-Nitro-4-(4-nitrophenoxy)benzene: a second monoclinic polymorph

In the title compound, C12H8N2O5, the aromatic rings are inclined to one another by 56.14 (7)°. The nitro groups are inclined by to the benzene rings to which they are attached by 3.86 (17) and 9.65 (15)°. In the crystal, mol­ecules are linked by C—H⋯O hydrogen bonds, forming a three-dimensional structure. The title compound is a new monoclinic polymorph, crystallizing in space group P21/c. The first polymorph crystallized in space group C2/c and the mol­ecule possesses twofold rotation symmetry. Two low-temperature structures of this polymorph (150 K and 100 K, respectively) have been reported [Meciarova et al. (2004). Private Communication (refcode IXOGAD). CCDC, Cambridge, England, and Dey & Desiraju (2005). Chem. Commun. pp. 2486–2488].

Influence of lone pair doping on the multiferroic property of orthorhombic HoMnO3: ab initio prediction

Using first principles density functional theory, we predict new multiferroic compounds Ho1/2A1/2MnO3 (A = As, Sb, Bi) with enhanced polarization. We find that doping of lone pair cations with different ionic radii, at the A-site of orthorhombic HoMnO3, results in a marked increase of the electronic polarization and its development along the b-axis. This development of electronic polarization along the b-axis is attributed to the breaking of the two-fold rotational symmetry which leads to the emergence of a polar b-axis. Furthermore, this symmetry breaking leads to the emergence of two inequivalent Mn ions (Mn(0) and Mn(1)) and the variance in their octahedral (Mn(0)O6 and Mn(1)O6) distortions. We rationalize the observed trends in the total polarization in terms of disparate eg electron hopping along the two different Mn(0) and Mn(1) chains. We expect large ionic polarization in the doped compounds due to the presence of 4s2 As, 5s2 Sb and 6s2 Bi lone pairs, but surprisingly the effect of the lone pairs seems to be inactive. This is attributed to the strong GdFeO3 distortions exhibited by the MnO6 octahedron which hinders polar displacement of the lone pair cations.

Reflection resonance switching in metamaterial twisted nematics cell

Electric switching of reflection resonances at near-IR spectral range is experimentally demonstrated in a reflective metamaterial twisted nematic liquid crystal cell. Reflective metamaterial composed of nano-sized double-split ring resonator aperture is fabricated by a focused ion beam milling. Two-fold rotational symmetry of double-split ring resonators allows for two orthogonal polarization-dependent reflection resonances in the reflective metamaterial. With an external voltage of 10V across 12μm cell gap, a full switching is achieved between two reflection resonances. Dynamic measurements show the time constants of switch-on and switch-off are in the order of 100ms and 10ms, respectively.

Observation of Robust FMR in Permalloy Quasiperiodic Arrays

We have used ferromagnetic resonance (FMR) at fixed frequency 9.7 GHz to investigate permalloy thin films patterned with antidots to form a five-fold quasicrystal pattern. For DC applied fields H below 3.5 kOe, we observed robust FMR spectra exhibiting two-fold rotational symmetry instead of the five-fold symmetry expected for quasicrystals. Dynamic micromagnetic simulations of the FMR spectra performed at <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm H}=1\ {\rm kOe}$</tex></formula> and 12 kOe exhibit two-fold and ten-fold rotational symmetry, respectively. The ten-fold symmetry observed in simulations is consistent with a saturated state of isolated write fields. We infer the two-fold rotational symmetry observed for <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm H}=1\ {\rm kOe}$</tex></formula> is due to an unsaturated state of five-fold write fields closely spaced on a square lattice.

Bloch surface eigenstates within the radiation continuum

From detailed numerical calculations, we demonstrate that in simple photonic crystal structures, a discrete number of Bloch surface-localized eigenstates can exist inside the continuum of free-space modes. Coupling to the free space causes the surface modes to leak, but the forward and back-reflected leakage may interfere destructively to create a perfectly bound surface state with zero leakage. We perform analytical temporal coupled-mode theory analysis to show the generality of such phenomenon and its robustness from variations of system parameters. Periodicity, time-reversal invariance, two-fold rotational symmetry and a perfectly reflecting boundary are necessary for these unique states. Inside the periodic structure of a photonic crystal, photons behave analogously to electrons in solid-state materials. Localized light patterns can be found at the interface between a photonic crystal and the surrounding air. Such surface states can exist even when the light has a possibility of escaping into the air, as Marin Soljacic and co-workers at the Massachusetts Institute of Technology and Harvard University, USA, now show in a theoretical study. They demonstrate that under certain conditions, different leakage channels from the crystal surface interfere destructively and thus cancel each other completely. These findings should help in the design of photonic-crystal structures for applications such as sensing and spectroscopy, where strongly localized light states are desired.

(2Z)-3-(2,4-Dichlorophenyl)-3-hydroxy-N-phenylprop-2-enethioamide

In the title mol­ecule, C15H11Cl2NOS, the dihedral angle between the phenyl and benzene rings is 72.24 (1)°. In the crystal, pairs of N—H⋯S hydrogen bonds form dimers with twofold rotational symmetry. The dimers are connected by weak C—H⋯O hydrogen bonds, forming a two-dimensional network parallel to (001). An intra­molecular O—H⋯S hydrogen bond is also observed.

Pallidol hexaacetate ethyl acetate monosolvate

The entire mol­ecule of pallidol hexa­acetate {systematic name: (±)-(4bR,5R,9bR,10R)-5,10-bis­[4-(acet­yloxy)phen­yl]-4b,5,9b,10-tetra­hydro­indeno­[2,1-a]indene-1,3,6,8-tetrayl tetra­acetate} is completed by the application of twofold rotational symmetry in the title ethyl acetate solvate, C40H34O12·C4H8O2. The ethyl acetate mol­ecule was highly disordered and was treated with the SQUEEZE routine [Spek (2009 ▶). Acta Cryst. D65, 148–155]; the crystallographic data take into account the presence of the solvent. In pallidol hexa­acetate, the dihedral angle between the fused five-membered rings (r.m.s. deviation = 0.100 Å) is 54.73 (6)°, indicating a significant fold in the mol­ecule. Significant twists between residues are also evident as seen in the dihedral angle of 80.70 (5)° between the five-membered ring and the pendent benzene ring to which it is attached. Similarly, the acetate residues are twisted with respect to the benzene ring to which they are attached [C—O(carb­oxy)—C—C torsion angles = −70.24 (14), −114.43 (10) and −72.54 (13)°]. In the crystal, a three-dimensional architecture is sustained by C—H⋯O inter­actions which encompass channels in which the disordered ethyl acetate mol­ecules reside.

Multiferroic behavior in elemental selenium below 40 K: effect of electronic topology.

The quasi-one-dimensional, chiral crystal structure of Selenium has fascinating implications: we report simultaneous magnetic and ferroelectric order in single crystalline Se microtubes below ≈40 K. This is accompanied by a structural transition involving a partial fragmentation of the infinite chains without losing overall crystalline order. Raman spectral data indicate a coupling of magnons with phonons and electric field, while the dielectric constant shows a strong dependence on magnetic field. Our first-principles theoretical analysis reveals that this unexpected multiferroic behavior originates from Selenium being a weak topological insulator. It thus exhibits stable electronic states at its surface, and magnetism emerges from their spin polarization. Consequently, the broken two-fold rotational symmetry permits switchable polarization along its helical axis. We explain the observed magnetoelectric couplings using a Landau theory based on the coupling of phonons with spin and electric field. Our work opens up a new class of topological surface-multiferroics with chiral bulk structure.