Boundary effects in chiral polymer hexatics

Boundary effects in liquid-crystalline phases can be large due to long-ranged orientational correlations. We show that the chiral-hexatic phase can be locked into an apparent three-dimensional N+6 phase via such effects. Simple numerical estimates suggest that the recently discovered "polymer hexatic" may actually be this locked phase.     

轻柔扭转DNA

DNA是细胞中遗传信息的存储媒介,它也越来越多地在生物和纳米技术中用做结构单元。这些情形中,DNA的扭转刚度这一性质至关重要。这一特性源自DNA分子标志性的右手双螺旋结构,它将复制、转录或DNA修复等过程与旋转运动和扭转力矩联系起来。分子的刚度控制着扭转力矩如何影响分子的形状：超过某个阈值,扭转会导致双螺旋链形成相缠超螺旋(plectonemic supercoils)的高阶结构,就像老式电话的螺旋线不可避免地缠结在一起。     

Dynamics of twist point defects with stretching in a polymer crystal

A molecular-dynamics simulation of the behavior of a twist point defect with stretching in a chain of an equilibrium polymer crystal (“united” atoms approximation for polyethylene) is performed for immobile and mobile neighboring chains. It is shown that such a defect in a cold polymer crystal possesses soliton-type mobility. The upper limit of the spectrum of soliton velocities is found, and it is the same for both cases. The maximum possible velocity of defects is three times lower than the theoretical limit of the spectrum (which is equal to the velocity of “ torsional” sound in an isolated chain). An explanation of the reason for this discrepancy is proposed: because of the interaction of two “degrees of freedom” of the defect (twisting and stretching) the energy of a nonlinear wave is dissipated in the linear modes of the system, which results in effective friction whose magnitude depends strongly on the velocity of the defect. The “boundary of the spectrum of soliton velocities” determines the transition between regimes of strong and weak braking of defects.     

Refusing to twist: demonstration of a line hexatic phase in DNA liquid crystals

We report conclusive high resolution small angle x-ray scattering evidence that long DNA fragments form an untwisted line hexatic phase between the cholesteric and the crystalline phases. The line hexatic phase is a liquid-crystalline phase with long-range hexagonal bond-orientational order, long-range nematic order, but liquidlike, i.e., short-range, positional order. So far, it has not been seen in any other three dimensional system. By line-shape analysis of x-ray scattering data we found that positional order decreases when the line hexatic phase is compressed. We suggest that such anomalous behavior is a result of the chiral nature of DNA molecules.     

Extracting DNA twist rigidity from experimental supercoiling data

We use an elastic rod model with contact to study the extension versus rotation diagrams of single supercoiled DNA molecules. We reproduce quantitatively the supercoiling response of overtwisted DNA and, using experimental data, we obtain an estimate of the effective supercoiling radius and of the twist rigidity of B-DNA. We find that the twist rigidity of DNA seems to vary widely with the nature and concentration of the salt buffer in which it is immersed.     

Model of inversion of DNA charge by a positive polymer: fractionalization of the polymer charge

We model one strand of DNA by a one-dimensional lattice (ODL) of negative charges and consider the problem of inversion of its charge by a positive polyelectrolyte (PE). In the neutral state of the ODL-PE complex, each of the ODL charges is locally compensated by a PE charge. When an additional PE molecule is adsorbed by ODL, its charge gets fractionalized into monomer charges of defects (tails and arches) on the background of the perfectly neutralized ODL. Defects spread all over the ODL, eliminating the self-energy of PE. For DNA this fractionalization mechanism leads to a substantial inversion of charge, a phenomenon which is widely used for gene delivery.     

Role of tension and twist in single-molecule DNA condensation

Using magnetic tweezers, we study in real time the condensation of single DNA molecules under tension. We find that DNA condensation occurs via discrete nucleated events. By measuring the influence of an imposed twist, we show that condensation is initiated by the formation of a plectonemic supercoil. This demonstrates a strong interplay between the condensation transition and externally imposed mechanical constraints.     

Graphene bilayer with a twist: electronic structure

We consider a graphene bilayer with a relative small angle rotation between the layers--a stacking defect often seen in the surface of graphite--and calculate the electronic structure near zero energy in a continuum approximation. Contrary to what happens in an AB stacked bilayer and in accord with observations in epitaxial graphene, we find: (a) the low energy dispersion is linear, as in a single layer, but the Fermi velocity can be significantly smaller than the single-layer value; (b) an external electric field, perpendicular to the layers, does not open an electronic gap.     

Electrochemical synthesis and characterization of a new conducting polymer: Polyrhodanine

An antimicrobial drug, rhodanine (Rh), was electrochemically polymerized on a Pt electrode using cyclic voltammetry (CV). The high quality and homogeneous polyrhodanine (pRh) films with a dark-purple color were obtained. The chemical structure characterization was investigated by Fourier transform infrared spectroscopy (FTIR) and UV-vis spectroscopy techniques. Further, thermogravimetric analysis (TGA) and differential thermal analysis (DTA) techniques used to investigate thermal properties of the film. It is found that thermal stability of pRh films is relatively high. It is also observed that tetrahydrofurane (THF) and N-methyl-2-pyrrolidone (NMP) are good solvents for the polymer.     

Chiral Schwinger model with new regularization

A recently proposed version of the chiral Schwinger model is studied in detail in this paper. It is shown that a suitable Pauli-Villars regularization can be devised to reproduce the bosonized form of the effective action that was earlier written down. It is then shown how this anomalous gauge theory can be made gauge invariant by the introduction of a Wess-Zumino field. The equations of motion of this theory are explicitly solved in Lorentz covariant gauges. Finally, the operator solution of the fermionic form of the theory is constructed.     

Universal teleportation with a twist

We give a transfer theorem for teleportation based on twisting the entanglement measurement. This allows one to say what local unitary operation must be performed to complete the teleportation in any situation, generalizing the scheme to include overcomplete measurements, non-Abelian groups of local unitary operations (e.g., angular momentum teleportation), and the effect of nonmaximally entangled resources.     

Electronic structure of polyisonaphtothiophene: Design of a new conjugated organic polymer

ab initio band structure results of polyisonaphtothiophene (PINTP) are presented. Comparison of its electronic properties such as band gap, ionisation potential and electron affinity with those of polyisothianaphthene (PITN) and polythiophene (PTP) indicates PINTP to be the best intrinsic electrical conductor of these three polymers. PINTP is also predicted to be the most dopant-philic of them. The effect of the hetero substitution and the substitution at the naphthalene ring on the conduction properties of PINTP is also discussed.     

Two-dimensional group lattices with a twist

We study a statistical mechanics system defined on a particular two-dimensional group lattice. The lattice has local abstract dislocation defects. The partition function is explicitly computed and the phase transition points are enumerated. We consider several applications to physical systems.     

A new discrete Edwards model and a new polymer measure in two dimensions

The new discrete Edwards models in this paper are defined in terms of the so-called restricted intersection local times of the lattice random walk in two dimensions. We study the asymptotic behaviours of these new discrete Edwards models in the superrenormalizable cases. In particular, by approximating these models we can construct new polymer measures in two dimensions which are different from the original polymer measures obtained by approximating the original discrete Edwards models. The new discrete Edwards models can be thought of as zero-component lattice ω⁴-fields with different cutoffs in the free and interacting parts.     

A twist on chiral Potts

We show that the chiral Potts model may be formulated so that the rapidity lines carry a second integer variable-an increment or twist in each bond crossing it. This modification does not affect those properties of the chiral Potts model which lead to integrability, since it is equivalent to one of the automorphisms allowed for in the theory. In particular, transfer matrices still form commuting families and still satisfy hierarchies of functional equations. Surprisingly, the superintegrable case with twists retains the special algebraic properties which lead to its Ising-like spectra. The formalism should be useful for considering systems with twisted boundary conditions or embedded interfaces.     

Chiral electrostatics breaks the mirror symmetry of DNA supercoiling

DNA supercoiling plays a fundamental role in regulating cellular activity and in the packaging of genetic material. In this communication, we analyse the effect of attractive chiral forces on the conformation of a closed circular DNA molecule, arising due to the helical patterns of charges on the DNA. We propose a model for closed loop DNA which uses the results of the recent theory of electrostatic interactions of a braid of two free-ended DNA molecules. Our model reproduces the known features of DNA supercoiling in an environment of low ionic strength. In high salt conditions, and in the presence of counterions that have high affinity to the DNA grooves, helix-specific forces significantly affect the conformation of the molecule by favouring a state characterized by a central left-handed braided section where there is close contact between distant portions of the loop. In such an environment we predict a previously unexplored possibility that nicked or topologically relaxed DNA molecules adopt a writhed state. This prediction suggests an alternative explanation for experiments in which it was assumed that the most stable topoisomer is always an open circle. Our results also give the first plausible explanation for the occurrence of tightly interwound molecules observed in cryo-electron microscopy and atomic force microscopy in a high ionic strength environment. We suggest several new experiments to test the predictions of this theory.     

Chiral electrostatic interaction and cholesteric liquid crystals of DNA

Explicit expressions for electrostatic interaction between stiff DNA duplexes of finite length are obtained. These expressions allow for the helical symmetry of charge distribution on DNA molecules and reveal chiral and non-chiral interaction terms. Asymptotic expressions at small twist angles are applied to evaluate the cholesteric pitch and the twist elastic constant and their dependence on the length of DNA fragments. These estimates suggest an explanation for the large value of the cholesteric pitch and its nonmonotonic variation with the density of the liquid crystal. An analysis of biaxial correlations rationalizes the driving force of the transition from the cholesteric to hexagonal phase upon dehydration.