Direct observation of counterion organization in F-actin polyelectrolyte bundles

Attractions between like-charged polyelectrolytes have been observed in a variety of systems (W.M. Gelbart, R.F. Bruinsma, P.A. Pincus, V.A. Parsegian, Phys. Today 53, September issue, 38 (2000)). Recent biological examples include DNA, filamentous viruses, and F-actin. Theoretical investigations on idealized systems indicate that counterion correlations play a central role, but no experiments that specifically probe such correlations have been performed. Using synchrotron X-ray diffraction, we have directly observed the organization of multivalent ions on cytoskeletal filamentous actin (a well-defined biological polyelectrolyte) and found an unanticipated symmetry-breaking collective counterion mechanism for generating attractions. Surprisingly, the counterions do not form a lattice that simply follows actins helical symmetry; rather, the counterions organize into frozen ripples parallel to the actin filaments and form structures reminiscent of charge density waves. Moreover, these 1D counterion charge density waves form a coupled mode with twist deformations of the oppositely charged actin filaments. This counterion organization is not sensitive to thermal fluctuations in temperature range accessible to protein-based polyelectrolyte systems. Moreover, the counterion density waves are pinned to the spatial periodicity of charges on the actin filament even if the global filament charge density is varied, indicating the importance of charge periodicity on the polyelectrolyte substrate.     

水合作用与pH调控聚电解质刷的构象转变

我们基于Flory-Huggins理论,建立理论模型研究水合作用与pH调控聚电解质刷的构象转变.理论模型考虑聚电解质链与水分子间的作用(聚电解质链的水合作用)、体系中的静电作用.研究发现,随着水合作用的改变,聚电解质刷出现由溶胀到塌缩的构象转变.由此表明了水合作用可在很大程度调节聚电解质刷的相变.通过分析pH的调控效应我们还发现,在碱性环境中(pH=8),聚电解质链单体的解离度增大,静电排斥会使得聚电解质刷溶胀.由此表明,聚电解质刷内水合作用与静电效应的耦合,将会共同决定聚电解质刷的构象转变特性.理论结果深刻揭示了水合作用的改变,会使得聚电解质刷体系发生相变,pH可在很大程度上改变其相变特性.     

Morphologies of a spherical bimodal polyelectrolyte brush induced by polydispersity and solvent selectivity

It is commonly realized that polydispersity may significantly affect the surface modification properties of polymer brush systems. In light of this, we systematically study morphologies of bidisperse polyelectrolyte brush grafted onto a spherical nanocolloid in the presence of trivalent counterions using molecular dynamics simulations. Via varying polydispersity, grafting density, and solvent selectivity, the effects of electrostatic correlation and excluded volume are focused, and rich phase behaviors of binary mixed polyelectrolyte brush are predicted, including a variety of pinned-patch morphologies at low grafting density and micelle-like structures at high grafting density. To pinpoint the mechanism of surface structure formation, the shape factor of two species of polyelectrolyte chains and the pair correlation function between monomers from different polyelectrolyte ligands are analyzed carefully. Also, electrostatic correlations, manifested as the bridging through trivalent counterions, are examined by identifying four states of trivalent counterions. Our simulation results may be useful for designing smart stimuli-responsive materials based on mixed polyelectrolyte coated surfaces.     

Effects of spin fluctuations in itinerant electron ferromagnets

Effects of spin fluctuations on the paramagnetic spin susceptibility, magnetization and specific heat are calculated in gaussian statistics as a power series of k_BT. The results are applied to nickel and iron. It is shown that in effect spin fluctuations reduce the molecular field coefficient above the Curie temperature.     

Effects of Pauli paramagnetism on superconducting fluctuations

We discuss the effects of the Pauli paramagnetism on the excess conductivity σ^fl due to fluctuations of the superconducting order parameter. We derived a formula for σ^fl for a thin film placed in a magnetic field of an arbitrary orientation α. It was found that σ^fl has a universal behavior as a function of some parameterp which depends on α and ΔH=H-H _c. If ΔH is kept constant and σ^fl is measured as a function of α, in the absence of the Pauli paramagnetism σ^fl is maximum when the field is parallel to the film and is minimum when the field is perpendicular. But in high field superconductors due to the effect of the Pauli paramagnetism σ^fl becomes maximum at some intermediate field orientation. We also discussed the excess conductivity in magnetic alloys in which impurity spins are aligned by an external magnetic field. It was shown that in this case one should expect, with certain strengths of the external field, the excess conductivity which is non-monotonic in temperature.     

Compression of polyelectrolyte brushes in a salt-free theta solvent

This paper examines the normal force between two opposing polyelectrolyte brushes and the interpenetration of their chains that is responsible for sliding friction. It focuses on the special case of semi-dilute brushes in a salt-free theta solvent, for which Zhulina and Borisov (J. Chem. Phys. 107, 5952 (1997)) have derived analytical predictions using the classical strong-stretching theory (SST). Interestingly, SST predicts that the brushes contract as they are compressed together maintaining a polymer-free gap, which provides an explanation for the ultra-low frictional forces observed in experiment. We examine the degree to which the SST predictions are affected by chain fluctuations by employing self-consistent field theory (SCFT). While the normal force is relatively unaffected, fluctuations are found to have a strong impact on brush interpenetration. Even still, the contraction of the brushes does significantly prolong the onset of interpenetration, implying that a sizeable normal force can be achieved before the sliding friction becomes significant.     

Multilayering of a pseudo polyelectrolyte (PVPh) with a strong polyelectrolyte (PDMAC) from aqueous media

The introduction of pseudo polyelectrolytes (pPE) into the field of multilayer thin films has recently been achieved with the successful combination of poly(4-vinylphenol) (PVPh) with the weak polyelectrolyte (WPE), polyallylamine hydrochloride (PAH). This paper examines the stretching of this limit by exploring the extremes of using the pPE with the strong polyelectrolyte (SPE), poly(diallyldimethylammonium chloride) (PDMAC). UV-Vis absorbance and atomic force microscopy (AFM) topography data reveal a linear growth trend in film thickness that depends critically upon the assembly pH. At an assembly pH of 11.0 the multilayer was five times thicker compared to that assembled at pH 12.0. AFM topography images also show that the surface roughness of the films increases as the assembly pH decreases.     

Counterion distribution in a spherical charged sparse brush

The counterion distribution within a spherical polyelectrolyte sparse brush was measured by small-angle X-ray scattering using contrast variation with different counterions by means of ion dialysis. The brush was made by self-association of charged diblock copolymers. Thanks to the contrast variation method, we were able to separate the signal due to the monomers and the signal due to the counterions. At a small length scale, it is demonstrated that the system behaves as independent charged rods whose counterion distribution follows the Poisson-Boltzmann model. Received 14 February 2001 and Received in final form 2 May 2001     

Effects of strong coupling on pairing fluctuations in layered superconductors

The influence of short inelastic lifetimes due to strong coupling of fermionic quasiparticles to bosons on superconducting fluctuation effects aboveT _c is calculated. Considering a strong-coupling model for a layered superconducting metal, it is shown that pairing fluctuation corrections to the spin-lattice relaxation rate in weak coupling and very strong coupling are qualitatively different if the pairing fluctuation spectrum has s-wave symmetry. For weak coupling the corrections are positive, whereas for very strong coupling γ = 2? d ω α² F(ω)/ω > 2 the corrections are negative. In contrast, the Pauli spin susceptibility is insensitive to strong-coupling corrections.     

Effects of nuclear charge fluctuations on dilepton photoproduction

Fluctuations of dilepton production from two photon interactions γγ→l~+l~- are studied in semicentral and peripheral nuclear collisions.Based on the Weizsacker-Williams approach,electromagnetic(EM) fields generated by moving nuclear charges are approximated as quasi-real photons.As fluctuating EM fields in each collision event are hard to be measured directly in experiments due to its short lifetime,we study the dilepton photoproduction with fluctuating EM fields,which are crucial for the EM fields induced chiral and charged particle evolutions,and calculate the relative standard deviation of the dilepton mass spectrum with the event-by-event fluctuating nuclear charge distributions.This fluctuation effect becomes smaller in more peripheral collisions where the shift of proton positions is implicit for EM fields outside the nucleus.The uncertainty of effective impact parameter △b on the standard deviation is also studied,and its effect is around one-third of the effect of nuclear charge fluctuations when △b is taken as～1 fm.     

Effects of quantum fluctuations on electromagnetic properties in triaxial nuclei

Effects of quantum fluctuations, like wobbling and γ-fluctuations, are examined in even- as well as in odd-particle systems by making use of a simple model based on the Bohr hamiltonian. The strong signature dependence observed in ΔI = 1 E2 transitions in odd nuclei is studied in the context of an axially asymmetric deformation. We point out the importance of the quantum fluctuations especially at low-spin values and / or for shapes with small deviations from axial symmetry. It is also demonstrated that the J⁽²⁾ moment of inertia may be larger than J⁽¹⁾ due to the wobbling motion.     

Variational theory for a single polyelectrolyte chain

Variational methods are applied to a single polyelectrolyte chain. The polymer is modeled as a Gaussian chain with screened electrostatic repulsion between all monomers. As a variational Hamiltonian, the most general Gaussian kernel, including the possibility of a classical or mean polymer path, is employed. The resulting self-consistent equations are systematically solved both for large and small monomer-monomer separations along the chain. In the absence of screening, the polymer is stretched on average. It is described by a straight classical path with Gaussian fluctuations around it. If the electrostatic repulsion is screened, the polymer is isotropically swollen for large separations, and for small separations the polymer correlation function is calculated as an analytic expansion in terms of the monomer-monomer separation along the chain. The electrostatic persistence length and the electrostatic blobsize are inferred from the crossover between distinct scaling ranges. We perform a global analysis of the scaling behavior as a function of the screening length and electrostatic interaction strength , where is the Bjerrum length and A is the distance of charges along the polymer chain. We find three different scaling regimes. i) A Gaussian-persistent regime with Gaussian behavior at small, persistent behavior at intermediate, and isotropically swollen behavior at large length scales. This regime occurs for weakly charged polymers and only for intermediate values of the screening length. The electrostatic persistence length is defined as the crossover length between the persistent and the asymptotically swollen behavior and is given by and thus disagrees with previous (restricted) variational treatments which predict a linear dependence on the screening length .ii) A Gaussian regime with Gaussian behavior at small and isotropically swollen behavior at large length scales. This regime occurs for weakly charged polymers and/or strong screening, and the electrostatic repulsion between monomers only leads to subfluent corrections to Gaussian scaling at small separations. The concept of a persistence length is without meaning in this regime. iii) A persistent regime , where the chain resembles a stretched rod on intermediate and small scales. Here the persistence length is given by the original Odijk prediction, , if the overstretching of the chain is avoided. We also investigate the effects of a finite polymer length and of an additional excluded-volume interaction, which modify the resultant scaling behavior. Applications to experiments and computer simulations are discussed. Received 24 December 1997     

Distinctive fluctuations in a confined geometry

Spurred by recent theoretical predictions [Phys. Rev. E 69, 035102(R) (2004)10.1103/PhysRevE.69.035102; Surf. Sci. Lett. 598, L355 (2005)10.1016/j.susc.2005.09.023], we find experimentally using STM line scans that the fluctuations of the step bounding a facet exhibit scaling properties distinct from those of isolated steps or steps on vicinal surfaces. The correlation functions go as t0.15 +/- 0.03 decidedly different from the t0.26 +/- 0.02 behavior for fluctuations of isolated steps. From the exponents, we categorize the universality, confirming the prediction that the nonlinear term of the Kardar-Parisi-Zhang equation, long known to play a central role in nonequilibrium phenomena, can also arise from the curvature or potential-asymmetry contribution to the step free energy.     

Rhodamine photoreduction in ethanol with an electron-donor counterion

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 51, No. 6, pp. 991–996, December, 1989.     

Heat fluctuations in a nonequilibrium bath

We measure the energy fluctuations of a Brownian particle confined by an optical trap in an aging gelatin after a very fast quench (less than 1 ms). The strong nonequilibrium fluctuations due to the assemblage of the gel are interpreted, within the framework of fluctuation theorem, as a heat flux from the particle towards the bath. We derive an analytical expression of the heat probability distribution, which fits the experimental data and satisfies a fluctuation relation similar to that of a system in contact with two baths at different temperatures.     

Thermal fluctuations in a piezodielectric plate

The mechanism of origin is clarified and the spectral density of the emf of the thermal noise of a piezoelectric plate of a crystal with C_6V symmetry is calculated. It is shown that in the case in which the C₆ axis is oriented parallel or perpendicularly to the plate the problem of fluctuations in the external circuit reduces to a one-dimensional one.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 95–101, June, 1978.     

Complexation and distribution of counterions in a grafted polyelectrolyte layer

The complexation and the distribution of various cations, bound to a poly(styrene sulfonate) brush, have been investigated using infra-red spectroscopy and neutron reflectivity. Small counterions (like tetremethylammonium) are distributed throughout the brush in such a way that a local electroneutrality is ensured. They also exchange readily with other bulk small cations. On the other hand, model polycations are irreversibly trapped to the brush despite a relative small number of ionic bonds involved in the complexation. These complexed polycations are localized at the outer border of the brush, forming a macromolecular barrier. However, this spatial segregation does not allow the buildup of polyelectrolyte multilayers. Cationic surfactants are associated stoichiometrically with the brush sulfonates but unlike small counterions, this complexation is “irreversible” and induces a restructuring of the polymer interface. Received 22 August 2000     

Spectroscopic study of a brush cathode plasma in a magnetic field

The brush cathode helium discharge in the magnetic field has been operated stably at discharge currents larger than those without magnetic field. The diameter of the plasma column has been determined by the configuration of the magnetic field. The measurements of the spectral intensities of the recombination continuum followed by the 2³S-n³P series reveals that the electron density is 1·8 × 10¹³ cm^-3 and the electron temperature is 0·17 eV at a discharge current of 500 mA and a pressure of 0·9 torr for a magnetic flux density of 1·3 kG. The principal quantum number for line merging is 20.     

Vacuum lightcone fluctuations in a dielectric

A model for observable effects of time modulated electromagnetic vacuum fluctuations is presented. The model involves a probe pulse which traverses a slab of nonlinear optical material with a nonzero second order polarizability. We argue that the pulse interacts with the ambient vacuum fluctuations of other modes of the quantized electric field, and these vacuum fluctuations cause variations in the flight time of the pulse through the material. The geometry of the slab of material defines a sampling function for the quantized electric field, which in turn determines that vacuum modes whose wavelengths are of the order of the thickness of the slab give the dominant contribution. Some numerical estimates are made, which indicate that fractional fluctuations in flight time of the order of 10⁻⁸$10^{-8}$ are possible in realistic situations. The model presented here is both an illustration of a physical effect of vacuum fluctuations occurring in a finite interval of time, and an analog model for the lightcone fluctuations predicted by quantum gravity.