Chromosome segregation in Escherichia coli division: a free energy-driven string model

Although the mechanisms of eukaryotic chromosome segregation and cell division have been elucidated to a certain extent, those for bacteria remain largely unknown. Here we present a computational string model for simulating the dynamics of Escherichia coli chromosome segregation. A novel thermal-average force field accounting for stretching, bending, volume exclusion, friction and random fluctuation is introduced. A Langevin equation is used to simulate the chromosome structural changes. The mechanism of chromosome segregation is thereby postulated as a result of free energy-driven structural optimization with replication introduced chromosomal mass increase. Predictions of the model agree well with observations of fluorescence labeled chromosome loci movement in living cells. The results demonstrate the possibility of a mechanism of chromosome segregation that does not involve cytoskeletal guidance or advanced apparatus in an E. coli cell. The model also shows that DNA condensation of locally compacted domains is a requirement for successful chromosome segregation. Simulations also imply that the shape-determining protein MreB may play a role in the segregation via modification of the membrane pressure.     

Solvent self-diffusion,polymer NMR relaxation,and free volume in polymer solutions

The self-diffusion coefficient of chloroform in poly(isopropyl acrylate)—chloroform solutions has been studied as a function of concentration and temperature by using the pulsed-field-gradient spin-echo NMR method. It is found that the self-diffusion coefficient of the solvent can be adequately fitted by using a simple free-volume approach with either a concentration or temperature superposition. It was noted that the free-volume parameters derived from the self-diffusion data are the same as those derived from deuterium NMR transverse relaxation-time measurements of the polymer in the same system. The equality of these two sets of experiments suggests a fundamental relationship between the two different processes. The simplest explanation is that the free volume necessary for the local segmental motion of the polymer and the translation of the solvent are similar.     

Surface segregation and surface tension of polydisperse polymer melts

The effect of polydispersity on surface segregation of a lower molecular weight polymer component in a higher molecular weight linear polymer melt host is investigated theoretically. We show that the integrated surface excess zM of a polymer component of molecular weight M satisfies a simple relation zM=2Ue(M/Mw-1)phiM, where Mw is the weight averaged molecular weight, phiM is the polymer volume fraction, and Ue is the attraction of polymer chain ends to the surface. Ue is principally of entropic origin, but also reflects any energetic preference of chain ends to the surface. We further show that the surface tension gammaM of a polydisperse melt of high molar mass components depends on the number average degree of polymerization Mn as, gammaM=gammainfinity+2UerhobRT/Mn. The parameter gammainfinity is the asymptotic surface tension of an infinitely long polymer of the same chemistry, rhob is the bulk density of the polymer, R is the universal gas constant, and T is the temperature. The predicted gammaM compare favorably with surface tension values obtained from self-consistent field theory simulations that include equation of state effects, which account for changes in polymer density with molecular weight. We also compare the predicted surface tension with available experimental data.     

Macromolecular dynamics and free volume in polymer melts

The temperature dependence of the excess free volume, derived from the quasi-lattice theory of Simba and Somcynsky, above the glass-transition temperature for the non-crystalline component of four semicrystalline polymers [polyoxymethylene, poly(ethyleneterephthalate), polyethylene, and nylon-661] is used to facilitate the interpretation of ¹H-n.m.r. data. This method gives values for the crystalline content that agree well with values obtained by other methods. The correlation between the excess free vlume of the quasi-lattice and the segmental mobility of the noncrystalline component above the glass-transition temperature is discussed.     

Positron annihilation and free volume studies in polymer glasses

In this article, the interconnections between the ortho-positronium (the bound state of the positron and electron having parallel orientation of spins, ^TPs) pick-off annihilation characteristics and concentration and effective size of elementary free volumes in polymer structures are discussed. Free volume parameters are responsible for many important properties of polymers such as permeability to gases, selectivity, ageing mechanical strength, etc. However, the ways of quantitative estimations of size distributions of the free volume elements on the bases of experimental data are sometime not obvious. Various approaches to this problem are analyzed in this review mostly on examples of glassy polymer membrane materials. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2485–2503, 2008     

Entanglement,friction, and free volume between dissimilar chains in compatible polymer blends

Dynamic oscillation, shear creep, and time-temperature superposition are used to study the entanglement, friction, and free volume between dissimilar chains in compatible polymer blends: PMMA/PEO, PMMA/PVF2, PMMA/SAN, and PS/PPO. It is found that interchain specific interactions, responsible for the compatibility, tend to reduce the entanglement but increase the friction between dissimilar chains. The former appears to arise from local reduction of chain convolution due to segmental alignment and the latter from increased interchain attraction. The entanglement probability and the friction coefficient between dissimilar chains correlate with the strength of specific interactions. On the other hand, the free volume tends to be linearly additive but may deviate either positively or negatively, apparently influenced by segmental conformation and packing rather than specific interactions. The reduced entanglement and the free volume additivity tend to reduced the melt viscosity, while the increased friction tends to increase it. The former two effects are often stronger than the latter, resulting in a net reduction of melt viscosity in many cases.     

Diffusivity of solvent in a polymer solution—expansive free volume effect

The concentration dependency of the diffusivity of a solvent in a polymer solution is derived on the basis of a free volume theory. Applying a molecular kinetics approach, the Fujita-Doolittle equation is modified. The result of numerical simulation reveals that the diffusivity of solvent in a polymer solution depends largely on both the polymer chain structure and its concentration. The applicability of the analytical expression derived is justified by fitting the experimental data for n-alkyl acetate-(poly-methyl acrylate) polymer solutions in the literature.     

The non-random distribution of free volume in fluids: polydisperse polymer systems

A new equation-of-state model is presented that accounts for the non-random distribution of free volume in multicomponent fluid mixtures. The classical quasi-chemical approach is used in connection with the Lattice Fluid (LF) model. The model is extended via continuous thermodynamics to polydisperse polymer systems. The algorithm for the application of the model to phase equilibrium calculations is also presented. The model is applied to both solvent—polydisperse polymer and to polydisperse polymer—polydisperse polymer systems. Besides phase equilibrium calculations, the distributions of the polymeric species in principal and conjugate phases are obtained. The comparison with available experimental data is satisfactory.     

Distribution of chain ends at the surface of a polymer melt: Compensation effects and surface tension

We consider the surface of a nearly incompressible polymer melt, extending the usual ground-state analysis of self-consistent field theory to describe finite length polymers in the ground-state potential. To maintain self-consistency, further corrections to the potential are calculated within linear response theory. From this, we find an excess of ends near the surface, followed by a compensating depletion on the R_g length scale, which relies crucially on the finite compressibility of the melt. The attraction of ends to the surface can be described as resulting from a surface potential for ends with a strength on the order of k_BT. Our results address the long-standing controversies of the distribution of chain ends, the chain-length dependence of the surface tension, and the interaction between objects immersed in a melt. © 1995 John Wiley & Sons, Inc.     

Liquid-crystalline polymer brushes: deformation and microphase segregation

A statistical theory of the structure and thermodynamics of a planar brush (‘accordion’) formed by bridged polymer chains containing mesogenic segments and immersed in a solvent is developed. It is shown that deformation of an accordion can lead to the formation of a two-phase structure with coexisting liquid-crystalline (LC) and swollen microphases. Phase diagrams for accordions with different grafting densities are obtained. The influence of anisotropic interaction between mesogenic segments on the structure of phase diagrams is investigated.     

Association and segregation in aqueous polymer/polymer,polymer/surfactant,and surfactant/surfactant mixtures: similarities and differences

Recent experimental findings on the phase behaviour of aqueous polymer/surfactant mixtures are reviewed and compared with the phase behaviour of “analogous” polymer/polymer or surfactant/surfactant mixtures, which is also reviewed. Polyelectrolyte effects are given special consideration. Attention is drawn to the polymer aspect of a surfactant aggregate, and, also, to the surfactant aspect of an hydrophobe-modified polymer. It is proposed that a consideration of these aspects should be helpful in predicting the phase behaviour of polymer/surfactant mixtures.     

Positron annihilation study of free volume holes in polymers and polymer blends

Positron lifetime spectra were measured as a function of the time for metallocene polyethylene (mPE), poly(methyl methacrylate) (PMMA), polyamide (PA), and polycarbonate (PC). A decrease in o-Ps intensity with the elapsed time was observed in mPE and PC measured at room temperature and in PMMA measured at 225 K. Formation of free radicals has been supposed to be one of the causes of this effect. The effect of maleic-anhydride (maH) grafted copolymers and its ionomers in mPE/PA blends was also studied. The change in the positron lifetime distribution with increasing maH and the ionomer content revealed an enhanced interaction between mPE and PA phase and the decrease of dispersed mPE particles, which reflected good compatibility of the blend.     

Influence of the organic compounds addition in the polymer free volume,gas sorption and diffusion

Three low molecular compounds were added to a glassy polymer, poly(amino-ether) resin, in order to change its free volume and to study the subsequent effect in transport properties. Free volume characterization was carried out using the positron annihilation lifetime spectroscopy technique. Two electrobalances were used to obtain carbon dioxide sorption kinetic. Diffusion coefficient variation is in good agreement with the additive antiplasticizer character, i.e. the reduction of the free volume fraction. However, solubility is affected by other variables too, as the balance of the interactions between penetrant, additive and polymer as well as the effect of coming closer the sorption temperature to the glass transition temperature in the sorption mode.     

GTI-space: the space of generalized topological indices

A new extension of the generalized topological indices (GTI) approach is carried out to represent “simple” and “composite” topological indices (TIs) in an unified way. This approach defines a GTI-space from which both simple and composite TIs represent particular subspaces. Accordingly, simple TIs such as Wiener, Balaban, Zagreb, Harary and Randić connectivity indices are expressed by means of the same GTI representation introduced for composite TIs such as hyper-Wiener, molecular topological index (MTI), Gutman index and reverse MTI. Using GTI-space approach we easily identify mathematical relations between some composite and simple indices, such as the relationship between hyper-Wiener and Wiener index and the relation between MTI and first Zagreb index. The relation of the GTI-space with the sub-structural cluster expansion of property/activity is also analysed and some routes for the applications of this approach to QSPR/QSAR are also given.     

Voronoi cell volume distribution and configurational entropy of hard-spheres

The Voronoi cell volume distributions for hard-disk and hard-sphere fluids have been studied. The distribution of the Voronoi free volume vf, which is the difference between the actual cell volume and the minimal cell volume at close packing, is well described by a two-parameter (2gamma) or a three-parameter (3gamma) gamma distribution. The free parameter m in both the 2gamma and 3gamma models is identified as the "regularity factor." The regularity factor is the ratio of the square of the mean and the variance of the free volume distribution, and it increases as the cell volume distribution becomes narrower. For the thermodynamic structures, the regularity factor increases with increasing density and it increases sharply across the freezing transition, in response to the onset of order. The regularity factor also distinguishes between the dense thermodynamic structures and the dense random or quenched structures. The maximum information entropy (max-ent) formalism, when applied to the gamma distributions, shows that structures of maximum information entropy have an exponential distribution of vf. Simulations carried out using a swelling algorithm indicate that the dense random-packed states approach the distribution predicted by the max-ent formalism, though the limiting case could not be realized in simulations due to the structural inhomogeneities introduced by the dense random-packing algorithm. Using the gamma representations of the cell volume distribution, we check the numerical validity of the Cohen-Grest expression [M. H. Cohen and G. S. Grest, Phys. Rev. B 20, 1077 (1979)] for the cellular (free volume) entropy, which is a part of the configurational entropy. The expression is exact for the hard-rod system, and a correction factor equal to the dimension of the system, D, is found necessary for the hard-disk and hard-sphere systems. Thus, for the hard-disk and hard-sphere systems, the present analysis establishes a relationship between the precisely defined Voronoi free volume (information) entropy and the thermodynamic entropy. This analysis also shows that the max-ent formalism, when applied to the free volume entropy, predicts an exponential distribution which is approached by disordered states generated by a swelling algorithm in the dense random-packing limit.     

Molecular weight dependence of diblock copolymer segregation at a polymer/polymer interface

Utilizing forward recoil spectrometry (FRES), we have determined the segregation isotherm which describes the interfacial excess z_i^* of diblock copolymers of poly (d₈-styrene-b-2-vinylpyridine) (dPS-PVP) at the interface between the homopolymers PS and PVP as a function of ?_∞, the volume fraction of diblock copolymer remaining in the host homopolymer. All the samples were analyzed after annealing at temperatures and times sufficient to achieve equilibrium segregation. The effect of the degree of polymerization of both the diblock copolymers and the host homopolymers on the segregation isotherm is investigated. When the degree of polymerization of the homopolymer is much larger than that of the diblock copolymer, the normalized interfacial excess (z_i^*/R_g), where R_g is the radius of gyration of an isolated block copolymer chain, is a universal function of that portion of the block copolymer chemical potential due to chain stretching. The existence of such a universal function is predicted by theory and its form is in good agreement with self-consistent mean field calculations. Using these results, one can predict important aspects of the block copolymer segregation (e.g., the saturation interfacial excess) without recourse to the time-consuming numerical calculations. © 1994 John Wiley & Sons, Inc.