Strength Of Attractive Interaction Research Articles

Equilibrium self-assembly of small RNA viruses.

We propose a description for the quasiequilibrium self-assembly of small, single-stranded (ss) RNA viruses whose capsid proteins (CPs) have flexible, positively charged, disordered tails that associate with the negatively charged RNA genome molecules. We describe the assembly of such viruses as the interplay between two coupled phase-transition-like events: the formation of the protein shell (the capsid) by CPs and the condensation of a large ss viral RNA molecule. Electrostatic repulsion between the CPs competes with attractive hydrophobic interactions and attractive interaction between neutralized RNA segments mediated by the tail groups. An assembly diagram is derived in terms of the strength of attractive interactions between CPs and between CPs and the RNA molecules. It is compared with the results of recent studies of viral assembly. We demonstrate that the conventional theory of self-assembly, which does describe the assembly of empty capsids, is in general not applicable to the self-assembly of RNA-encapsidating virions.

Multiparticle Bound-State Formation following a Quantum Quench to the One-Dimensional Bose Gas with Attractive Interactions.

We consider quantum quenches from an ideal Bose condensate to the Lieb-Liniger model with an arbitrary attractive interaction strength. We focus on the properties of the stationary state reached at late times after the quench. Using recently developed methods based on integrability, we obtain an exact description of the stationary state for a large number of bosons. A distinctive feature of this state is the presence of a hierarchy of multiparticle bound states. We determine the dependence of their densities on interaction strength and obtain an exact expression for the stationary value of the local pair correlation g_{2}. We discuss ramifications of our results for cold atom experiments.

Attractive Hubbard model: Homogeneous Ginzburg–Landau expansion and disorder

We derive Ginzburg - Landau (GL) expansion in disordered attractive Hubbard model within the combined Nozieres - Schmitt-Rink and DMFT+Sigma approximation. Restricting ourselves to the case of homogeneous expansion, we analyze disorder dependence of GL expansion coefficients on disorder for the wide range of attractive potentials U, from weak BCS coupling region to the strong coupling limit, where superconductivity is described by Bose - Einstein condensation (BEC) of preformed Cooper pairs. We show, that for the case of semi - elliptic "bare" density of states of conduction band, disorder influence on GL coefficients A and B before quadratic and fourth -- order terms of the order parameter, as well as on the specific heat discontinuity at superconducting transition, is of universal nature at any strength of attractive interaction and is related only to the general widening of the conduction band by disorder. In general, disorder growth increases the values of coefficients A and B, leading either to the suppression of specific heat discontinuity (in the weak coupling limit), or to its significant growth (in the strong coupling region). However, this behavior actually confirms the validity of the generalized Anderson theorem, as disorder dependence of superconducting critical temperature T_c, is also controlled only by disorder widening of conduction band (density of states).

Effect of homopolymer matrix on diblock copolymer grafted nanoparticle conformation and potential of mean force: A molecular simulation study

ABSTRACTWe study the effect of homopolymer A or B matrix on the conformations and effective interactions of AB diblock copolymer grafted particles using coarse‐grained molecular dynamics simulations. In an A homopolymer matrix we observe patchy conformations within the AB diblock copolymer grafted layer, where the number of B patches is controlled by the A‐A attractive interaction strength. In a B homopolymer matrix the grafted particle takes on a core‐corona conformation, where the inner A block aggregates near the particle surface and the outer B block forms a corona that interacts with the B matrix. The potential of mean force (PMF) between two particles in an A homopolymer matrix has a long‐ranged attractive well with a minima at intermediate distances corresponding to the location of the outer B block patches. The PMF between two particles in a B homopolymer matrix has an attractive well at short interparticle distances corresponding to the size of the inner A block. We isolate the contribution of the homopolymer matrix on the PMF between the two diblock copolymer grafted particles, by deducting the PMF in the absence of a matrix, assuming the contributions of the grafted particle and matrix to the PMF to be additive. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 76–88

Role of anisotropic interactions for proteins and patchy nanoparticles.

Protein–protein interactions are inherently anisotropic to some degree, with orientation-dependent interactions between repulsive and attractive or complementary regions or “patches” on adjacent proteins. In some cases it has been suggested that such patch–patch interactions dominate the thermodynamics of dilute protein solutions, as captured by the osmotic second virial coefficient (B22), but delineating when this will or will not be the case remains an open question. A series of simplified but exactly solvable models are first used to illustrate that a delicate balance exists between the strength of attractive patch–patch interactions and the patch size, and that repulsive patch–patch interactions contribute significantly to B22 for only those conditions where the repulsions are long-ranged. Finally, B22 is reformulated, without approximations, in terms of the density of states for a given interaction energy and particle–particle distance. Doing so illustrates the inherent balance of entropic and energetic contributions to B22. It highlights that simply having strong patch–patch interactions will only cause anisotropic interactions to dominate B22 solution properties if the unavoidable entropic penalties are overcome, which cannot occur if patches are too small. The results also indicate that the temperature dependence of B22 may be a simple experimental means to assess whether a small number of strongly attractive configurations dominate the dilute solution behavior.

Weak competing interactions control assembly of strongly bonded TCNQ ionic acceptor molecules on silver surfaces

The energy scales of interactions that control molecular adsorption and assembly on surfaces can vary by several orders of magnitude, yet the importance of each contributing interaction is not apparent a priori. Tetracyanoquinodimethane (TCNQ) is an archetypal electron acceptor molecule and it is a key component of organic metals. On metal surfaces, this molecule also acts as an electron acceptor, producing negatively charged adsorbates. It is therefore rather intriguing to observe attractive molecular interactions in this system that were reported previously for copper and silver surfaces. Our experiments compared TCNQ adsorption on noble metal surfaces of Ag(100) and Ag(111). In both cases we found net attractive interactions down to the lowest coverage. However, the morphology of the assemblies was strikingly different, with two-dimensional islands on Ag(100) and one-dimensional chains on Ag(111) surfaces. This observation suggests that the registry effect governed by the molecular interaction with the underlying lattice potential is critical in determining the dimensionality of the molecular assembly. Using first-principles density functional calculations with a van der Waals correction scheme, we revealed that the strengths of major interactions (i.e., lattice potential corrugation, intermolecular attraction, and charge-transfer-induced repulsion) are all similar in energy. The van der Waals interactions, in particular, almost double the strength of attractive interactions, making the intermolecular potential comparable in strength to the diffusion potential and promoting self-assembly. However, it is the anisotropy of local intermolecular interactions that is primarily responsible for the difference in the topology of the molecular islands on Ag(100) and Ag(111) surfaces. We anticipate that the intermolecular potential will become more attractive and dominant over the diffusion potential with increasing molecular size, providing new design strategies for the structure and charge transfer within molecular layers.

Solvent isotope effect on sol–gel transition of methylcellulose studied by DSC

Methylcellulose (MC), a hydrophobically modified cellulose derivative, in an aqueous solution undergoes sol-to-gel and gel-to-sol transitions on heating and cooling, respectively. Using differential scanning calorimetry, MC in light (H2O) and heavy (D2O) water solutions has been investigated to elucidate the solvent isotope effect on the transitions. As a result, their transition temperatures are higher in H2O by about 4 °C than D2O. This phenomenon is rationalized in terms of the strength of the hydrophobic attractive interaction; the strength is enhanced by D2O. We discuss the reason for the enhancement and the difference in the isotope effect between MC and a poly(N-isopropylacrylamide) polymer which shows an opposite trend to MC.

Systematic Coarse-Graining of the Dynamics of Self-Attractive Semiflexible Polymers

We develop a coarse-graining procedure, using bead–rod chains with varying numbers of rods per Kuhn length (Nr,K), to model the conformational dynamics of polymer chains with bending stiffness and self-attraction. We find the conditions under which increased chain resolution leads to converged results for the effect of bending stiffness, attractive interaction strength (e), and shearing flow on the conformational dynamics. As chain stiffness increases, we find a collapse transition that becomes steeper as attractive strength increases. The collapsed state depends not only on the net dimensionless attractive strength e* = eNr,K between each Kuhn step of the chain but also on the ratio of bead diameter to Kuhn length (σ*). At high e* (>0.8/σ*), collapsed globules are produced at moderate chain diameter σ* = 1/4, while for fat chains with σ* = 1, helices are formed, and for thin ones with σ* = 1/16, tori, folded bundles, and finally globules are formed as e* increases.

Effect of interactions with the chaperonin cavity on protein folding and misfolding.

Recent experimental and computational results have suggested that attractive interactions between a chaperonin and an enclosed substrate can have an important effect on the protein folding rate: it appears that folding may even be slower inside the cavity than under unconfined conditions, in contrast to what we would expect from excluded volume effects on the unfolded state. Here we examine systematically the dependence of the protein stability and folding rate on the strength of such attractive interactions between the chaperonin and substrate, by using molecular simulations of model protein systems in an idealised attractive cavity. Interestingly, we find a maximum in stability, and a rate which indeed slows down at high attraction strengths. We have developed a simple phenomenological model which can explain the variations in folding rate and stability due to differing effects on the free energies of the unfolded state, folded state, and transition state; changes in the diffusion coefficient along the folding coordinate are relatively small, at least for our simplified model. In order to investigate a possible role for these attractive interactions in folding, we have studied a recently developed model for misfolding in multidomain proteins. We find that, while encapsulation in repulsive cavities greatly increases the fraction of misfolded protein, sufficiently strong attractive protein-cavity interactions can strongly reduce the fraction of proteins reaching misfolded traps.

Driving forces for the self-assembly of graphene oxide on organic monolayers.

Graphene oxide (GO) flakes were self-assembled from solution on surfaces of self-assembled monolayers (SAMs), varying in the chemical structure of their head groups. The coverage density of GO relates to strength of attractive interaction, which is largest for Coulomb interaction provided by positively charged SAM head groups and negatively charged GO. A rough surface enhances the coverage density but with the same trend in driving force dependency. The self-assembly approach was used to fabricate field-effect transistors with reduced GO (rGO) as active layer. The SAMs as attractive layer for self-assembly remain almost unaffected by the reduction from GO to rGO and serve as ultra-thin gate dielectrics in devices, which operate at low voltages of maximum 3 V and exhibit a shift of the Dirac voltage related to the dipole moment of the SAMs.

Computer Simulation of Micellization for Ring-like Block Copolymers

The micellization behaviour of ring-like AB diblock copolymers in a selective solvent was compared with the corresponding linear ABA triblock copolymers via Monte Carlo simulation. The simulation results show that the difference in the critical micellization concentration (cmc) values between ring-like copolymers and linear copolymers with same compositions is closely related to the content of A blocks, f(A), and the strength of attractive interaction between B monomers, epsilon. When fA is relatively small and e is big, the cmc values of ring-like block copolymers are smaller than those of the corresponding linear ones; whereas when fA is big and e is small, the cmc values of triblock copolymers are smaller than those of the corresponding ring-like ones. To further understand the effect of fA and e on the micellization behavior, we calculated the entropy and the potential energy contributions to the micellization free energy, respectively. The results show that in the studied range of fA and 8, the entropy loss in micellization for ring-like block copolymers is always smaller than that for triblock copolymers, so it seems that the micellization of ring-like block copolymers is always easier than that of the linear ones if we only consider the entropy contributions to the free energy. But if we calculate the potential energy contributions to the free energy, it can be found when fA is relatively small and 8 is big, the potential energy decreases greatly during the micellization of ring-like block copolymers. Considering the less unfavorable contribution of entropy part to the free energy for ring-like block copolymers, the micellization of ring-like block copolymers should be much easier than that of the linear ones. When f(A), is big and e is small, the potential energy decreases greatly when the micelles form for linear block copolymers. Although the entropic contribution to the free energy is unfavorable for linear ones, the potential energy still shows the dominative contribution to the free energy and therefore the micellization of linear block copolymers is much easier than the ring-like ones. The above analysis of the free energy will help us understand the micellization behavior of ring-like block copolymers and linear block copolymers.

Collective dynamics of pedestrians interacting with attractions

In order to investigate collective effects of interactions between pedestrians and attractions, this study extends the social force model. Such interactions lead pedestrians to form stable clusters around attractions, or even to rush into attractions if the interaction becomes stronger. It is also found that for high pedestrian density and intermediate interaction strength, some pedestrians rush into attractions while others move to neighboring attractions. These collective patterns of pedestrian movements or phases and transitions between them are systematically presented in a phase diagram. The results suggest that safe and efficient use of pedestrian areas can be achieved by moderating the pedestrian density and the strength of attractive interaction, for example, in order to avoid situations involving extreme desire for limited resources.

Un-Fermi liquids: Unparticles in strongly correlated electron matter

Since any non-trivial infrared dynamics in strongly correlated electron matter must be controlled by a critical fixed point, we argue that the form of the single-particle propagator can be deduced simply by imposing scale invariance. As a consequence, the unparticle picture proposed by Georgi\cite{georgi} is the natural candidate to describe such dynamics. Unparticle stuff is scale-invariant matter with no particular mass. Scale invariance dictates that the propagator has an algebraic form which can admit zeros and hence is a candidate to explain the ubiquitous pseudogap state of the cuprates. The non-perturbative electronic state formed out of unparticles we refer to as an un-Fermi liquid. We show that the underlying action of the continuous mass formulation of unparticles can be recast exactly as an action in anti de Sitter space. We find that this mapping fixes the scaling dimension of the unparticle to be $d_U=d/2+\sqrt{d^2+4}/2$ and ensures that the corresponding propagator has zeros with $d$ the spacetime dimension of the unparticle field. Should $d=2+1$, unparticles acquire the non-trivial phase $2\pi d_U$ upon interchange. Because $d_U$ is non-integer and in general not half-integer, clockwise and counterclockwise interchange of unparticles do not lead to the same phase and time reversal symmetry is broken spontaneously as reported in numerous experiments in the pseudogap phase of the cuprates. The possible relevance of this mechanism to such experiments is discussed. We then formulate the analogous BCS gap using unparticles and find that in contrast to the Fermi liquid case, the transition temperature increases as the attractive interaction strength decreases, indicating that unparticles are highly susceptible to a superconducting instability.

Integral Equation Prediction of Surface-Induced Layering Transition of Polymer Nanocomposites

The polymer reference interaction site model (PRISM) integral equation for inhomogenous polymers was applied to investigate layering transitions of nanoparticle/polymer blends near solid surfaces. The equation has the advantage over other theoretical approaches in describing the chemical and morphologic details of polymers. Construction of a novel bridge function derived from a free-energy functional allowed the PRISM equation to be solved with a modified hypernetted chain approximation. Tested by the density functional theory as well as molecular simulations, the equation is quantitatively reliable for the inhomogeneous blends containing flexible or semiflexible polymer chains. Accordingly, the effects of particle size, the attractive interaction strength between particles and polymer monomers, and the stiffness of polymer chains on the layering transitions were investigated to decipher the contribution of the packing and configurational entropies.

Self-consistent phonon theory of the crystallization and elasticity of attractive hard spheres

We propose an Einstein-solid, self-consistent phonon theory for the crystal phase of hard spheres that interact via short-range attractions. The approach is first tested against the known behavior of hard spheres, and then applied to homogeneous particles that interact via short-range square well attractions and the Baxter adhesive hard sphere model. Given the crystal symmetry, packing fraction, and strength and range of attractive interactions, an effective harmonic potential experienced by a particle confined to its Wigner-Seitz cell and corresponding mean square vibrational amplitude are self-consistently calculated. The crystal free energy is then computed and, using separate information about the fluid phase free energy, phase diagrams constructed, including a first-order solid-solid phase transition and its associated critical point. The simple theory qualitatively captures all the many distinctive features of the phase diagram (critical and triple point, crystal-fluid re-entrancy, low-density coexistence curve) as a function of attraction range, and overall is in good semi-quantitative agreement with simulation. Knowledge of the particle localization length allows the crystal shear modulus to be estimated based on elementary ideas. Excellent predictions are obtained for the hard sphere crystal. Expanded and condensed face-centered cubic crystals are found to have qualitatively different elastic responses to varying attraction strength or temperature. As temperature increases, the expanded entropic solid stiffens, while the energy-controlled, fully-bonded dense solid softens.

General pairing theory for condensed and noncondensed Cooper pairs of a superconductor in a high magnetic field

We extend Gor'kov theory to address superconducting pairing at high magnetic fields and general temperatures with arbitrary attractive interaction strength. This analysis begins with a new interpretation of the high-field Gor'kov gap equation which we associate with an instability in a generalized particle-particle ladder series. Importantly, this interpretation of the non-linear gap equation enables a treatment of pairing which is distinct from condensation. We also show how to consolidate two distinct fermionic pairing schemes in real and momentum space, both corresponding to an Abrikosov lattice. Numerical results for the fermionic local density of states demonstrate that gapless structure in a field is robust and presumably relevant to quantum oscillation experiments. We find that despite their differences, both pairing schemes contain very similar physics. Our formalism is designed to explore a variety of magnetic field effects in the so-called pseudogap phase and throughout the BCS-BEC crossover.

Weak adhesion at the mesoscale: particles at an interface

Adhesion of colloidal particles (synthetic or biological) to one another or to other surfaces, typically immersed in a fluid medium, is frequently a mesoscale phenomenon, intermediate between microscopic and macroscopic scales. Similar to other mesoscale phenomena, gaining a useful understanding based on ideas from either continuum or atomistic concepts alone is limited. Combination of experimental techniques like optical trapping and scanning probe techniques with simultaneous imaging provides direct insight into the space- and time-dependent dynamics of the adhesion process in the relevant scales. The strength of adhesion typically evolves with time; the details of the processes depend on the overall strength of attractive interaction among the adhering objects as well as its spatial fluctuations governed by the randomness of the surfaces involved and their own time-dependent reconstruction due to varying local stresses. This review focuses on situations where the adhesive inter-object interactions are “weak” compared to the cohesive intra-object interactions whereby all processes occur, to a good approximation, in the interfacial region. In these situations, the dynamics of adhesion is affected by both the “quenched” spatial disorder of the surfaces as well as the thermal fluctuations. A local microrheological characterization of the stress-coupling in the interfacial region provides a heuristic description of the adhesion process. The discrete, thermally assisted “punctuated descent” of the effective adhering system down a rough energy landscape through a hierarchy of finite metastable minima reflects the mesoscopic nature of adhesion. We review these experimental observations of particle adhesion in the weak limit and explore possible common mechanisms underlying apparently disparate phenomena and processes.

The self-consistent field study of the adsorption of flexible homo-polymers onto two neutral cylindrical objects

Using the continuum self-consistent field theory (SCFT), the adsorption of flexible homo-polymers onto the surfaces of two identical cylinders immersed in a neutral polymer solution is studied. The effects of various system parameters (the chain length, the radius of the cylinders, the number density of objects, the intensity of short-region field, etc.) on the bridging chain conformation and the total amount of the adsorbed chains are investigated. The efficient multigrid method is adopted to solve the modified diffusion equation. The “masking” technique is applied to deal with the interface between the cylinders and the polymer solution, which enables the use of the Cartesian grid. Simulation results show that, the total amount of the adsorbed chains scales linearly with respect to the radius of the cylinders. On the other hand, the total amount of the bridging chain conformation does not follow a linear scaling relation with the radius of the cylinder due to the curvature effect of the cylinders. Simulation results reveal that, the total amount of the bridging chain conformation increases with both the chain length and the strength of the short-range attractive interaction between the object surface and the monomers, but eventually saturates.

Compositeness of dynamically generated states in a chiral unitary approach

The structure of dynamically generated states in the chiral unitary approach is studied from a viewpoint of their compositeness. We analyze the properties of bound states, virtual states, and resonances in a single-channel chiral unitary approach, paying attention to the energy dependence of the chiral interaction. We define the compositeness of a bound state using the field renormalization constant which is given by the overlap of the bare state and the physical state in the nonrelativistic quantum mechanics, or by the residue of the bound state propagator in the relativistic field theory. The field renormalization constant enables one to define a normalized quantitative measure of compositeness of the bound state. Applying this scheme to the chiral unitary approach, we find that the bound state generated by the energy-independent interaction is always a purely composite particle, while the energy-dependent chiral interaction introduces the elementary component, depending on the value of the cutoff parameter. This feature agrees with the analysis of the effective interaction by changing the cutoff parameter. A purely composite bound state can be realized by the chiral interaction only when the bound state lies at the threshold or when the strength of the two-body attractive interaction is infinitely large. The natural renormalization scheme, introduced by the property of the loop function and the matching with the chiral low-energy theorem, is shown to generate a bound state which is dominated by the composite structure when the binding energy is small.

Cell‐penetrating peptide induces various deformations of lipid bilayer membrane: Inverted micelle, double bilayer, and transmembrane

AbstractWe investigated the deformation of lipid bilayer membrane by cell‐penetrating peptide using coarse‐grained molecular dynamics simulation. The simulations have been performed with different values of two parameters, strength of angle potential of lipid and strength of attractive interaction between peptide and lipid heads. We obtained the phase diagram of deformation of membrane and found that there are four kinds of structures such as surface dent, inverted micelle, double bilayer, and transmembrane. The results suggest that one peptide can make various structures on lipid bilayer membrane. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012