Overcharging of a macroion by an oppositely charged polyelectrolyte

The complexation of a polyelectrolyte with an oppositely charged spherical macroion is studied for both salt free and salty solutions. When a polyelectrolyte winds around the macroion, its turns repel each other and form an almost equidistant coil. It is shown that this repulsive correlations of turns lead to the charge inversion: more polyelectrolyte winds around the macroion than it is necessary to neutralize it. The charge inversion becomes stronger with increasing concentration of salt and can exceed 100%. This paper confirms that correlations are the universal mechanism of charge inversion.Comment: Monte-Carlo simulation details added. Many corrections and references added. Accepted to Physica

Adsorption of polyelectrolytes from semi-dilute solutions on an oppositely charged surface

We propose a detailed description of the structure of the layer formed by polyelectrolyte chains adsorbed onto an oppositely charged surface in the semi-dilute regime. We combine the mean-field Poisson-Boltzmann-Edwards theory and the scaling functional theory to describe the variations of the monomer concentration, the electrostatic potential, and the local grafting density with the distance to the surface. For long polymers, we find that the effective charge of the decorated surface (surface plus adsorbed polyelectrolytes) can be much larger than the bare charge of the surface at low salt concentration, thus providing an experimental route to a "supercharging" type of effect.Comment: 14 pages, 6 figure

Access of Extracellular Cations to their Binding Sites in Na,K-ATPase: Role of the Second Extracellular Loop of the α Subunit

Na,K-ATPase, the main active transport system for monovalent cations in animal cells, is responsible for maintaining Na+ and K+ gradients across the plasma membrane. During its transport cycle it binds three cytoplasmic Na+ ions and releases them on the extracellular side of the membrane, and then binds two extracellular K+ ions and releases them into the cytoplasm. The fourth, fifth, and sixth transmembrane helices of the α subunit of Na,K-ATPase are known to be involved in Na+ and K+ binding sites, but the gating mechanisms that control the access of these ions to their binding sites are not yet fully understood. We have focused on the second extracellular loop linking transmembrane segments 3 and 4 and attempted to determine its role in gating. We replaced 13 residues of this loop in the rat α1 subunit, from E314 to G326, by cysteine, and then studied the function of these mutants using electrophysiological techniques. We analyzed the results using a structural model obtained by homology with SERCA, and ab initio calculations for the second extracellular loop. Four mutants were markedly modified by the sulfhydryl reagent MTSET, and we investigated them in detail. The substituted cysteines were more readily accessible to MTSET in the E1 conformation for the Y315C, W317C, and I322C mutants. Mutations or derivatization of the substituted cysteines in the second extracellular loop resulted in major increases in the apparent affinity for extracellular K+, and this was associated with a reduction in the maximum activity. The changes produced by the E314C mutation were reversed by MTSET treatment. In the W317C and I322C mutants, MTSET also induced a moderate shift of the E1/E2 equilibrium towards the E1(Na) conformation under Na/Na exchange conditions. These findings indicate that the second extracellular loop must be functionally linked to the gating mechanism that controls the access of K+ to its binding site

Local Alignment Refinement Using Structural Assessment

Homology modeling is the most commonly used technique to build a three-dimensional model for a protein sequence. It heavily relies on the quality of the sequence alignment between the protein to model and related proteins with a known three dimensional structure. Alignment quality can be assessed according to the physico-chemical properties of the three dimensional models it produces

Polyelectrolyte Adsorption on Charged Particles in the Debye−Hückel Approximation. A Monte Carlo Approach

Monte Carlo simulations are used to study in the Debye−Hückel approximation the complexation between a polyelectrolyte and an oppositely charged spherical particle. Attention is focused on the effect of chain length and ionic concentration on (i) the adsorption/desorption limit, (ii) the interfacial structure of the adsorbed layer, and (iii) the overcharging issue. In particular, we are interested in polyelectrolyte adsorption on particles whose surface area is small to allow the polyelectrolyte to spread to the same extent on a flat surface. The extent of polyelectrolyte adsorption is found to be the result of two competing effects: the electrostatic repulsion between the chain monomers which forces the polyelectrolyte to adopt extended conformations in solutions and limits the number of monomers which may be attached to the particle, and the electrostatic attractive interactions between the particle and the monomers forcing the chain to undergo a structural transition and collapse at the particle surface. To overcome the loss of entropy per monomer due to adsorption, it is shown that a stronger electrostatic attraction, with decreasing ionic concentration, is needed for the short chains. Below that critical ionic concentration, it is found that the degree of adsorption increases with the decrease in both the chain length and ionic strength. Trains are favored at low degrees of chain polymerization while loops are favored more when increasing the size of the chain. Above a critical chain length, electrostatic repulsions between the adsorbed monomers force the polyelectrolyte to form a protuding tail in solution. Charge inversion is also observed. Indeed, depending on the polyelectrolyte length, the number of monomers close to the particle surface is higher than it is necessary to neutralize it. Charge inversion is found to increase with the ionic concentration of the solution.</p

Collapse transitions of a supersized neutral chain due to irreversibly adsorbed small colloidal particles

We performed Monte Carlo simulations to study the destabilization processes of large neutral and flexible polymer chains due to irreversibly adsorbed colloidal particles attached to the chains like beads on a necklace. The particles are modeled as charged spherical units which interact with each other via repulsive electrostatic and attractive van der Waals (vdW) potentials. The usual Monte Carlo search procedure is extended and carefully checked to completely sample the chain conformational space and achieve dense conformations in the limit of both strong attractive and repulsive interaction potentials. Configurational properties, such as the radius of gyration, the end-to-end length, and the Kuhn length, are calculated as a function of the intensity of the vdW interactions and ionic strength values. It is observed that chains exhibit a new range of possible conformations compared to the classical random walk and self avoiding walk chains or polyelectrolytes. In the limit of low salt concentration, by gradually increasing vdW interactions, chains undergo a cascade of transitions from extended structures to dumbbells, from dumbbells to pearl necklaces, and from pearl necklaces to collapsed coils. Because of strong competition between the vdW and electrostatic forces, the distance along the chain between the interacting particles, and the sampling limitations, these transitions are found to sample metastable domains and to depend on the initial conformations. To gain insight into the spatial organization of the collapsed conformations, the pair correlation functions of both monomers and particles are calculated. It is shown that collapsed conformations which are the result of strong particle–particle interactions exhibit two distinct parts: a hard core mainly composed of particles and a surrounding polymeric shell composed of loops and tails. Possible effects of such a collapsed transition on the kinetics of flocculation of a mixture containing large flexible chains and small adsorbing colloidal particles are discussed