Multiple Scale Reorganization of Electrostatic Complexes of PolyStyrene Sulfonate and Lysozyme

We report on a SANS investigation into the potential for these structural reorganization of complexes composed of lysozyme and small PSS chains of opposite charge if the physicochemical conditions of the solutions are changed after their formation. Mixtures of solutions of lysozyme and PSS with high matter content and with an introduced charge ratio [-]/[+]intro close to the electrostatic stoichiometry, lead to suspensions that are macroscopically stable. They are composed at local scale of dense globular primary complexes of radius ~ 100 {\AA}; at a higher scale they are organized fractally with a dimension 2.1. We first show that the dilution of the solution of complexes, all other physicochemical parameters remaining constant, induces a macroscopic destabilization of the solutions but does not modify the structure of the complexes at submicronic scales. This suggests that the colloidal stability of the complexes can be explained by the interlocking of the fractal aggregates in a network at high concentration: dilution does not break the local aggregate structure but it does destroy the network. We show, secondly, that the addition of salt does not change the almost frozen inner structure of the cores of the primary complexes, although it does encourage growth of the complexes; these coalesce into larger complexes as salt has partially screened the electrostatic repulsions between two primary complexes. These larger primary complexes remain aggregated with a fractal dimension of 2.1. Thirdly, we show that the addition of PSS chains up to [-]/[+]intro ~ 20, after the formation of the primary complex with a [-]/[+]intro close to 1, only slightly changes the inner structure of the primary complexes. Moreover, in contrast to the synthesis achieved in the one-step mixing procedure where the proteins are unfolded for a range of [-]/[+]intro, the native conformation of the proteins is preserved inside the frozen core

In silico physicochemical characterization and comparison of two intrinsically disordered phosphoproteins : β-casein and acidic PRP-1

A coarse-grained model has been implemented using the Metropolis-Hastings Monte Carlo method to simulate β-casein and acidic proline-rich protein 1 (PRP-1). The aim of the study is to directly compare the properties and behavior of β-casein and PRP-1, in both bulk solution and in the presence of a negatively charged surface, in order to evaluate the possibility of using β-casein as a replacement for PRP-1 in, e.g. saliva substitutes and, possibly, dental products. The results are obtained by studying the effect of varying pH, monovalent salt concentration and with/without charge saturation of the phosphorylated serines. The electrostatic properties in bulk are found to be very similar for the two proteins, especially at physiological pH and with simulated calcium saturation. When studying surface adsorption it is observed that both proteins attach to the surface in similar ways, relatively to their size. Surface adsorption seems to be stronger for PRP-1, but β-casein is shown to adsorb closer to the surface. The adsorption of both proteins onto the negatively charged surface is strikingly similar under physiological pH and higher, near physiological, salt concentrations. The effect of calcium saturation on surface adsorption is almost negligible for both proteins at high salt concentration

Flocculated Laponite-PEG/PEO Dispersions with Multivalent Salt : A SAXS, Cryo-TEM, and Computer Simulation Study

The aim of this study is to scrutinize the mechanism behind aggregation, i.e., tactoid formation of nanostructures with the shape of a platelet. For that purpose, the clay minerals Laponite and montmorillonite have been used as model systems. More specifically, we are interested in the role of: the platelet size, the electrostatic interactions, and adsorbing polymers. Our hypothesis is that the presence of PEG is crucial for tactoid formation if the system is constituted by small nanometric platelets. For this purpose, SAXS, USAXS, Cryo-TEM, and coarse-grained molecular dynamics simulations have been used to study how the formation and the morphology of the tactoids are affected by the platelet size. The simulations indicate that ion-ion correlations are not enough to induce large tactoids solely if the platelets are small and the absolute charge is too low, i.e., in the size and charge range of Laponite. When a polymer is introduced into the system, the tactoid size grows, and the results can be explained by weak attractive electrostatic correlation forces and polymer bridging. It is shown that when the salt concentration increases the long-ranged electrostatic repulsion is screened, and a free energy minimum appears at short distances due to the ion-ion correlation effects. When a strongly adsorbing polymer is introduced into the system, a second free energy minimum appears at a slightly larger separation. The latter dominates if the polymer is relatively long and/or the polymer concentration is high enough. (Graph Presented)

Temperature Dependence of Intrinsically Disordered Proteins in Simulations : What are We Missing?

The temperature dependence of the conformational properties in simulations of the intrinsically disordered model protein histatin 5 has been investigated using different combinations of force fields, water models, and atomistic and coarse-grained methods. The results have been compared to experimental data obtained from NMR, SAXS, and CD experiments to assess the accuracy and validity of the simulations. The results showed that neither simulations completely agreed with the experimental data, nor did they agree with each other. It was however possible to conclude that the observed conformational changes upon variations in temperature were not at all driven by electrostatic interactions. The final conclusion was that none of the simulations that were investigated in this study was able to accurately capture the temperature induced conformational changes of our model IDP

Bovine β-casein has a polydisperse distribution of equilibrium micelles

β-casein, a self-associating protein, has been extensively studied over the years, and it is a molecule that is of academic, industrial, and clinical relevance. Therefore it is of interest to understand the structural and conformational properties of the assemblies, also denoted micelles. Here we show that β-casein possess a polydisperse distribution of equilibrium micelles, which has, to the authors knowledge, not been published before

Structure of polyelectrolytes in 3 : 1 salt solutions

Polyion conformation and the distribution of small ions near the polyion have been investigated using Monte Carlo simulations. The systems of interest contained one polyion and its monovalent counterions, and variable amount of a 3:1 salt. With monovalent counterions only, the polyion is strongly extended. As salt is added, the polyion folds, and the most compact and spherical-like structure appears at a three-fold excess of the trivalent counterions. The polyion exerts a strong influenc on the nearest-neighbor distance among the trivalent ions, an effect being relevant for energy transfer reactions. (C) 2003 American Institute of Physics

Spontaneous formation of cushioned model membranes promoted by an intrinsically disordered protein

In this article, it is shown that by exposing commonly used lipids for biomembrane mimicking studies, to a solution containing the histidine-rich intrinsically disordered protein histatin 5, a protein cushion spontaneously forms underneath the bilayer. The underlying mechanism is attributed to have an electrostatic origin, and it is hypothesized that the observed behavior is due to proton charge fluctuations promoting attractive electrostatic interactions between the positively charged proteins and the anionic surfaces, with concomitant counterion release. Hence, we anticipate that this novel “green” approach of forming cushioned bilayers can be an important tool to mimic the cell membrane without the disturbance of the solid substrate, thereby achieving a further understanding of protein–cell interactions

Impact of arginine−phosphate interactions on the reentrant condensation of disordered proteins

Re-entrant condensation results in the formation of a condensed protein regime between two critical ion concentrations. The process is driven by neutralization and inversion of the protein charge by oppositely charged ions. Re-entrant condensation of cationic proteins by the polyvalent anions, pyrophosphate and tripolyphosphate, has previously been observed, but not for citrate, which has similar charge and size compared to the polyphosphates. Therefore, besides electrostatic interactions, other specific interactions between the polyphosphate ions and proteins must contribute. Here, we show that additional., attractive interactions between arginine and tripolyphosphate determine the re-entrant condensation and decondensation boundaries of the cationic, intrinsically disordered saliva protein, histatin 5. Furthermore, we show by small-angle X-ray scattering (SAXS) that polyvalent anions cause compaction of histatin 5, as would be expected based solely on electrostatic interactions. Hence, we conclude that arginine−phosphate-specific interactions not only regulate solution properties but also influence the conformational ensemble of histatin 5, which is shown to vary with the number of arginine residues. Together, the results presented here provide further insight into an organizational mechanism that can be used to tune protein interactions in solution of both naturally occurring and synthetic proteins

Understanding cooperative protein adsorption events at the microscopic scale: a comparison between experimental data and Monte Carlo simulations

Cooperative effects play a vital role in protein adsorption events on biological interfaces. Despite a number of studies in this field molecular adsorption mechanisms that include cooperativity are still under debate. In this work we use a Monte Carlo-type simulation to explore the microscopic details behind cooperative protein adsorption. The simulation was designed to implement our previously proposed mechanism through which proteins are not necessarily rejected if they approach the surface to an occupied region. Instead, we suggest that proteins can be tracked laterally for a certain distance due to the influence of preadsorbed proteins in order to reach the nearest available binding site. The simulation results were compared with experimental data obtained by using the supercritical angle fluorescence (SAF) microscopy technique. It was found that the tracking distance may be up to 2.5 times the protein’s diameter depending on the investigated system. The general validity of this tracking mechanism is supported by a number of linear or upward concave adsorption kinetics reported in the literature which are consistent with our simulation results. Furthermore, the self-organization of proteins adsorbing under cooperative conditions on the surface is shown to necessarily cause density inhomogeneities in the surface distribution of proteins which is also in agreement with experimental observations