A biomimetic motility assay provides insight into the mechanism of actin-based motility

Abiomimetic motility assay is used to analyze the mechanism of force production by site-directed polymerization of actin. Polystyrene microspheres, functionalized in a controlled fashion by the N-WASP protein, the ubiquitous activator of Arp2/3 complex, undergo actin-based propulsion in a medium that consists of five pure proteins. We have analyzed the dependence of velocity on N-WASP surface density, on the concentration of capping protein, and on external force. Movement was not slowed down by increasing the diameter of the beads (0.2 to 3 μm) nor by increasing the viscosity of the medium by 105-fold. This important result shows that forces due to actin polymerization are balanced by internal forces due to transient attachment of filament ends at the surface. These forces are greater than the viscous drag. Using Alexa®488-labeled Arp2/3, we show that Arp2/3 is incorporated in the actin tail like G-actin by barbed end branching of filaments at the bead surface, not by side branching, and that filaments are more densely branched upon increasing gelsolin concentration. These data support models in which the rates of filament branching and capping control velocity, and autocatalytic branching of filament ends, rather than filament nucleation, occurs at the particle surface

Aluminosilicate network formation during geopolymerization followed by in-situ 27Al nutation NMR

In classical cement systems, hydration reactions can typically be stopped by a solvent exchange (such as isopropanol) or by drying1. Subsequently, the chemical reactions are studied by separating and characterizing independently the solid and the liquid phases at different times, to follow their respective compositions and to establish a reaction process by finding chemical intermediates and products. As for geopolymers, they are formed by a dissolution-condensation mechanism resulting from the mixing a solid aluminosilicate source (for example metakaolin) with a highly concentrated alkali-silicate solution. The properties of the suspension do not allow to employ phase separation. This is the reason why the reaction mechanism leading to geopolymers is still said to be unclear, because it has only been studied by indirect methods so far, such as calorimetry, time-resolved rheology or small-angle scattering2 for instance. In-situ static 27Al NMR has already been used as a direct method to probe and quantify the aluminate species in the liquid phase during geopolymerization, using the quadrupolar nature of 27Al nuclei. Aluminum is not present in the liquid state at the very beginning of the process but goes to the initial aluminosilicate powder to the final solid product, naturally making it the nucleus of interest for an NMR study. While dissolved species are mobile enough for the quadrupolar interaction to be averaged, the quadrupolar coupling persists in less mobile species or in solids, leading to different nutation behaviors. In the present study, it will be demonstrated that a nutation experiment, which simply consists in varying the pulse length and measuring the resulting signal, allows filtering out the reactant aluminosilicate source from the 27Al NMR signal to detect reaction intermediates, and apparently also products. The evolution of the 27Al NMR signal was followed over longer periods of time up to several days during the geopolymerization process of metakaolin-based systems. It was shown that more than two steps can be identified in the geopolymerization process, depending on the frequency of the radiofrequency field applied during the experiment. Simulation of nutation curves at different times of the reactions allowed to follow the evolution of the quadrupolar coupling constant, and gave insight on the aluminate intermediates. Finally, the NMR results were confronted to time-resolved rheology and isothermal calorimetry in order to understand processes occurring on different time scales. 1- Collier et al. The influence of water removal techniques on the composition and microstructure of hardened cement pastes, Cement and Concrete Research, 38(6) (2008) pp. 737-744. 2- Steins et al. Structural Evolution during Geopolymerization from an Early Age to Consolidated Material, Langmuir, 28 (2012) pp. 8502-8510. 3- Favier et al., Mechanical properties and compositional heterogeneities of fresh geopolymer pastes, Cement and Concrete Research, 48 (2013) pp. 9-16

Fractionnement des asphaltenes par chromatographie d'exclusion sterique : caracterisation des fractions par diffusion des neutrons aux petits angles

SIGLECNRS T 62340 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Viscoelasticity of Supramolecular Center-functionalized Polymer: Effect of the strength of Hydrogen Bonding Stickers

International audienceDuring the last decade, supramolecular chemistry (1) showed its great potential for the elaboration of innovative materials, with features such as self-healing or stimuli-responsiveness. Supramolecular materials are composed of polymer chains functionalized by strongly interacting moieties. These moieties, called “stickers”, can associate by non covalent interactions (hydrogen bonds, ionic…) and thus, form a transient network in the polymer matrix. While the self-assembly of stickers favors elastic behavior in the case of telechelic polymer chains (2), our group focused on the control of dissipative properties of center-functionalized polymers at small and large deformation (3), an important property for soft adhesives.The specificity of our project is to adjust the viscoelastic properties of non or lightly entangled center-functionalized polymers by varying the supramolecular parameters, such as the strength and the density of stickers. Monodisperse poly(butylacrylate) chains below and above their average molecular weight between entanglements, center-functionalized by multiple hydrogen bonding stickers were synthesized by ATRP. Their structure (by AFM and SAXS) and linear viscoelastic properties in the melt state were systematically characterized. Stickers are observed to self-assemble into cylindrical aggregates packed into a hexagonal structure. This supramolecular structure gives a gel-like character at low molecular weight and high sticker density while the entanglements’ dynamics makes them highly dissipative over a large frequency range (5 decades) for higher Mw and lower sticker density. The transition from one regime to another is easily adjusted by varying independently the length of the side chains and the nature of stickers. Such a fine control of the dissipative properties via chemical non random structures makes the supramolecular center-functionalized polymers promising to target fine-tuned innovative soft materials.(1)Seiffert, S. et al, J. Chem. Soc. Rev. 2012 (2) Folmer, B. J. B. et al, Adv. Mater. 2000 (3) Courtois, J. et al Adv. Funct. Mater. 2010

Viscoelastic Properties of Poly(vinyl alcohol) Hydrogels Having Permanent and Transient Cross-Links Studied by Microrheology, Classical Rheometry, and Dynamic Light Scattering

International audienceDynamics of poly(vinyl alcohol) (PVA) hydrogels having chemical and physical transient cross-links simultaneously (dual cross-link PVA gels) were studied by microrheology based on diffusing wave spectroscopy (DWS), classical rheology and single dynamic light scattering (DLS), and compared with those of corresponding chemical and physical PVA gels. Three different relaxation modes (fast, intermediate and slow modes) are observed for physical gels, while one mode (fast mode) is found for chemical gels, and two (fast and intermediate) for dual cross-link gels. The three modes are attributed respectively to Brownian diffusion of PVA polymer or collective diffusion of the network or gel mode (fast mode), macroscopic stress relaxation (intermediate mode whose characteristic time shows q(0) dependence) and Brownian diffusion of aggregates (slow mode). Microrheological measurements are in good agreement with macrorheological showing segmental Rouse mode dynamics in the high frequency range. For physical gels, we found Maxwell type viscoelasticity characterized by a crossover frequency (maximum of G '') and G' similar to omega(2) and G '' similar to omega(1) in the lower frequency range. The chemical gels displayed an elastic plateau with low G '' at low frequency. For the dual cross-link gel a maximum of G '' was observed, and its characteristic time agrees with that of the intermediate mode measured by DLS. We show that this relaxation mode corresponds to the associative Rouse mode characterized by G' = G '' similar to omega(0.5), depending on the dissociation rate of the reversible transient cross links We propose a stress relaxation mechanism of the PVA chains in the presence of elastically inactive but associative transient cross links which induces incomplete stress relaxation

Stress–Strain Relationship of Highly Stretchable Dual Cross-Link Gels: Separability of Strain and Time Effect

International audienceWe studied the stress-strain relation of model dual cross-link gels having permanent cross-links and transient cross-links over an unusually wide range of extension ratios lambda and strain rates (epsilon) over dot (or time t = (lambda - 1)/(epsilon) over dot). We propose a new analysis method and separate the stress into strain- and time-dependent terms. The strain-dependent term is derived from rubber elasticity, while the time-dependent term is due to the failure of transient cross-links and can be represented as a time-dependent shear modulus which shows the same relaxation as in small strain. The separability is applicable except for the strain stiffening regimes resulting from the finite extensibility of polymer chains. This new analysis method should have a wide applicability not only for hydrogels but also for other highly viscoelastic soft solids such as soft adhesives or living tissues

Rational Design of Urea-Based Two-Component Organogelators

International audienceLow molecular weight gelators are versatile and responsive gel-forming systems. However, it is still a challenge to develop a new organogelator for a precise application, i.e., to gel a predetermined liquid. We propose a simple concept of a two-component gelling system that can be rationally adaptedto gel liquids ranging in polarity from silicone oil to acetonitrile

Anions as Efficient Chain Stoppers for Hydrogen‐Bonded Supramolecular Polymers

International audienceThe chain length of hydrogen-bonded supramolecular polymers and thus their rheological properties can be controlled by the presence of so-called chain stoppers: these monofunctional monomers are able to interact with the monomers and break the polymer chains. In this letter, we show that the use of anions, strong hydrogen bond competitors, instead of precisely designed complementary units is a very simple approach to tuning the rheology of a bisurea-based hydrogen-bonded supramolecular polymer. All of the anions tested were able to break the supramolecular chains, resulting in a dramatic drop in the viscosity of the solutions and were found to be more efficient than a previously described organic stopper. A careful study of the rheological properties of bisurea solutions in the presence of H2PO4,N(C4H9)4 showed that the presence of this ion does not modify the nature of the bisurea supramolecular assembly. For a molar fraction of stopper of only 10−5, the viscosity of bisurea solutions decreases by a factor of 10 as a result of the formation of shorter supramolecular assemblies