Molecular motors robustly drive active gels to a critically connected state

Living systems often exhibit internal driving: active, molecular processes drive nonequilibrium phenomena such as metabolism or migration. Active gels constitute a fascinating class of internally driven matter, where molecular motors exert localized stresses inside polymer networks. There is evidence that network crosslinking is required to allow motors to induce macroscopic contraction. Yet a quantitative understanding of how network connectivity enables contraction is lacking. Here we show experimentally that myosin motors contract crosslinked actin polymer networks to clusters with a scale-free size distribution. This critical behavior occurs over an unexpectedly broad range of crosslink concentrations. To understand this robustness, we develop a quantitative model of contractile networks that takes into account network restructuring: motors reduce connectivity by forcing crosslinks to unbind. Paradoxically, to coordinate global contractions, motor activity should be low. Otherwise, motors drive initially well-connected networks to a critical state where ruptures form across the entire network.Comment: Main text: 21 pages, 5 figures. Supplementary Information: 13 pages, 8 figure

Redundancy and cooperativity in the mechanics of compositely crosslinked filamentous networks

The actin cytoskeleton in living cells has many types of crosslinkers. The mechanical interplay between these different crosslinker types is an open issue in cytoskeletal mechanics. We develop a framework to study the cooperativity and redundancy in the mechanics of filamentous networks with two types of crosslinkers: crosslinkers that allow free rotations of filaments and crosslinkers that do not. The framework consists of numerical simulations and an effective medium theory on a percolating triangular lattice. We find that the introduction of angle-constraining crosslinkers significantly lowers the filament concentrations required for these networks to attain mechanical integrity. This cooperative effect also enhances the stiffness of the network and suppresses non-affine deformations at a fixed filament concentration. We further find that semiflexible networks with only freely-rotating crosslinks are mechanically very similar to compositely crosslinked flexible networks with both networks exhibiting the same scaling behavior. We show that the network mechanics can either be redundant or cooperative depending on the relative energy scale of filament bending to the energy stored in the angle-constraining crosslinkers, and the relative concentration of crosslinkers. Our results may have implications for understanding the role of multiple crosslinkers even in a system without bundle formation or other structural motifs.Comment: 21 pages, 5 figure

Comparison and improvement of the determinations of actinide low activities using several alpha liquid scintillation spectrometers

We applied three procedures using two or liquid scintillation spectrometers (PERALS and TRI-CARB) and two scintillation cocktails (Alphaex and Ultima Gold LLT) for the determination of a-emitter low activities. For each procedure, the limit of detection, the resolution, the separation factor, and the Fischer coefficient were determined in order to perform U-232-U-234-U-238 isotopic measurements. The deconvolution usually performed is clean when the PERALS spectrometer is used. This is not possible for the TRI-CARB spectrometer using the Ultima Gold LLT scintillation cocktail. This problem was solved by combining the advantages of both techniques using the Alphaex scintillation cocktail in the TRI-CARB spectrometer, Under these conditions, the limit of detection was improved, the resolution decreased from 500-800 to 420-590 keV, and the separation factor increased from 0.9 to 1.1-1.2. This third procedure was applied with success for U-232-U-234-U-238 isotopic experiments

Etude par ESI-FTMS du fractionnement d'un acide fulvique terrestre et de l'effet sur la sorption des lanthanides sur l'hématite et la kaolinite

International audienc

Electrochemical behaviour of technetium in acetic buffer

Tn order to understand the complex reduction mechanism of pertechnetate ions on mercury electrode we have developed electrochemical techniques which use an additional time parameter to the classical methods applied on mercury microelectrode: 3 Dimensional polarography and 3D differential capacity measurements have been investigated in addition with coulometry, cyclic voltammetry, and radioactivity determinations. We have concluded that the first VII --> III reduction wave, which has been described in literature, has to be associated with the successive VII --> V reactions followed by the disproportionation of Tc(V) and formation of Tc(VI) and insoluble tetravalent species which are adsorbed on the electrode and reduced to trivalent ions. Then, reduction of Tc(III) to Tc(0) is reached through Tc(II) and Tc(I) intermediate species. The intermediates are characterised depending on the time window of observation

Electrochemical behaviour of technetium in acetic buffer

Tn order to understand the complex reduction mechanism of pertechnetate ions on mercury electrode we have developed electrochemical techniques which use an additional time parameter to the classical methods applied on mercury microelectrode: 3 Dimensional polarography and 3D differential capacity measurements have been investigated in addition with coulometry, cyclic voltammetry, and radioactivity determinations. We have concluded that the first VII --> III reduction wave, which has been described in literature, has to be associated with the successive VII --> V reactions followed by the disproportionation of Tc(V) and formation of Tc(VI) and insoluble tetravalent species which are adsorbed on the electrode and reduced to trivalent ions. Then, reduction of Tc(III) to Tc(0) is reached through Tc(II) and Tc(I) intermediate species. The intermediates are characterised depending on the time window of observation