The “Roll and Lock” Mechanism of Force Generation in Muscle

SummaryMuscle force results from the interaction of the globular heads of myosin-II with actin filaments. We studied the structure-function relationship in the myosin motor in contracting muscle fibers by using temperature jumps or length steps combined with time-resolved, low-angle X-ray diffraction. Both perturbations induced simultaneous changes in the active muscle force and in the extent of labeling of the actin helix by stereo-specifically bound myosin heads at a constant total number of attached heads. The generally accepted hypothesis assumes that muscle force is generated solely by tilting of the lever arm, or the light chain domain of the myosin head, about its catalytic domain firmly bound to actin. Data obtained suggest an additional force-generating step: the “roll and lock” transition of catalytic domains of non-stereo-specifically attached heads to a stereo-specifically bound state. A model based on this scheme is described to quantitatively explain the data

Structure of nano-composite (fibers-PEO) materials in extensional Flow

Extensive researches on nano-composite materials submitted to shear flow or extensional flow, have been motivated by their spectacular rheological behavior and vast industrial applications (deposition, ultrafiltration, extrusion, etc.). Indeed, these materials under extensional flow can exhibit time dependent viscoelastic properties. Instabilities such as localization, phase separation, segregation, can occur which are not well understood. The scientific objective of this research is to elucidate the relationship between the extensional flow properties (strain and stress fields) and the dynamics of the induced structures (orientation, aggregation, phase transition, etc.). The nano-composite materials studied are composed of sepiolite clay particles (rigid rods 1 micrometer long and 0.010 micrometers in diameter) in a water-soluble polymer, polyethylene oxide. The trajectory of the strain field within the sample volume has been well characterized by particle imaging velocimetry, in order to apply pure extensional flows in the desired extensional rate and viscosity domains. A new extensional flow device developed at the "Laboratoire de Rhéologie", has allowed to combine extensional flow with SAXS on ID02 at the ESRF. Time-resolved information has been obtained on the orientation dynamics of the fibers in the polymer matrix. Analysis of extensional transient and steady state regimes revealed the main characteristics of the extensional flow of these nano-composite suspensions. Under steady states, the fibers are aligned along the extensional direction. The evolution of the orientation order parameter versus strain rate has allowed defining the rate of organization of the fibers at increasing extensional flow rates. Under transient states, time evolution of the order parameter permitted to identify the time scales at which the fibers get aligned in the extensional field and the time scales over they relax in the polymer matrix

Phase separation and nanocrystallization in Al₉₂Sm₈ metallic glass

International audienceCrystallization of Al92Sm8 metallic glass was investigated in-situ using combined small-angle and wide-angle X-ray scattering (SAXS/WAXS) techniques during isothermal annealing at temperatures close to crystallization point. A continuously growing interference maximum shifting progressively toward lower angles was found to develop in SAXS regime. Simultaneously taken WAXS spectra reveal formation of fcc-Al nanocrystalline phase. The analysis of the SAXS/WAXS data indicate that amorphous phase separation is responsible for the nanocrystalline microstructure formation. The primary fcc-Al crystals nucleate inside the Al-rich amorphous regions formed during alloy decomposition and their growth is constrained by the region size

Design of liquid-crystalline aqueous suspensions of rutile nanorods : evidence of anisotropic photocatalytic properties.

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Structural Changes in the Myosin Motors During Activation of Skeletal Muscle

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Structure and orientation dynamics of sepiolite fibers - poly(ethylene oxide) aqueous suspensions under extensional and shear flow, probed by in-situ SAXS

International audienceThe structure and orientation dynamics of sepiolite clay fibers about 1000 nm long and 10 nm thick, suspended in an aqueous poly(ehtylene oxide) matrix of 100 000 g/mol molecular mass, have been studied under control extensional and shear flow. A new extensional flow cell developed at the "Laboratoire de Rhéologie" and the combined rheology and small angle x-ray scattering (Rheo-SAXS) set-up available at the European Synchrotron Radiation Facility, have allowed to access to in-situ and time resolved fibers orientations and structure properties in the volume of suspensions under flow. In the volume fractions and shear rate domains for which the suspensions exhibit shear-thinning properties, two regimes of orientation separated by a critical strain rate have been identified under extensional flow

Extensional and shear flow properties of sepiolite fibers - poly(ethylene oxide) suspensions probed by in-situ SAXS

Présentation orale dans le cadre du projet ANR ANISO (Analysis of Non Isotropic Suspensions Organisation) (Nancy

Determination of anisotropy in turbid media

Concentrated suspensions of anisotropic or deformable particles show partial orientation when submitted to an external field, like shear, as for instance, transition from isotrope to nematic phase. Apart from these two extreme phases, partial orientation can occur. Direct information of this intermediate regime is difficult for micrometric suspensions, since the medium is generally turbid. Indeed, microscopy provides a partial information at interface, that is not representative of the average volumic orientation at the sample scale. A good and pertinent tool is 2D Small Angle X Scattering (SAXS), leading to the determination of the average orientation of single particles. The aim of this presentation is to show that anisotropic (suggested: scattering) of incoherent light can also give an average information on the anisotropy of the medium. The experimental device is composed by a focused laser (100µm) at the sample surface and incoherent light transport is then measured by a camera in the backscattering geometry (1,2). The acquired image size is 1cm, very large compared to the impinging laser. For spherical particles, or randomly oriented anisotropic particles, the acquired image possesses an angular symmetry. When the scatterers are globally oriented, this incoherent image is deformed. We have performed experiment on a sepiolite suspension (0.5%), known as developing a fiber like system under shear (3). We show here that the measurement of its anisotropy through incoherent light transport is very similar to the order parameter obtained by 2D SAXS measurements performed in shear flow geometry at the European Synchrotron Radiation Facility, on beamline ID02 (fig.1). This shows that the average time and volumic information obtained by both techniques are identical. This very simple technique is then used in another system composed by non Brownian calibrated glass fibers (6µm diameter, 12µm length). Average orientation of particles is followed versus shear at various concentrations