The diffusion constant of a labeled protein sliding along DNA

Long ago inferred by biochemists, the linear diffusion of proteins along DNA has recently been observed at a single-molecule level using fluorescence microscopy. This imaging technique requires labeling the protein of interest with a fluorophore, usually an organic nanosized dye that is not supposed to impact the dynamics of the protein. Yet individual proteins can also be tracked using much larger labels, like quantum dots or beads. We investigate here the impact of such a large label on the protein diffusion along DNA. Solving a Fokker-Planck equation, we estimate the diffusion constant of a protein-label complex diffusing in a periodic potential that mimics the DNA-protein interaction, the link between the protein and the label being modeled as a Hookean spring. Our results indicate that the diffusion constant can generally be calculated by considering that the motion of the protein in the DNA potential is decoupled from the Brownian motion of the label. Our conclusions are in good agreement with the experimental results we obtained with the restriction enzyme EcoRV, assuming a rotation-coupled diffusion of the enzyme along DNA.     

A new class of epoxy thermosets

The reinforcing strategies of epoxy thermosets rely on the control of the phase separation between the additive and the growing thermoset. With standard additives, such as reactive liquid rubbers, the length scale of the resulting domains is the micrometer. Here, we present a route that enable a control of the morphology down to the nanometer scale. This strategy is based upon the self-assembly process of blends of epoxy and SBM triblock copolymers, namely Poly(Styrene-b-1,4 Butadiene-b-Methyl methacrylate). It relies on the respective affinities between the epoxy precursors and each of the three blocks. Liquid epoxy has a strong affinity for PMMA, whilst it is not miscible with polystyrene nor polybutadiene at standard processing temperatures. Thus, within the reactive system, microphase separation leads to a regular network of S-B domains. This nanostructure is governed by thermodynamics. The size and geometry of the dispersed domains are controlled by the concentration and the ratio between blocks lengths. The domain size is of the order of magnitude of the chain length, ranging typically from 10 to 30 nanometers. What controls the blend's morphology throughout the curing process of the thermoset was one topic on which we focused our interest. Nanostructured thermosets have been obtained. These supramolecular architectures yield significant toughness improvements while preserving the transparency of the material. The reinforcing mechanisms are not yet fully understood : it is intriguing to induce significant toughening with elastomer domains smaller than 30 nanometers in diameter. Besides being efficient epoxy tougheners, SBM can broaden the scope of applications of thermosets due to specific rheological behaviors. Thanks to the self assembly process taking place in the blend of the SBM block copolymers with the epoxy thermosets precursors, the reactive solvent can be turned into a reactive gel or solid (before curing). This physical gelation is induced by the microphase separation and is thus thermoreversible. At relatively moderate loadings of block copolymers the reactive blend behaves like a thermoplastic material, with adjustable modulus and tackiness. These results evidence that SBM block copolymers open a broad area for designing new class of thermoset materials.