Understanding the motion of nanoparticles in polymer solutionsand melts is a problem of broad importance, with applications tomany different fields, such as material science, biophysics, andmedicine. If the nanoparticles are larger than the polymer's radiusof gyration, their structure and dynamics can be well described interms of effective pair potentials. However, much remains to beunderstood in the so-called “protein limit”, where the size of thenanoparticles becomes comparable to or smaller than that ofthe polymers. Moreover, most of the previous study consideringthis size range have only focused on the dilute nanoparticleregime, which is easier to handle since inter-nanoparticleinteraction can be neglected and the properties of the polymersolution/melt are expected to be unchanged.Using molecular dynamics simulations, we study the dynamic andstructural properties of a semidilute polymer solution containingwell dispersed spherical nanoparticles of size smaller than thepolymer's radius of gyration. We consider various nanoparticlediameters and a broad range of nanoparticle volume fractions,up to values for which the inter-nanoparticle interactionbecomes important.We find that the polymers slow down when the nanoparticleconcentration is increased, in qualitative agreement with theconfinement parameter theory (Choi et al.,ACS Macro Lett.2013,2,485−490), according to which polymers slow downbecause they have to squeeze through “bottlenecks” created bythe presence of the nanoparticles. Also the nanoparticles slowdown when their concentration is increased, with the magnitudeof the slowing down depending in a non-trivial way on their size.Surprisingly, if the concentration of the nanoparticles is increasedpast the range in which the nanoparticle dispersion is good, thediffusivities of polymers and nanoparticles reach a minimum andthen start to increase.

Une configuration de microscopie holographique qui utilise trois faisceaux d'illumination est proposée. Elle permet d'imager en 3D la positions des globules rouges en mouvement à partir d'un hologramme, ainsi que la structure 3D des vaisseaux sanguins perfusé à partir d'une séquence d'hologrammes.

Proteins are the basis of cellular functions, yet key parameters regulating protein synthesis remain elusive. Understanding the fine mechanisms of regulation is a major goal of molecular and systems biology, and this knowledge will support many synthetic biology applications.We have the ambitious goal of providing a biophysical modelling framework of one of the last steps in protein synthesis, namely mRNA translation. Our work focuses on translation initiation and elongation, which relative role is highly debated in the literature: is the ribosome recruitment or the codon bias determining the expression of a gene? We explain how the transcript efficiency can be dictated by ribosome abundances, codon usage and transcript length.We propose analytical and simulation methods to investigate translation models based on an inhomogeneous exclusion process, the prototypical non-equilibrium traffic model in one dimension. We show that the first codons, together with the value of the initiation rate, are the main determinants of protein production. Finally, we interpret the obtained analytical results based on the evolutionary role of codons' choice for regulating translation rates and ribosome usage.

Dense active materials