Expriments in soft condensed matter

Membranes (natural or artificial) are ubiquitous. They ensure compartmentalization, in living organisms at all scales (nucleus, mitochondria, cell, organ, organism), but also in all artificial electrochemical devices (batteries, power cells). Basically, the role of this compartmentalization is to slow down the chemical reactions between constituents of each compartment. But these chemical reactions are nevertheless necessary for the device to operate. A membrane plays thus a double and contradictory role : it must separate, but not too much. For this, membranes are generally equipped with pores (holes with length and diameter of the same order of magnitude) or channels (length much greater than diameter) allowing the transport of ions or molecules from one compartment to the other. We are interested to these latter.

The interaction of cell membranes with the molecules adsorbed on their surface and the way in which the incorporation of these molecules takes place is still poorly understood. We are interested in antimicrobial peptides that form the keystone of the innate immune system of multicellular organisms. Basically, these peptides make membranes permeable by forming pores in them. Their universal presence in the animal and plant kingdoms, their non-specific and broad-spectrum action as well as their very elementary structure suggest a mode of action according to physical mechanisms that are also very general and universal. We have addressed this problem through patch-clamp experiments (consisting in measuring the transmembrane ionic current while a given voltage is applied) coupled with neutron reflectivity. In particular, we showed that pore openning exhibits a slow dynamics typically observed near the glass transition, which could be governed by concentration fluctuations of peptides on the surface as it is predicted by the RSA model (“Random Sequential Adsorption”). We are also interested in the thermodynamics of pore opening as a function of temperature and applied voltage. Our results allow us to propose a new physical mecanism where adsorption of peptide and the electric field both contribute to membrane bending. With this mecanism the main entropy cost to pore opening comes from an “exclude area” effect for lipid translation.

The extracellular matrix is a gel. That is to say a solid network of macromolecuiles linked together, here mainly proteins like collagen. This matrix surrounds and delimits organs. So that, cell invasion and tumor vascularization necessarily imply its crossing by cells, which for this produce proteolytic enzymes (proteinases) which break the links of this network. Uderstanding how they work is therefore important to fight against the spread of cancer cells.

Associative polymers assemble into supramolecular structures whose formation and arrangement result from the delicate balance between contradictory forces. Among these, the entropic forces related to the loss of degrees of freedom of molecules, for example freedom of translation or freedom of conformation etc. They perfectly illustrate the various problems encountered in the study of soft matter. Mainly studied from the 1980s in the absence of solvent, my contributions in the field relate to the structures obtained in solution.

A theory is an economy of thought (read E. Mach , P. Duhem , A. Einstein ). We make a representation of reality by establishing unlikely links between things or phenomena that are a priori very different. In the 1970s, P.G. de Gennes achieves a major advance of this type by noting that polymers were critical objects whose self-similarity makes it possible to establish universal laws of behavior (called scaling laws) for observable macroscopic quantities such as osmotic pressure, viscosity or elasticity when expressed as a function of carefully chosen variables (called reduced variables). This was followed in France (particularly at Saclay) for about twenty years by an abundant experimental activity which had a lasting impact on the physics of soft matter.