Biomimetic hybrid membranes, mixing lipids and an amphiphilic block copolymer

Abstract

Biological membranes determine the cell boundaries and regulate the transport and communication between the cells and the environment. They are also the site of many important reactions, since they host a lot of proteins which acts as enzymes. For their fundamental role in cell life, the interest in better understand the membranes processes is increased in the recent years. Natural membranes are commonly the results of the self-assembly of different types of amphiphilic lipids and proteins, forming a really complex system. Due to this complexity, the research is driven in the direction of developed simple membrane models, in order to build an environment where it would be possible the study of one process at the time. The first examples of membranes model where formed using biological derived lipids, which are biocompatible, and forms structures showing the lateral fluidity typical of natural membranes. To overcome the mechanical instability of lipids membranes, amphiphilic block copolymers can be used to form membrane systems. Block copolymes opens the doors to many possibilities in changing the physico-chemicals properties of the membranes, thanks to their chemical versatilities and to the possibility to easily modify their hydrophilic and hydrophobic structure. In this thesis project the possibility to form different membranes model is investigate, using mixtures of an amphiphilic polymer and two different lipids. The polymer studied is composed by nine repetitions of poly(ethylene oxide) as the hydrophilic head group, and twelve repetitions of poly(butadiene) forming the hydrophobic tail, and it will be named OB9-12. Different mixtures of OB9-12 and DPPC are characterized in form of vesicles to assess the homogeneity or not of the bilayer formed. It will be found that a certain molar ratio between the two components is necessary to form stable and spherical vesicles. Moreover the possibility to form different bilayers changing the concentration of lipid and polymer will be shown, finding a molar ratio at which a phase separation is visible in giant unilamellar vesicles observed with fluorescence microscopy. The formation of lipids rafts is an important feature of biological membranes, helping the lateral protein organization and the regulation of the membrane mobility. Using the differential scanning calorimetry, the interaction between DPPC molecules and the polymer in form of vesicles will be investigated. The same mixtures will be deeply studied in form of monolayers, through the Langmuir-Blodgett technique, and the way in which the two components interact is observed also for these systems. A third interesting membrane model is in form of supported bilayer on a surface, which allows the use of sensitive surface techniques, like the quartz crystal microbalance. In this work, supported hybrid bilayers were successfully formed using mixtures of POPC and OB9-12 up to the 50% molar fraction of polymer. The difficulties in forming the bilayer via vesicle fusion, found for the pure polymer vesicles, were overcome by mixing it with a fluid lipid. In this way it was possible to form supported bilayer with a variable lateral fluidity, modifying the concentration of the polymer, as observed with fluorescence recovery after photobleaching measurements. The higher was the polymer molar fraction, the lower was the diffusion coefficient of the membrane. The bilayers formed were also characterized with the atomic force microscopy, both with force spectroscopy and imaging modes.