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Abstract

Wetting is an ubiquitous phenomenon not only present in nature and biology but also of key importance in industry and medical applications. In biological systems, cell adhesion can also be understood within the framework of wetting of complex, non- Newtonian fluids. During the past several decades, physical chemistry of wetting and interfacial interactions of biological interfaces has been developed with aid of new experimental and analytical tools. The primary aim of this thesis is to study the wetting of model cell membranes by using the combination of various physicochemical techniques. In section 4 the wetting (adhesion) of bio-inspired, stimulus responsive polymer brushes with simple model cells, giant vesicles,were studied. First, the switching of polymer brush conformation was monitored by using high energy specular X-ray reflectivity. Then, the global shape of giant vesicles at relaxed and compacted states were monitored by confocal fluorescence microscopy. As the shape of vesicles is not determined by the surface tension (like the case of Newtonian fluid), the shape (height profile) of vesicle near the surface, dominated by the membrane elasticity, was analyzed by microinterferometry. This enables the quantitative calculation of the adhesion free energy of vesicles. Moreover, owing to the capability of the microinterferometry to determine the height fluctuation in nm accuracy, the interfacial potential between the polymer brush and the giant vesicle could be calculated quantitatively. This line of study was further extended in section 5, where the dynamic switching of the vesicle-brush interaction was monitored. The transition between from non-wetting (off) to wetting (on), and vice versa, was was monitored in real time by the combination of microfluidics and microinterferometry with the time resolution of 30 ms. It is well known that not only the adhesion but also the binding of proteins changes the mechanical properties of lipid membranes. In section 6, the influence of subtle changes in molecular structures on the protein-membrane interaction and hence the membrane mechanics was investigated using the same membrane model (giant vesicles). As a biologically relevant model, the modulation of the membrane mechanics by the binding of the acute phase inflammatory C-reactive protein (CRP) to the vesicles containing oxidized lipids was investigated. First, the potential role of electrostatic interaction was studied by measuring the zeta potential of lipid vesicles, suggesting that CRP binds to the vesicles containing lipids with a higher oxidative level. In the next step, the influence of CRP binding on the mechanical properties of the membranes was calculated from the Fourier analysis of the membrane fluctuation, indicating that the binding of CRP to highly oxidized lipids caused the most prominent change in the bending rigidity of the membrane. The obtained results demonstrated how the subtle changes on the molecular level, such as the compaction of polymer brushes and the oxidation of lipids, significantly modulates the wetting and mechanical properties of lipid membranes.