Modulation of Substrate-Membrane Interactions by Linear Poly(2-methyl-2-oxazoline) Spacers Revealed by X-ray Reflectivity and Ellipsometry

Hydrated polymer interlayers between planar lipid membranes and solid substrates provide a water reservoir and thus maintain a finite membrane-substrate distance. Linear polymer spacers attached to lipid head groups (lipopolymer tethers) can be used as a defined model of oligo- and polysaccharides covalently anchored on cell surfaces (glycocalyx). They can offer a unique advantage over membranes physisorbed on polymer films (called polymer-cushioned membranes), owing to their ability to control both the length and density of polymer chains. In this study, a lipopolymer tether composed of a stable ether lipid moiety and a hydrophilic poly (2-methyl-2-oxazoline) spacer with a length of 60 monomer units is used to fabricate supported membranes by the successive deposition of proximal (lower) and distal (upper) leaflets. Using specular X-ray reflectivity and ellipsometry, we systematically investigate how the lateral density of polymer chains influences the membrane-substrate interactions. The combination of two types of reflectivity techniques under various conditions enables the calculation of quantitative force-distance relationships. Such artificial membrane systems can be considered as a half-model of cell-cell contacts mediated via the glycocalyx, which reveals the influence of polymer chain density on the interplay of interfacial forces at biological interfaces.