Stability of lipid layer systems under high adhesion

Abstract

The plasma membrane is the outermost component of cellular structures and as such it protects the cell interior from the external environment. Due to its wide range of biochemical functionality, the composition of this lipid bilayer structure is extremely complex and also differs from cell to cell. Supported lipid bilayers (SLBs) are greatly simplified model system for the plasma membrane and enable one to investigate the very basic properties of lipid bilayer structures. Due to the underlying substrate, the applicability of a variety of measurement techniques on the system is considerably enhanced. In this research, the stability of such synthetic lipid bilayer systems under high adhesion was investigated. For this purpose, the surface forces apparatus (SFA) was applied, yielding the distance-dependent interaction force of the deposited layer with an opposite surface and a qualitative measure of its mechanical compressibility. Time-stability and resistance of the systems to high adhesion forces could be detected by comparing sequential measurements performed on the same sample. In addition, the obtained data could be compared to the expected distance dependence of the interaction force according to DLVO theory. Atomic force microscopy (AFM) was further applied to measure the surface topographies of the systems which were then related to the results obtained from SFA experiments. The first measurements were performed on a mica supported 1,2-dipalmitoyl-snglycero-3-phosphocholine (DPPC) bilayer as a typical SLB system which was later modified in order to gain system stability. It was found that by introducing an inner layer with stronger interaction with the substrate material, a higher mechanical stability of the deposited layer structure under pressure could be achieved. This was shown for 1,2-dipalmitoyl-3-trimethylammonium-propane (DPTAP) as the inner layer, giving an electrostatic attraction to the substrate, and, to a greater extent, for a self-assembled monolayer (SAM) of hexadecanethiol on gold as the inner layer, yielding a strong covalent bond to the substrate.