Probing the stability of S-layer-supported planar lipid membranes

Isolated protein subunits of the crystalline bacterial cell surface layer ( S-layer) of Bacillus coagulans E38-66 have been recrystallized on one side of planar black lipid membranes (BLMs) and their influence on the electrica l properties, rupture kinetics and mechanical stability of the BLM was inve stigated. The effect on the boundary potential, the capacitance or the cond uctance of the membrane was negligible whereas the mechanical properties we re considerably changed. The mechanical stability was characterized by appl ying voltage pulses or ramps to induce irreversible rupture. The amplitude of the voltage pulse leading to rupture allows conclusions on the ability o f membranes to resist external forces. Surprisingly, these amplitudes were significantly lower for composite S-layer/lipid membranes compared to undec orated BLMs. In contrast, the delay time between the voltage pulse and the appearance of the initial defect was found to be drastically longer for the S-layer-supported lipid bilayer. Furthermore, the kinetics of the rupture process was recorded. Undecorated membranes show a fast linear increase of the pore conductance in time, indicating an inertia-limited defect growth. The attachment of an S-layer causes a slow exponential :increase in the con ductance during rupture, indicating a viscosity-determined widening of the pore. In addition, the mechanical properties on a longer time scale were in vestigated by applying a hydrostatic pressure across the BLMs. This causes the BLM to bulge, as monitored by an increase in capacitance. Compared to u ndecorated BLMs, a significantly higher pressure gradient has to be applied on the S-layer face of the composite BLMs to observe any change in capacit ance.