Cell Membranes and Solid Supported Bilayers

Cell membrane and model systems. Supported bilayers.

According to the fluid mosaic model, cell membrane (picture on the left) is envisaged as a lipid bilayer (~ 5 nm thick) with transmembrane proteins embedded in it and peripheral proteins bound to it. The model is discussed in detail by Singer and Nicolson in Science 175, 720 (1972). A modern revision of this model, discussed by Jacobson et al. in Science 268, 1441 (1995) may also be of interest. The structure performs a wide variety of barrier, selective transport, and signaling functions in the cells. Lipids can not be thought of as a passive solvent for the proteins. Instead, they play an active role in the functioning of the proteins and in the functioning of the membrane as a whole.

Pathways for supported bilayer formation from vesicles in solution:

Solid-supported lipid bilayers are a versatile model system for studying cell membranes and for studying interactions between biological systems and inorganic materials. The process of their formation remains enigmatic however. We studied it by time-resolved atomic force microscopy. (The image on the left shows sonicated liposomes adsorbed on mica imaged with an AFM in contact mode in liquid).

Liposome - TiO2 interactions. Supported bilayer formation on TiO2

We investigated interactions between zwitterionic phospholipids (e.g., DOPC) and negatively charged phospholipids (DOPS) with TiO2 by quartz crystal microbalance, atomic force, and fluorescence microscopy.

Properties of TiO2 - supported lipid bilayers: aminophospholipid asymmetry.

In this scheme, a liposome composed of phosphatidyl serine (PS, green) and phosphatidyl choline (red) is shown to adsorb on the TiO2 surface in the presence of Ca2+. The bilayers that form are asymmetric with respect to the distribution of PS between the two bilayer leaflets: most of PS is sequestered in the surface-facing (proximal) leaflet and its mobility is restricted.

Patterns of supported lipid bilayers on SiO2/TiO2 pre-patterned surfaces.

By relying on the detailed knowledge of the lipid-surface interactions, we could prepare patterns of bilayers of different compositions on the pre-patterned Si/Ti substrates.