Silica colloidal particles with functionalized surfaces are used, for example, in studies of membrane proteins or for drug delivery, where novel applications are based on the use of particles covered by lipid membrane bilayers. The mechanism by which such supported lipid bilayers are formed on spherical support is not fully understood. Here, we present results from studies of this process using a new method based on flow cytometry. The approach enabled us to detect particle populations coated and uncoated with lipids in the same sample according to the vesicle:particle surface area ratio. The data suggest that DOPC lipid vesicles efficiently break upon interaction with the silica colloidal particle surface; only a small fraction of the adsorbed vesicles remain unbroken. Furthermore, the data support earlier observations showing that formation of the lipid bilayer at the surface is a cooperative process, where bilayer formation is catalyzed by previously bound membrane fragments.

We have investigated formation of a proteolipid membrane surrounding mesoporous silica particles with a diameter of 550 nm and pore sizes of 3.0 nm. A multisubunit redox-driven proton pump, cytochrome c oxidase, was incorporated into the membrane and we show that the enzyme is fully functional, both with respect to catalysis of O₂ reduction to water, and charge separation across the membrane. The orientation of cytochrome c oxidase in the membrane was found to be the same (~70/30 %) in the lipid vesicles and in the silica-particle supported lipid membrane, which provides information on the mechanism by which the vesicles adsorb to the surface. Furthermore, cytochrome c oxidase could maintain a proton electrochemical gradient across the supported proteomembrane, i.e. the membrane system was proton tight, defining an interior particle compartment that is separated from the surrounding aqueous media. Such a biofunctional cellular interface, supported onto a colloid that has a connected interior cytoskeleton-like pore structure, provides a basis for functional studies of membrane-bound transport proteins, and also for applications within pharmaceutical drug delivery.

Energy conversion in biological systems is underpinned by membrane-bound proton transporters that generate and maintain a proton electrochemical gradient across the membrane. The free energy stored in this gradient is used, for example, for transport of molecules or ions by other transporters as well as for generation of ATP by the ATP synthase. Understanding the overall process requires functional studies of the coupled reactions, which involve co-reconstitution of e.g. a proton pump and a transporter that utilizes the proton gradient. This process is likely to be further influenced by the composition of the membrane, which, for example, may facilitate lateral proton transfer. In the present study, we have co-reconstituted the proton pump bo₃ ubiquinol oxidase with ATP synthase, both from E. coli, in liposomes with different membrane compositions. The coupled proton pumping and ATP-synthesis activities were investigated. We found that the ATP synthesis was significantly higher in a membrane composed of pure DOPC lipids than in the presence of DOPA, DOPE, DOPG or cardiolipin. The drop in activity was considerably more pronounced upon addition of the negatively charged head groups (PA, PG or cardiolipin) than upon addition of the zwitterionic PE. The origin of these effects is discussed.