mimetic chemistry, amplification of biomolecular recogni-Isotherms for adsorption of bilayer-forming synthetic amphition, and design of biosensors. However, the assembly of philes or phospholipids from vesicles onto hydrophilic silica partibilayer-forming molecules at interfaces is not yet well cles (Aerosil OX-50) are obtained over a range of experimental understood. conditions. Phosphatidylcholine (PC), dipalmitoylphosphatidyl-Unilamellar phosphatidylcholine vesicles disrupt and adcholine (DPPC), dihexadecylphosphate (DHP), and dioctadecylhere to a mica surface to form a bilayer coating (1). Phosdimethylammonium bromide (DODAB) dispersed in 10 m M Tris,
pholipid monolayers with lipid haptens inserted can be sup-pH 7.4, as small unilamellar vesicles present affinities for silica ported by hydrophobic glass and are useful for specific following the sequence DODAB ΓΊ DPPC ΓΊ PC ΓΊ DHP. Among these, only DHP adsorption is below that expected for one bilayer adherence of macrophages and cell surface recognition deposition. Interaction at 65ΠC for 1 h between DPPC (or, at 25ΠC, studies, but cannot serve as hosts for transmembrane profor PC) vesicles and silica efficiently leads to bilayer deposition at teins (2). Very stable high-resistance bilayers were formed maximal adsorption, if Tris is the buffer used. Preliminary centrifby deposition of a covalently bound monolayer of mercapugation of the PC or DPPC vesicle dispersion is necessary to toalkyls on gold-covered substrates followed by transfer prevent overestimated adsorption. PC affinity for silica and its of a lipid monolayer via monolayer transfer ( 3 ) . Small deposition as a bilayer depend on the nature of buffer used being unilamellar phospholipid vesicles and polystyrene micromuch higher for Tris than for Hepes at pH 7.4. Formation of ion spheres interact in aqueous solution to form homodisperse pairs between protonated amino groups in Tris and silanol groups and stable phospholipid-covered latexes. First, there is covon silica may lead to Tris adsorption and an increase in density erage with one phospholipid monolayer with polar heads of {OH groups on the solid surface. Therefore, formation of cooperative hydrogen bridges between {P|O in the phosphati-uppermost on the latex, and thereafter, phospholipid bidyl of the phospholipid bilayer and the -OH groups of Tris adlayers deposit onto the monolayer-covered latex (4). These sorbed on silica increases PC affinity for silica in the presence of covered latexes are useful as hosts for receptors that medi-Tris. For Hepes as buffer, PC affinity for silica is much lower ate transmembrane signaling (5). Phospholipid-covered laand no plateau indicative of bilayer deposition is observed in the tex is also useful in promoting biomolecular recognition adsorption isotherm. Stabilization of supported phospholipid biand in modulating protein adsorption on the polymer (6). layers on solid surfaces requires several cooperative hydrogen brid-Charged synthetic amphiphile bilayers composed of dioctages between the phospholipid and the solid surface. DODAB addecyldimethylammonium chloride ( DODAC ) or dihexadesorption was unaffected by vesicle age and physical state of the cylphosphate ( DHP ) ( 7 ) are electrostatically attracted to bilayer vesicle. Adsorption isotherms for DODAB are of the highoppositely charged polystyrene microspheres, forming hoaffinity type with a maximum indicative of competition between intervesicle interactions and DODAB deposition on silica. Stabili-modisperse bilayer-covered latex (8-11).
zation of DODAB bilayer deposition requires surface charge densi-
Reports in the literature regarding the interaction between ties on silica higher than 01 mC/cm 2 . α§ 1997 Academic Press single phospholipid bilayers and glass or silica are controver-Key Words: bilayer-forming amphiphiles; phospholipids; catsial. Some authors report coverage with one lipid bilayer ionic and anionic surfactants; hydrophilic colloidal particles; Aerothat is separated from the bead surface by a water layer 1.5sil OX-50; vesicles; supported bilayers on silica.
2.0 nm thick (12, 13), whereas others indicate formation of one monolayer at the interface with polar head groups oriented into the aqueous phase (14,15). Deposition of a sup-