Biomembrane mimetic surfaces by phospholipid self-assembled monolayers on silica substrates

A method of covalently bonding phospholipid molecules to silica substrates followed by loading with free phospholipids is demonstrated to form well-organized and stable phospholipid self-assembled monolayers (SAMs). Surfaces of such SAMs, which structurally mimic the aqueous sides of phospholipid bilayer membranes, are characterized with X-ray photoelectron spectroscopy and atomic force microscopy. Dynamics of phospholipids and an adsorbed protein, lipase, in the SAMs are probed with the technique of fluorescence recovery after photobleaching (FRAP), in terms of lateral diffusion of both phospholipids and protein molecules. The lateral diffusion coefficient of the free lipids in the SAMs is found as (1.9 +/- 0.4) x 10(-9) cm(2)/s whereas that-of lipase is (2.7 +/- 0.4) x 10(-10) cm(2)/s; both are within an order of magnitude of those in cell membranes. The esterase activity of lipase on the SAM surfaces is confirmed by the hydrolysis reaction of a substrate, umbelliferone stearate.