Infrared spectroscopic characterization of lipid-alkylsiloxane hybrid bilayer membranes at oxide substrates


Atul N. Parikh, Jaime D. Beers, Andrew P. Shreve, Basil I. Swanson, Langmuir 15, 5369 (1999)


Successive depositions of precompressed Langmuir monolayers have been shown to allow reproducible formation of air-stable, lipid−alkylsiloxane hybrid bimolecular architectures at oxidic supports. Specifically, prepolymerized Langmuir−Blodgett films of n-octadecylsiloxane (OTS) monolayers on oxided silicon substrates were used as the hydrophobic templates, upon which compressed monolayers of dipalmitoyl-sn1-glycerophosphatidylcholine (DPPC) and monosialogangliosides (Gm1) were deposited from a low-temperature air−water interface by the horizontal deposition method. Structural features of each leaflet of the resultant bimolecular architectures, namely DPPC/OTS/SiO2/Si and Gm1/OTS/SiO2/Si, were characterized using a combined application of infrared spectroscopy, null-ellipsometry, and surface wetting measurements. In both cases, the outer lipid leaflet (DPPC or Gm1) was found to be structurally decoupled with respect to the inner OTS layer. The inner silane layer was composed of essentially untilted (cant angle, θ = 0−10°), all-trans chains at the dense packing of ∼19 Å2/molecule, consistent with the previously reported structure in solution-phase assembled OTS monolayers. The outer DPPC leaflet, however, was found to be composed of collectively tilted (θ = 36°), all-trans acyl chains at the lower chain-packing density (∼26−28 Å2/chain) whereas the outer Gm1 leaflet was concluded to have essentially untilted chains at similarly lower chain-packing densities (∼23−26 Å2/chain) but with the carbohydrate head-groups disposed in a topologically staggered conformation. The structural independence of the two leaflets in the two classes of bilayered architectures examined here confirms the possibility of independently manipulating the molecular structure in each leaflet of supported hybrid bilayers.