Analysis of Supramolecular Protein Assemblies on Binary Lipid Monolayers by Adsorption, Topographical, and Fluorescence Studies

Abstract

Binary films of 1-palmitoyl-2-(16-(S-methyldithio)-hexadecanoyl)-sn-glycero-3-phosphocholine (DSDPPC) and 1,2-Dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-(Cap Biotinyl) (biotin-Cap-DPPE) were formed at the air-water interface. These floating Langmuir monolayers were transferred to a solid gold-coated substrate by the Langmuir-Schaefer deposition method. The role of the DSDPPC lipids is to bind the monolayer covalently to the gold substrate. The embedded receptor molecules (biotin-Cap-DPPE) make the film reactive against streptavidin (SA). For the deposition of subsequent streptavidin layers, a film of biotinylated bovine serum albumin (BBSA) was utilized as an intermediate binding layer between the consecutive streptavidin monolayers. The structure of the deposited binary lipid monolayer as well as the immobilization of subsequent protein layers was studied by Quartz Crystal Microbalance (QCM), Scanning Probe Microscopy (SPM) and ToF-SIMS. Attention was paid on the de-convolution and roughness analysis of the SPM data in order to obtain more quantitative information about the immobilized proteins. Fluorescence from the Eu-labeled SA was finally studied as a function SA-gold substrate distance.