Electrochemical and PM-IRRAS studies of floating lipid bilayers assembled at the Au(111) electrode pre-modified with a hydrophilic monolayer

Abstract

A floating bilayer lipid membrane (fBLM) assembled on a 1-thio-β-D-glucose (β-Tg)-modified Au(111) surface was engineered to mimic the environment of a biological cell membrane. This bilayer consists of a mixture of the phospholipid (DMPC), polyethylene glycol lipid (DMPE-PEG350) and cholesterol. Prior to membrane adsorption, a monolayer of β-Tg was first self-assembled at the Au(111) surface to increase the hydrophilicity of the electrode surface. The asymmetric bilayer was then deposited onto the hydrophilic support using a combination of Langmuir–Blodgett and Langmuir–Schaefer techniques. The inner leaflet of the bilayer was composed of DMPC, DMPE-PEG350 and cholesterol, while the outer leaflet consisted of DMPC and cholesterol. The hydrophilic PEG spacer subunit of the DMPE-PEG350 molecule promotes the formation of a water-rich domain, which separates the bilayer from the β-Tg modified surface. The Langmuir compression isotherms were recorded to examine the miscibility of these mixed monolayers at the air-solution interface of the Langmuir trough. Differential capacitance measurements showed that this fBLM was remained stable on the surface of the modified-gold electrode for potentials ranging from −0.40 to 0.45V versus Ag/AgCl and achieved a minimum capacitance of ∼3.0μFcm−2. Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) was used to obtain information regarding the conformation and orientation of the acyl chains and measure the degree of hydration of the phospholipid polar head groups within the film. These spectra demonstrated that the acyl chains of the lipids assume a small angle with respect to surface normal and that the phospho-glycerol groups are well-hydrated. The IR spectra of the PEG subunits demonstrated that the spacer unit adopts an amorphous, disordered conformation, which favors the hydration of the inner layer. This work demonstrates that the fBLM, composed of DMPE-PEG350 molecules, forms a water-rich region that separates the bilayer from the metal surface and that these new biomimetic systems are promising for future investigations involving transmembrane proteins.