Tethered bilayer lipid membranes studied by simultaneous attenuated total reflectance infrared spectroscopy and electrochemical impedance spectroscopy.

The formation of tethered lipid bilayer membranes (tBLMs) from unilamelar vesicles of egg yolk phosphatidylcholine (EggPC) on mixed self-assembled monolayers (SAMs) from varying ratios of 6-mercaptohexanol and EO(3)Cholesteryl on gold has been monitored by simultaneous attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and electrochemical impedance spectroscopy (EIS). The influence of the lipid orientation (and hence the anisotropy) of lipids on a gold film on the dichroic ratio was studied by simulations of spectra with a matrix method for anisotropic layers. It is shown that for certain tilt angles of the dielectric tensor of the adsorbed anisotropic layer dispersive and negative absorption bands are possible. The experimental data indicate that the structure of the assemblies obtained varies with varying SAM composition. On SAMs with a high content of EO(3)Cholesteryl, tBLMs with reduced fluidity are formed. For SAMs with a high content of 6-mercaptohexanol, the results are consistent with the adsorption of flattened vesicles, and spherical vesicles have been found in a small range of surface compositions. The kinetics of the adsorption process is consistent with the assumption of spherical vesicles as long-living intermediates for surfaces of a high 6-mercaptohexanol content. No long-living spherical vesicles have been detected for surfaces with a large fraction of EO(3)Cholesteryl tethers. The observed differences between the surfaces suggest that for the formation of tBLMs (unlike supported BLMs) no critical surface coverage of vesicles is needed prior to lipid bilayer formation.