Monitoring of Bicelles Spreading into Floating Lipid Bilayers

Bicelles are hybrid, disk-shaped aggregates. Bicelles with a diameter of 11.8 ± 0.2 nm containing a long-chain lipid (1,2-dimyristoyl-sn-glycero-3-phosphocholine, DMPC) in their core and a short-chain, rim forming, lipid (1,2-diheptanoyl-sn-glycero-3-phosphocholine, DHPC) were prepared. A 100-fold dilution of the stock bicelle solution destabilizes the aggregate structure. Under this condition, the bicelles spread onto a β-thioglucose:6-mercaptohexanoic acid monolayer modifying Au(111) surface to form a free-standing floating DMPC bilayer while the DHPC molecules form micelles and diffuse into the electrolyte solution. Electrochemical impedance spectroscopy, electrochemically controlled quartz crystal microbalance, and polarization modulation infrared reflection absorption spectroscopy were applied to probe the macroscopic properties and potential-driven molecular scale changes in the floating DMPC bilayer. They are dependent on the sign of the membrane potential. The high values of the membrane resistance (ca. 2 MΩ cm²) indicate the formation of a compact, defect-free bilayer. At negative membrane potentials, the membrane resistance and tilt of the acyl chains (thickness of the membrane) change linearly as a function of potential, indicating that ion conduction occurs through the defect-free bilayer. A transition to positive membrane potentials leads to an abrupt decrease in the membrane resistance and tilting of acyl chains. These sudden reorientations lead to the formation of defects in the membrane structure as observed in electrochemical impedance spectroscopy experiments. The floating DMPC bilayer spread from bicelles provides ideal conditions to incorporate transmembrane proteins.