Characterization and control of lipid layer fluidity in hybrid bilayer membranes

The main gel-to-liquid-crystal (LC) phase transition temperature, T-m, of the distal lipid layer in hybrid bilayer membranes (HBMs) under water was investigated using vibrational sum frequency spectroscopy (VSFS). VSFS has unique sensitivity to order/disorder transitions in the lipid acyl chains and can determine T-m for the lipid monolayers in HBMs. We recently reported the observation that T-m is raised and the transition width is broadened for the overlying phospholipid monolayer in HBM systems formed on densely packed crystalline self-assembled monolayers (SAMs) as compared to that of vesicles in solution. In this report, we establish that T-m for the lipid layer of HBMs can be controlled by proper choice of the SAM underlayer. The SAM underlayer of the HBM was systematically altered by using an alkane thiol, a saturated thiolipid, a mixed SAM of a saturated lipid-pyridine disulfide, and finally a mixed SAM of an unsaturated lipid-pyridine disulfide. T-m was measured for two different chain length saturated phosphatidylcholine lipid overlayers on these SAMs. The values obtained show that T-m of the lipid layer of HBMs is sensitive to the composition and/or packing density of molecules in the underlying SAM.