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 Tm of the lipid layer of HBMs is sensitive to the composition and/or packing density of molecules in the underlying SAM.