Dissipation-enhanced quartz crystal microbalance studies on the experimental parameters controlling the formation of supported lipid bilayers

We report on the investigations of the transformation of spherically closed lipid bilayers to supported lipid bilayers in aqueous media in contact with SiO2 Surfaces. The adsorption kinetics of small unilamellar vesicles composed of dimyristoyl- (DMPC) and dipalmitoylphosphatidylcholine (DPPC) mixtures on SiO2 surfaces were investigated using a dissipation-enhanced quartz crystal microbalance (QCM-D) as a function of buffer (composition and pH), lipid concentration (0.01-1.0 mg/mL), temperature (15-37 degrees C), and lipid composition (DMPC and DMPC/DPPC mixtures). The lipid mixtures used here possess a phase transition temperature (T-m) of 24-33 degrees C, which is close to the ambient temperature or above and thus considerably higher than most other systems studied by QCM-D. With HEPES or Tris-HCl containing sodium chloride (150 mM) and/or calcium chloride (2 mM), intact vesicles adsorb on the surface until a critical density (Theta(c)) is reached. At close vesicle contact the transformation from vesicles to supported phospholipid bilayers (SPBs) occurs. In absence of CaCl2, the kinetics of the SPB formation process are slowed, but the passage through Theta(c) is still observed. The latter disappears when buffers with low ionic strength were used. SPB formation was studied in a pH range of 3-10, yet the passage through Theta(c) is obtained only for pH values above to the physiological pH (7.4-10). With an increasing vesicle concentration, Theta(c) is reached after shorter exposure times. At a vesicle concentration of 0.01-1 mg/mL, vesicle fusion on SiO2 proceeds with the same pathway and accelerates roughly proportionally. In contrast, the pathway of vesicle fusion is strongly influenced by the temperature in the vicinity of T-m. Above and around the T-m, transformation of vesicles to SPB proceeds smoothly, while below, a large number of nonruptured vesicles coexist with SPB. As expected, the physical state of the membrane controls the interaction with both surface and neighboring vesicles.