Formation of substrate-supported membranes from mixtures of long- and short-chain phospholipids

We studied the formation of substrate-supported planar phospholipid bilayers (SPBs) on glass and silica from mixtures of long- and short-chain phospholipids to assess the effects of detergent additives on SPB formation. 1,2-Hexyanoyl-sn-glycero-3-phosphocholine (DHPC-C6) and 1,2-heptanoyl-sn-glycero-3-phosphocholine (DHPC-C7) were chosen as short-chain phospholipids. 1-Palmitoyl-2-oleol-sn-glycero-3-phosphocholine (POPC) was used as a model long-chain phospholipid. Kinetic studies by quartz crystal microbalance with dissipation monitoring (QCM-D) showed that the presence of short-chain phospholipids significantly accelerated the formation of SPBs. Rapid rinsing with a buffer solution did not change the adsorbed mass on the surface if POPC/DHPC-C6 mixtures were used below the critical micelle concentration (cmc) of DHPC-C6, indicating that an SPB composed of POPC molecules remained on the surface. Fluorescence microscopy observation showed homogeneous SPBs, and the fluorescence recovery after photobleaching (FRAP) measurements gave a diffusion coefficient comparable to that for SPBs formed from POPC vesicles. However, mixtures of POPC/DHPC-C7 resulted in a smaller mass of lipid adsorption on the substrate. FRAP measurements also yielded significantly smaller diffusion coefficients, suggesting the presence of defects. The different behaviors for DHPC-C6 and DHPC-C7 point to the dual roles of detergents to enhance the formation of SPBs and to destabilize them, depending on their structures and aggregation properties.