Formation of supported phospholipid bilayers from unilamellar vesicles investigated by atomic force microscopy

Since their introduction through the work of McConnell et al. in the early 80s, supported phospholipid bilayers (SPBs) have proven to be a versatile m odel system for investigating a wide variety of phenomena. Despite their co ntinuous application in fundamental as well as applied research fields, the mechanism by which SPBs are formed from suspensions of unilamellar vesicle s remains poorly understood. Utilizing the ability of atomic force microsco py (AFM) to investigate processes in situ and in real time, we have studied the early stages of SPB formation on mica. Unilamellar vesicles of various sizes, composed of zwitterionic phospholipids, were prepared by sonication or extrusion. Vesicles of all sizes investigated were found to adsorb to m ica. Unruptured vesicles forming supported vesicular layers (SVLs), as well as disks, formed as a result of vesicle rupture, could be visualized by AF M. The behavior of the SVLs was found to depend on the vesicle size, the li pid concentration, and the presence or absence of Ca2+. The picture of the mechanism of SPB formation, which emerges from the results presented in thi s report, is critically compared with theoretical predictions and experimen tal results reported to date.