Lipid bilayer discs and banded tubules: Photoinduced lipid sorting in ternary mixtures

The self-assembly of biological amphiphiles has proved a fascinating topic in recent years, the hollow cylindrical lipid tubule morphology being of particular interest due to its potential applications in "soft" microtechnologies. Lateral coexistence of liquid-ordered (I-o) and liquid-disordered (I-d) phases, which may resemble raft formation in cell membranes, was investigated in lipid tubules, prepared from 1,2-dioleoylsn-glycero-3-phosphocholine, egg-sphingomyelin, and cholesterol. Fluorescence microscopy shows that the appearance of micrometer-scale 10 domains in the lipid tubule is not an intrinsic phase behavior of the system but a consequence of photoinduced lipid peroxidation. Most interestingly, new photoinduced bilayer structures: lipid discs, essentially stable flattened liposomes, were observed for the first time in a model membrane system. This investigation not only aids in our understanding of lipid sorting phenomena in cell membranes but also demonstrates how control of this process may provide a route to the generation of new, functional structures.