Order at the Edge of the Bilayer: Membrane Remodeling at the Edge of a Planar Supported Bilayer Is Accompanied by a Localized Phase Change

We present experimental evidence for the existence of a unique molecular-level order in the vicinity of the bilayer's edge. Discrete patches of substrate-supported lipid bilayers exhibiting stable edge defects are prepared by confining vesicle fusion to hydrophilic patches of a chemically patterned substrate exhibiting hydrophilic patches in hydrophobic surrounding, and edge properties are characterized by fluorescence and vibrational spectroscopy based measurements. Specifically, wide-field fluorescence microscopy using phase-sensitive dyes, temperature-programmed fluorescence recovery measurements, and temperature-dependent attenuated total reflection Fourier transform infrared spectroscopy measurements are performed to characterize the local chain conformational properties, local diffusional characteristics, and phase discrimination afforded by phase-sensitive Dil fluorescent probes. We find that the bilayer structure near the edge is characterized by (1) an increase in intramolecular conformational order; (2) reduced effective lateral mobility; and (3) a distinctly higher local, effective gel-fluid transition temperature in comparison to their bulk counterpart. Together, these features signal the emergence of unique ordering presumably triggered by the hemimicellar configuration of the edge. These results are consistent with simulations of lyso-lipid micelles predicting the presence of dynamic clusters of ordered lipids in comparable micellar topology and disagrees with some recent interpretations of mobility near the edges of supported bilayers. Our results also offer the structural basis for the stability of defects and edges in fluid supported bilayers, and may be relevant in understanding the ordering and stabilization of pores, edges, and defects generated in membrane bilayers by proteins, curvature-sensitive lipids, antimicrobial peptides, and detergents.