Direct submolecular scale imaging of mesoscale molecular order in supported dipalmitoylphosphatidylcholine bilayers.

Supported dipalmitoylphosphatidylcholine (DPPC) bilayers are widely used membrane systems in biophysical and biochemical studies. Previously, short-range positional and orientational order of lipid headgroups of supported DPPC bilayers was observed at room temperature using low deflection noise frequency modulation atomic force microscopy (FM-AFM). While this ordering was supported by X-ray diffraction studies, it conflicted with diffusion coefficient measurements of gel-phase bilayers determined from fluorescence photobleaching experiments. In this work, we have directly imaged mica-supported DPPC bilayers with submolecular resolution over scan ranges up to 146 nm using low deflection noise FM-AFM. Both orientational and positional molecular ordering were observed in the mesoscale, indicative of crystalline order. We discuss these results in relation to previous biophysical studies and propose that the mica support induces mesoscopic crystalline order of the DPPC bilayer at room temperature. This study also demonstrates the recent advance in the scan range of submolecular scale AFM imaging.