Electrostatic effects on deposition of multiple phospholipid bilayers at oxide surfaces
We investigated electrostatic effects on the formation of multiple supported phospholipid bilayers (SPB) by varying the oxide substrate, ionic strength, the presence of divalent Ca(2+), and phospholipid (PL) headgroup charge. Whereas the current understanding of processes and forces controlling SPB formation is based primarily on studies involving planar substrates, we report results from experiments using aqueous suspensions of quartz (α-SiO(2)) and corundum (α-Al(2)O(3)) particles. Using fluorescent dye-loaded dipalmitoylphosphatidylcholine (DPPC) vesicles, we determined that the vesicles underwent oxide particle-induced rupture and formed supported planar bilayers rather than a supported vesicle layer. Adsorption isotherms of DPPC at pH 7.2 in solutions of varying ionic strength set by NaCl, and with or without 2 mM Ca(2+), support our hypotheses that van der Waals forces predominantly account for two DPPC bilayers, and that adsorption beyond the second bilayer occurs at low ionic strength due to extension of the electric double-layer near the oxide surface. In contrast, adsorption isotherms of anionic dipalmitoylphosphatidylserine (DPPS) and cationic dipalmitoylethylphosphatidylcholine (DPEPC) show that adsorption of highly charged bilayers is decreased or prevented altogether due to bilayer-oxide and/or bilayer-bilayer repulsion. Results have potential implications for biomedical, industrial, and environmental remediation applications involving SPBs and for proto-cell stability in origin-of-life hypotheses.