Formation of supported phospholipid bilayers on molecular surfaces: role of surface charge density and electrostatic interaction.

Electrostatic interaction is known to play important roles in the adsorption of charged lipids on oppositely charged surfaces. Here we show that, even for charge neutral (zwitterionic) lipids, electrostatic interaction is critical in controlling the adsorption and fusion of lipid vesicles to form supported phospholipid bilayers (SPBs) on surfaces. We use terminally functionalized alkanethiol self-assembled monolayers (SAMs) to systematically control the surface charge density. Charge neutral egg phophatidylcholine (eggPC) vesicles readily fuse into SPBs on either a positively charged 11-aminino-1-undecanethiol SAM or a negatively charged 10-carboxy-1-decanethiol SAM when the density of surface charge groups is > or = 80%. These processes depend critically on the buffer environment: fusion of adsorbed vesicles to form SPBs on each charged molecular surface does not occur when the molecular ion of the buffer used is of the opposite charge type. We attribute this to the high entropic repulsion (electric double layer repulsion) due to the large size of molecular counterions. On the other hand, such a critical dependence on buffer type is not observed when charged lipids are used. This study suggests the general importance of controlling electrostatic interaction in the formation of stable SPBs.