Formation of Supported Phospholipid Bilayers on Molecular Surfaces: Role of Surface Charge Density and Electrostatic
Interaction
Cha, TaeWoon
ABSTRACT
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 ≥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.
INTRODUCTION
Supported phospholipid bilayers (SPBs) have received increasing attention due to their applications in biosensors and their importance as models for
biological membranes (1-4). A number of studies have characterized the spontaneous fusion of adsorbed vesicles to form SPBs on oxides and
polymeric supports by quartz crystal microbalance, atomic force microscopy, and fluorescence microscopy (5-7). To mimic natural membranes, it is
necessary to reconstitute membrane proteins into an SPB in a mobile form. A key factor is the two-dimensional mobility of lipid molecules within the
SPB. It has been shown that SPBs formed on oxide (SiO^sub 2^ and TiO^sub 2^) surfaces possess lateral fluidity, but they are unstable when
withdrawn from the air/water interface (5,8). In contrast, the use of polymer-cushioned substrates increases the stability of SPBs while reducing longrange
lateral mobility of lipid molecules (9,10). Recently, Cremer and co-workers reported a protein-covered lipid bilayer system which is both stable
upon exposure to the air and completely fluidic in lateral diffusion (11). These studies point to the importance of surface-vesicle interactions, including
van der Waals, electrostatic, hydration, and steric forces, in SPB formation. A number of studies have addressed mechanistic aspects of vesicle fusion
and SPB formation by varying pH, ionic strength, charge contents in lipids, and the concentration of bivalent metal ions (Ca^sup 2+^ or Mg^sup 2+^)
in solutions (6,12-14). Electrostatic interactions involving charged lipids (15), including the interaction between charged lipids and oppositely charged
surfaces (6), are well known. However, little is known about the possible role of electrostatic forces for charge neutral lipids.
