Structure and dynamics of phospholipid bilayer films under electrochemical control

Vesicle fusion was used to deposit mixed dimyristoyl phosphatidylethanolamine-dimyristoyl phosphatidylserine (DMPE-DMPS) phospholipid bilayers on Au electrodes. Bilayer structure and composition, when exposed to aqueous NaF and subject to an applied electrochemical potential, were studied using electrochemical, spectroscopic and neutron reflectivity (NR) techniques. Interfacial capacitance data indicate the formation of compact films. Chronocolometric data show that surface charge is significantly altered by the presence of lipid in the potential range -0.75 < E/V (Ag vertical bar AgCl) < 0.35. NR measurements were made on lipid films in which the hydrocarbon tails were either fully hydrogenous (h-DMPE-h-DMPS) or perdeuterated (d-DMPE-d-DMPS), in each case serially exposed to D2O and H2O electrolytes and subject to different applied potentials. Guided by simulations of candidate interfacial structures, these yield the spatial distributions of lipid and solvent within the layers. Adjacent to the electrode, a compact inner leaflet is formed, with potential-dependent solvent volume fraction in the range 0.09 < phi(S) < 0.19; there was no evidence of an intervening water layer. The outer leaflet contains rather more solvent, 0.52 < phi(S) < 0.55. NR-derived film thickness and PM-IRRAS intensity data show that the lipid molecules are tilted from the surface normal by ca. 26 degrees. Bilayer solvation and charge data show a strong correlation for the inner leaflet and very little for the outer leaflet.