A Spectroelectrochemical Study of the Effect of Asymmetry on the Electrochemical Response of Lipid Bilayers.

The effect of asymmetry in supported lipid bilayers on their electrochemical phase behavior has been studied using in situ Polarization Modulation Infrared Reflection Absorption Spectroscopy (PM-IRRAS). Dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylethanolamine (DMPE), which have the same tails and different headgroups, have been used to construct asymmetric bilayers on Au(111) electrodes. The organization and orientation of the hydrocarbon tails in each leaflet of the asymmetric bilayers have been characterized separately by deuterating the tails in the opposing leaflet. The vibrational frequencies of the chain methylene stretching modes show that DMPC is relatively ordered in asymmetric bilayers, and DMPE is relatively disordered, compared with their respective symmetric bilayers. The tail orientations in the as-deposited asymmetric bilayers are similar, showing the two monolayers influence each other, but the changes induced in each bilayer by the application of a potential difference across the bilayer are different and indicate that the bilayers decouple, with each monolayer responding separately to the imposed field. The results suggest that the behavior of previously reported symmetric systems may be more strongly influenced by the properties of the electrolyte-facing leaflet and highlight the value of using electrochemical perturbation of lipid bilayers in structural studies to provide additional insights into lipid-lipid interactions.