Electric field driven changes in the orientation and conformation of gramicidin in a lipid bilayer supported on a gold(111) electrode

In this work we have quantitatively studied the effect of an applied electrode potential comparable to the transmembrane potential on a DMPC phospholipid bilayer containing membrane-spanning gramicidin (GD) channels (DMPC:GD) supported on a Au(111) surface. Changes on the electric field affect the bilayer molecules. Langmuir-Blodgett bilayers containing 10% GD were initially characterized using charge density curves determined from chronocoulometry experiments. The bilayer undergoes a phase transition at E ∼ -500 mV vs. Ag/AgCl which defines two main states: an adsorbed state at more positive potentials and a desorbed state where the film is detached from the surface at more negative values of E. Molecular detailed information regarding the orientation, conformation and interactions between the components of the bilayer was obtained from PMIRRAS measurements and STM imaging. The presence of a 10% molar ratio of GD has a measurable impact on the behaviour of the lipid acyl chains as compared to pure DMPC and DMPC:cholesterol systems studied under similar conditions. In particular, the presence of GD causes an ordering of the bilayer seen in the red-shift of the peak positions upon desorption as well as a decrease in the tilt of the acyl chains from 37° to 20° when detached. There is also a significant difference in the interaction of the carbonyl stretch of the polar head group due to the presence of the gramicidin, compared to pure DMPC and even DMPC:cholesterol bilayers. It is likely that the cause of this difference is the formation of hydrogen bonds between the tryptophan side chains of GD and the DMPC carbonyl groups. The STM in situ images of a DMPC:GD monolayer showed that the sample behaves as a two dimensional solid of hexagonally ordered phospholipid molecules. In the high resolution images, the peptides are seen as characteristic triangular cavities. We have demonstrated that these images provide unique information concerning peptide conformation, aggregation and peptide-lipid interactions.