Electric Field Driven Conformational Changes of Gramicidin D in a Model Membrane Supported on a Au(111) Electrode Surface

Abstract

In this work, we show molecular resolution scanning tunnelling microscopy (STM) images of gramicidin, a model antibacterial peptide, inserted into a phospholipid matrix supported at a gold electrode surface. The resolution of the images is superior to that obtained in previous attempts to image gramicidin in a lipid environment using atomic force microscopy (AFM). This breakthrough has allowed visualization of individual peptide molecules surrounded by individual lipid molecules. We have observed several important features: the peptide molecules do not aggregate, the peptide molecules adopt a single conformation corresponding to a specific ion channel form, and the lipid molecules adjacent to the peptide molecules are systematically longer than those in the lipid matrix. These results constitute a new approach to obtain structural characteristics of antibiotic peptides in lipid assemblies that is necessary for the understanding of their biological activity.We then applied the polarization modulation infrared reflection absorption spectroscopy (PM IRRAS) to investigate the effect of the electric field on the conformation and orientation of gramicidin molecules in a bilayer supported at the gold electrode surface. We observed potential controlled changes in the orientation and conformation of the gramicidin molecules in the supported bilayer. Careful analysis of the IR data indicated that the potential applied to the electrode affects the bilayer structure and these changes cause reorientation and conformational transformations of gramicidin molecules.