LAUSR

Lipid bilayer membranes are composed of hundreds of lipids, sterols and proteins, which organize in to a heterogeneous, structural scaffold that controls critical elements of biological function. The interactions of all of these molecules are mostly non-covalent and electrostatic in nature. These interactions and the diversity of molecules, along with ordered water molecules at the lipid-water interface gives rise to a large electrostatic field that traverses the lipid bilayer. The whole field can be broken down in to three components- transmembrane field, surface field and dipole field. We have extensively studied the membrane dipole field, which propagates from the interior of the lipid bilayer to the lipid head group-water interface, using vibrational Stark effect spectroscopy paired with molecular dynamics simulations. With our knowledge of how the dipole field can be altered due to a change in lipid bilayer composition, we are exploring what role this field plays in controlling the transport of ions through a membrane via transmembrane protein channels (TPCs). Gramicidin has been accepted as a good model for TPCs due to its ability to selectively transport ions with a +1 charge through the pore it creates when in its channel conformation. Using this well characterized model TPC we hope to elucidate how the structure and function can be changed via non-covalent interactions with other lipids, water molecules and sterols, specifically arising from the membrane dipole field. Using VSE spectroscopy, we have been able to begin to measure how gramicidin alters the dipole field of a pure phospholipid bilayer in the form of small unilamellar vesicles (SUVs). We are studying how the transport of +1 cations through the gramicidin channel and the structure of the TPC are affected by both small and large perturbations of the electrostatic field by intercalating sterols in to the lipid bilayer.