Webb Group

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Electrostatic Fields in Lipid Bilayer Membranes

Summary: Lipid bilayer membranes control critical elements of biological function through structural and electrostatic mechanisms.  The structure of the lipid bilayer depends on numerous factors such as the chemical identity of lipids, the intercalation of small molecules such as sterols into the bilayer, and phase separation of lipid and protein-lipid domains.  This creates a complex electrostatic environment that has important consequences for interactions between the membrane and proteins associated with it.  The alignment of dipolar residues including water molecules, dipoles in the phospholipid head groups, and the carbonyls in the glycerol backbone create a dipole field as large as 1-10 MV/cm, located entirely within the low dielectric membrane interior.  In our laboratory, we have applied vibrational Stark effect (VSE) spectroscopy, coupled with molecular dynamics simulations with collaborators, to directly measure the magnitude and direction of an electric field within the membrane interior. We use the sensitivity of a nitrile vibrational chromophore, carefully placed at various locations across a lipid bilayer, to probe the local electrostatic environment inside the membrane.  This probe can be placed synthetically on a variety of membrane components, including on the lipid or other molecules intercalated in the membrane, without perturbing the native membrane structure or fluidity.  This is thus an ideal tool for studying the complex self-assembling organizational infrastructure of the membrane.

Strategy: We are currently focused on investigating how electrostatic fields in lipid membranes change as a function of the chemical composition of the bilayer using the experimental and computational methods developed in our laboratory and with collaborators.  Beginning with model bilayer membranes composed of phospholipids, we are systematically increasing the complexity of the membrane environment by adding small molecule intercalators such as sterols, more complex lipids, and membrane-associated peptides and proteins.  For example, we have successfully incorporated two structurally similar sterols, cholesterol and 6-ketocholestanol (6-kc) into a phospholipid bilayer and demonstrated that the presence of an extra ketone group on 6-kc causes distinct membrane fluidity and electrostatic profiles within the membrane. We have identified that the non-covalent electrostatic interactions between phospholipids,
sterols, and water molecules enveloping the membrane regulate the lipid packing density of a bilayer, and hence organizing into lipid order and disordered phases.  By exploring the complex network of electrostatic forces in a dynamic and heterogeneous biological membrane, we aim to better understand the underlying molecular mechanism of membrane structure, dynamics, and function, including vital functions such as unassisted transport of small molecules and peptides across the bilayer. Using a variety of characterization methods including FTIR, fluorescence, UV-vis, NMR, and circular dichroism spectroscopy, we are investigating membranes at a molecular level to better understand the mechanism of self-assembly and function in the complicated and dynamic structures that they create.

People:  Rebika Shrestha, Cari Anderson

Recent Publications: 
Cardenas, A. E.; Shrestha, R.; Webb, L. J.; Elber, R.  "Membrane Permeation of a Peptide: It is Better to be Positive."  J. Phys. Chem. B, 2015, 119, 6412-6420pdf

Shrestha, R.; Cardenas, A. E.; Elber, R. Webb, L. J.  "Measurement of the Membrane Dipole Electric Field in DMPC Vesicles Using Vibrational Shifts of p-Cyanophenylalanine and Molecular Dynamics Simulations."  J. Phys. Chem. B 2015, 119, 2869-2876. pdf