Poster Presentation 2014 International Biophysics Congress

How nanoscale bending affects structure, dynamics, phase properties and surface electrostatics of lipid bilayers (#600)

Alex I. Smirnov 1 , Antonin Marek 1 , Amir Koolivand 1 , Maxim Voinov 1
  1. North Carolina State University, Raleigh, NC, USA

Membrane-associated proteins are known to be capable of sensing local lipid bilayer curvature.  Highly curved lipid bilayers and small vesicles are thought to be involved in such important cellular processes as membrane fusion, endo- and exocytosis, and tubules’ formation.  While the significance of membrane curvature in cellular regulatory processes is emerging, limited biophysical data for highly curved lipid bilayers do exist.  Here we summarize the results of spin-probe/spin-labeling multifrequency EPR studies together with solid state NMR and DSC of unilamellar vesicles (SUV) with average diameter ranging from 200 to 30 nm and compare those with tubular bilayers formed inside ceramic nanopores of similar diameters.  Analysis of DSC data at multiple scan rates has revealed broadening and shifts of the main phase transition of DMPC. This indicates bilayer compression and an increase in local order parameter – phenomena that are revealed by EPR and oxygen accessibility measurements.  The surface electrostatics of lipid vesicles was assessed by EPR of a recently introduced phospholipid (IMTSL-PTE) bearing a pH-sensitive nitroxide covalently attached to the lipid head group. The magnitude of the negative surface electrostatic potential, Ψ, for POPG increased from -137 to -167 mV upon decrease in the vesicle diameter from ca. 110 to 30 nm even though zeta-potentials were nearly identical.  This effect could be again rationalized by an increase in lipid packing upon an increase in curvature for the bilayer in fluid phase.   However, the effect vanished for the gel phase.  We conclude that a biologically relevant fluid bilayer phase allows for a larger variability in the lipid packing density in the lipid polar head group region than a better ordered gel phase.  Supported by U.S. DOE Contract DE-FG02-02ER15354.