Membrane Structure and Intermembrane Forces Observed with Small Angle X-Ray Scattering

Abstract

Cellular functions rely on intermembrane interactions and forces that govern membrane structure and hence modulate lipid-protein interactions [1]. Moreover, the strengths of intermembrane forces vary with interlamellar distances. Here we address material properties of the membrane with structural deformation due to external stress using small-angle X-ray scattering (SAXS) spectroscopy. The SAXS technique has been extensively used to study membrane bilayers through application of osmotic pressure. However, distinguishing the effects of osmotic stress on intermembrane forces (separation force) and membrane deformation requires further investigation [2]. We subjected model membranes (DMPC) in the liquid-crystalline state to dehydration and high osmotic pressures (up to 25 MPa). The work of removal of water from the interlamellar region to the bulk water region restructures the membrane assembly and prompts us to examine membrane properties using complementary techniques. Using SAXS we were able to directly measure the interlamellar spacings and compare the results to solid-state 2H NMR data [1,3]. We correlated the influences of dehydration and osmotic pressure in SAXS results through the interlamellar spacing. This approach allowed us to gauge the strength of intermembrane forces for a given hydration state. The combined techniques allowed us to estimate the area per lipid and structural deformation at the molecular level. Under high osmotic pressure or low hydration we found large area deformations up to 15% [1]. Temperature variation with this approach is used to discern entropic-based forces (lipid protrusions) and ordering-based forces (the hydration force). These findings show significant area deformation of membranes and provide insight into the forces that govern intermembrane interactions.[1] K.J. Mallikarjunaiah et al. (2011) BJ 100, 98-107.[2] V.A. Parsegian et al. (1979) PNAS 76, 2750-2754.[3] H.I. Petrache and M.F. Brown (2007) Meth. Mol. Biol. 400, 341-353.