Highly Resolved Structure of a Floating Lipid Bilayer: Effects of Calcium Ions and Temperatue

Abstract

Biological membranes are heterogeneous and highly dynamical organizations of lipids and proteins that define the outer boundary of a cell. Solid supported lipid bilayers have often been studied as model systems to understand the structures and properties of such cellular membranes. Such systems are constrained to a planar geometry and the strong support-membrane interaction could disrupt the inherent structural and functional properties of the membranes. To accurately mimic the biological membranes with their natural thermal fluctuations, curvature deformity and in-plane mobility of lipid molecules, a soft support is required. Hence, we have studied a double bilayer where the upper floating bilayer has the structural freedom to reproduce the morphology of a cellular membrane. Electron density profiles obtained from 1,2- dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayers by reflectivity studies using synchrotron x-rays provide unprecedented structural details of this system. Further, the effects of Ca2+ ions on the structure of these systems and the distribution of these ions near the bilayer have been investigated. These ions are preferentially bound to the head group region of the bilayer, which leads to a tight packing of lipids in the film. They cause the bilayer to thicken by increasing the hydrophobic core of the bilayer. Again, these ions are observed to intensify the flexibility of the bilayers, which is exhibited by the increased interfacial roughness. With the added Ca2+ ions, the inter bilayer separation is found to increase as a function of temperature and finally the floating bilayer unbinds from the adsorbed one. Such an effect was not observed in the absence of these ions.Acknowledgments. This work was supported by Office of Basic Energy Sciences, US Department of Energy, via Grant No. DE-FG02-04ER46173.