Published: February 21, 2011 r2011 American Chemical Society 2783 dx.doi.org/10.1021/la104504p | Langmuir 2011, 27, 2783–2788 ARTICLE pubs.acs.org/Langmuir Lipid Domain Depletion at Small Localized Bends Imposed by a Step Geometry Matthew I. Hoopes, † Roland Faller, †,‡ and Marjorie L. Longo* ,†,‡ † Biophysics Graduate Group, and ‡ Department of Chemical Engineering and Materials Science, University of California, Davis, California 95616, United States ’ INTRODUCTION Lipid molecules self-assemble into fluid membranes only two molecules thick to form a dual leaflet structure with fascinating properties. These dynamic assemblies not only provide for com- partmentalization and transportation in the three-dimensional world of biological cells but also play a part in the organization of a two-dimensional assembly of molecules in the membrane itself. Model membrane studies have indicated that membrane curva- ture plays an important role in the location of lipid species in the cell. While individual molecules appear unsusceptible to curva- ture sorting, 1 when heterogeneous lipid compositions form in the membrane, they will localize to regions with curvatures that are energetically favorable. 2 Systems of lipids, proteins, sugars, and other biological molecules are governed by the laws of soft matter physics. Understanding the relationships between geo- metry, energy, and time scale in these systems is necessary for insight into how dynamic processes, such as lipid sorting, function in the cell. In addition, these insights can be used to engineer soft matter systems that use geometry to control energy and dynamics. Similar past work shows the relationship of lateral domain organization to curvature on double-supported lipid bilayers, 3 as well as curvature sorting of lipid domains in giant unilamellar vesicles (GUVs) and microtubule-like tethers. 1 Consensus in these studies indicates that the differences in the bending stiffness of the lipid phases experiencing curvature lead to the aggregate lipid sorting behavior to lower the system energy. This means that curvature precedes the localization of specific lipid species aggregates or aggregates of classes of lipids with specific physical properties. In our work, as in past work, this property is the ben- ding stiffness of the membrane, which can be derived directly from the area compressibility modulus, K A , 4 of an elastic lipid bilayer. The purpose of the present work is to offer an application of elastic theory to the deflection of adsorbed membranes and to demonstrate how this affects lipid organization of phase-sepa- rated liquid-ordered, L o , domains surrounded by a liquid-dis- ordered, L d , phase. The study of supported lipid bilayers is a convenient method for analyzing the membrane because of the planar geometry. One consideration with this method is that the adsorption to the substrate will influence the membrane beha- vior. On the basis of changes to area compressibility, computa- tional models have shown that single supported bilayers will have a higher bending stiffness to that of free bilayers 5 and past expe- rimental work has used double bilayers to reduce the substrate influence on the membrane in question. 3 To address this, our work uses a multilamellar stack of bilayers, where the upper Received: November 11, 2010 Revised: January 22, 2011 ABSTRACT: Natural processes in biological cells rely on molecules to be in the right place at the right time to maintain the dynamics of living processes. When lipids in bilayer membranes move and mix, they experience kinetic and thermo- dynamic barriers that affect the time scales of their locations and associations with each other. One of these barriers is that of the membrane shape. Using spin coating as a deposition technique, we formed multilamellar supported lipid bilayers on topologi- cally patterned substrates with defined step rise heights of 13 and 27 nm measured by atomic force microscopy. Each step rise imposed two ridges on the lipid bilayers, and the ridge angles were measured by atomic force microscopy. The lipid composition of this system was 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and cholesterol (4:4:2), doped with a fluorescent lipid, which displays liquid-ordered-liquid-disordered (L o -L d ) phase coexistence upon cooling to 25 °C. The DPPC-rich L o domains in the upper bilayers were established to have boundaries and positions that responded to local forces. We found that these L o domains were depleted at the location of each step rise. We employed an equation for local bending at a ridge and demonstrate that L o domain densities at each rise correspond to these energies. Remarkably, an energy barrier greater than 1k B T is erected at a small deflection (1.3°) from planar geometry at the ridge, resulting in depletion of the majority of the optically visible L o domains from the step rise. This work provides a means to design substrates that, in conjunction with supported lipid bilayers, provide defined localized topological energy barriers that can be used in biomembrane engineering. It also provides a method for easily analyzing the energetics of cusp-like shapes in cellular membrane structures.