Sensing Curvature: Lipid Phase Coexistence at Planar and Curved Monolayers

Abstract

Current approaches for the biophysical investigation of ‘curved’ cell membranes, like giant unilamellar vesicles and pipette-pulled membrane tubes of controlled radius, offer a narrow range of curvature tuning to explore the impact on membrane structure and behavior. We have designed and fabricated a novel bubble and drop based air-aqueous or oil-aqueous interface setup, where spherical monolayers of broad range of curvatures (25-1000 µm radii) can be created besides planar monolayer. The curved monolayers formed under controlled conditions can be gradually or instantly switched to higher or lower curvatures. Our unique approach combines 3-D imaging capabilities of confocal microscopy with this setup, to resolve early structures and domain evolution in curved and planar monolayers with respect to the surface tension and time. Moreover, an oscillation mechanism is included to induce sinusoidal waves of surface area in curved monolayers, causing dilatation and compression of the membrane, which assists in deciphering the alterations in mechanical strength of the monolayer with curvature and/or lateral structure.Using a biologically relevant monolayer, formed from Survanta (a clinically approved pulmonary surfactant, and a complex mixture of lipids and proteins that rapidly adsorb on the interface), we will present the first-ever experimental elucidation of the impact of induced macro to microscale curvatures on the phase/raft morphology of membrane. Interestingly, the Survanta components show the formation of distinct patterns of phase-coexistence as they adsorb on the planar interface and when on the spherical interfaces of different curvatures. Modulation of the local phase structure with the background curvature is found reversible up to certain extent, however it certainly has impact on the mechanics of membrane. We will present a theoretical model to explain how electrostatic (dipole:dipole) interaction energy between phases and within domains compete with the line tension energy on a curved surface and yield distinct phase patterns in the monolayer and bilayer membranes.This study has importance in understanding such impacts on the raft morphology and the mechanics of cell membrane during cellular processes involving large variations in membrane curvature. Additionally, it may have implications in modulating the self-assembly process of amphiphiles at the interface, leading to formation of distinct two-dimensional patterns useful for the nanotech industries.