Interfacial Curvature Effects on Morphology and Dynamics of Monolayer Membrane

For decades, phospholipid monolayers at the ‘planar’ air-water interface of Langmuir trough have been used as model systems for understanding the structure and function of ‘curved’ membranes of biological cells, assuming minimal impacts of curvature even though a curved monolayer-covered interface has additional thermodynamic degrees of freedom compared to the planar interface. Approaches for the biophysical investigation of ‘curved’ cell membranes, like giant unilamellar vesicles and pipette-pulled membrane tubes of controlled radius, also offer only a narrow range of curvature tuning to explore the impact on membrane structure and behavior.

Here, we present a systematic investigation of the impact of curvature on the phase morphology of a monolayer membrane and its mechanics. We have designed and fabricated a novel bubble based air-aqueous interface setup, where spherical monolayers of broad range of curvatures (25-1000 µm radii) are created beside planar monolayer. The curved monolayers formed under controlled conditions can be gradually or instantly switched to higher or lower curvatures. 3-D imaging capabilities of confocal microscopy have been uniquely combined with this setup to resolve structures and domain evolution in curved as well as planar monolayers with respect to time.

Using a biologically relevant multi-component lipids-proteins system that forms coexisting ordered, ‘solid-like’ domains in the disordered, ‘liquid-like’ phase in a monolayer on a planar interface, we show that as the curvature of interface approaches a critical range (micron scale) the molecular organization in the monolayer changes and yields a contrasting equilibrium phase-coexistence pattern where size, shape and connectivity of the domains dramatically change. Surprisingly, the background curvature-mediated dramatic shift in phase-coexistence pattern impacts the dynamic properties of monolayer as well, in particular the complex dilatational modulus. Modulation of local phase structure and dynamics with the interfacial curvature is found reversible up to certain extent. While there are examples of coupling molecular shape to the local curvature at the nanometer scale, such as spontaneous vesicles or budding, we are not aware of such example where curvature on the 100 micron scale modifies properties of monolayer or bilayer membranes. We 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 interface and yield distinct phase morphology in the monolayers.

This study has importance in understanding such impacts on the raft morphology and the mechanics of cell membranes during cellular processes involving large variations in membrane curvature.