Open Access Kent State (OAKS)

Complex morphologies in lipid membranes typically arise due to chemical heterogeneity, but in the tilted gel phase, complex shapes can form spontaneously even in a membrane containing only a single lipid component. We explore this phenomenon via experiments and coarse-grained simulations on giant unilamellar vesicles of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine. When cooled from the untilted Lα liquid-crystalline phase into the tilted gel phase, vesicles deform from smooth spheres to disordered, highly crumpled shapes. We propose that this shape evolution is driven by nucleation of complex membrane microstructure with topological defects in the tilt orientation that induce nonuniform membrane curvature. Coarse-grained simulations demonstrate this mechanism and show that kinetic competition between curvature change and defect motion can trap vesicles in deeply metastable, defect-rich structures.

Complex morphologies in lipid membranes arise typically due to chemical heterogeneities. For example, clustering of different lipid species can result in membrane domains with different intrinsic curvatures (1) and transmembrane proteins can induce local curvature (2). Here we explore another mechanism that produces complex shapes in membranes of a single lipid component without chemical heterogeneity: the formation of topological defects in a membrane with in-plane orientational order and their trapping to produce highly disordered morphologies.

In this paper, we present coordinated experimental and computational studies of giant unilamellar vesicles (GUVs), i.e., single-bilayer shells, as they cool from the untilted liquid-crystalline phase (Lα) into the tilted gel phase . In the gel phase, the local molecular tilt has orientational order; i.e., the molecules point in a particular direction within the 2D plane. Hence, the gel phase exhibits microstructural point defects, vortices in the tilt direction, which can be considered as positive or negative topological “charges” according to which way the tilt direction rotates about the defect core (3). A spherical vesicle must have defects with a total topological charge of +2, as shown by the Gauss–Bonnet theorem (just as one cannot comb the hair on a coconut without leaving at least two defects). In general, there is a fundamental geometric connection between 2D order and defects within a membrane and the 3D shape of the membrane: Curvature drives formation of topological defects, and conversely, defects can induce curvature (4⇓–6). This interaction has been explored in liquid crystals (7, 8), faceted block copolymer vesicles (9), liquid-crystalline elastomers (10), colloidal crystals (11), and superfluids (12), and we investigate how it affects the shape of GUVs.