Myelin Figures: Far-from-Equilibrium Assembly of Lipid Amphiphiles

Phase behaviours of amphiphiles, such as surfactants, lipids, and block copolymers, as well as their mixtures at thermodynamic equilibrium are now well-understood. But how these mixtures organize within dynamic morphologies – such as those that appear under far from equilibrium conditions while dissipating energy – is incompletely understood. This talk describes the spontaneous emergence of a spatio-temporal organization characterized by an extended spatial pattern of chemical composition within the dynamic myelin-like morphology, which protrudes from the lipid-water interface during the hydation of dry lipid mass consisting of mixtures of cholesterol and lipids. The invasion of water into the initially dry lipid pool expends the osmotic energy giving rise to a transient hierarchical organization of the amphiphilic mixture producing long-lived metastable cylindrical liquid crystals or myelin figures. These emergent protrusions – tens of micrometers wide and hundreds of micrometers long – exhibit a long-range smectic ordering of thousands of alternating layers of lipid bilayers and water in a multi-cylindrical geometry. Strikingly, molecules comprising individual tubules segregate differentially across different lamellae – in stark contrast to intralamellar phase separation in equilibrated structures – producing a continuous radial gradient of cholesterol (and saturated lipid or sphingomyelin) concentration. This in turn gives rise to corresponding interlamellar gradients of lateral fluidity, bending rigidity, and water permeability within the smectic phase. Moreover, the populations of cylindrical tubules grow by a two-dimensional reptation-like motion producing dense dynamic arrays reminiscent of labyrinthine patterns of many disparate complex fluids. We propose that this emergent, higher-order spatio-temporal self-organization, reflecting complex and dynamic synergy between fluidity, permeability, and deformability of hierarchically organized and energy dissipating lipid tubules, is driven not by any preexisting phase separation of the lipids in the dry mass. But rather non-equilibrium information processing of curvature preferences of individual molecules while expending energy dictates this transient, but long-lived, response of lipid mixtures during hydration.