Dynamical clustering and a mechanism for raft-like structures in a model lipid membrane

We use molecular dynamics simulations to examine the dynamical heterogeneity of a model single-component lipid membrane using a coarse-grained representation of lipid molecules. This model qualitatively reproduces the known phase transitions between disordered, ordered, and gel membrane phases, and the phase transitions are accompanied by significant changes in the nature of the lipid dynamics. In particular, lipid diffusion in the liquid-ordered phase is hindered by the transient trapping of molecules by their neighbors, similar to the dynamics of a liquid approaching its glass transition. This transient molecular caging gives rise to two distinct mobility groups within a single-component membrane: lipids that are transiently trapped, and lipids with displacements on the scale of the intermolecular spacing. Most significantly, lipids within these distinct mobility states spatially segregate, creating transient "islands" of enhanced mobility having a size and time scale compatible with lipid "rafts," dynamical structures thought to be important for cell membrane function. Although the dynamic lipid clusters that we observe do not themselves correspond to rafts (which are more complex, multicomponent structures), we hypothesize that such rafts may develop from the same universal mechanism, explaining why raft-like regions should arise, regardless of lipid structural or compositional details. These clusters are strikingly similar to the dynamical clusters found in glass-forming fluids, and distinct from phase-separation clusters. We also show that mobile lipid clusters can be dissected into smaller clusters of cooperatively rearranging molecules. The geometry of these clusters can be understood in the context of branched equilibrium polymers, related to percolation theory. We discuss how these dynamical structures relate to a range observations on the dynamics of lipid membranes.