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

Abstract

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. Further examination shows 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 the statistics percolation theory. We discuss how these dynamical structures relate to a range observations on the dynamics of lipid membranes.

Partition, orientation and mobility of ubiquinones in a lipid bilayer

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Quantitative relations between cooperative motion, emergent elasticity, and free volume in model glass-forming polymer materials

“…These well-defined string clusters provide a concrete realization of the CRRs. Notably, these "strings" have been observed for all of the systems we examine here, as well as in other materials, ranging from the grain boundaries of crystals and the interfacial dynamics of nanoparticles (42)(43)(44), to driven granular systems (45) to lipid membranes (46). The ubiquity of the phenomenon suggests that the dynamics of dense, strongly interacting particle systems may be generally characterized by string-like collective motion.…”

Heterogeneous rotational diffusion of a fluorescent probe in lipid monolayers

“…Such dynamic clusters have been observed for a large range of strongly interacting condensed matter systems 39–41 . These dynamic fluctuations are very small (∼10 nm) and short lived (10  μ s) 45 . Dynamic clustering in lipid membranes could have a large impact on membrane recognition, transport, and protein self-assembly in biological systems.…”