Lipid bilayer membranes
We have also worked on other soft matter systems, specifically, (1) self-organization and non-equilibrium dynamics in lipid bilayers, (2) molecular structure and properties of lipid bilayers, and (3) self-assembly of detergents in bulk and on surfaces. The first figure from the left shows a typical microstructure in a three component lipid bilayer undergoing phase separation in the presence of non-equilibrium flux of lipid to and from the monolayer in view. This “lipid recycling” leads to a steady-state where the domains attain a characteristic size dependent on the recycling rate. We argue that this recycling-induced steady-state can explain the presence of the elusive “lipid rafts” thought to exist in the mammalian plasma membrane [Fan et al., PRL 2008]. The middle frame demonstrates the effects of spatially-varying recycling rate on domain formation. In particular, rapid changes in the local recycling rate may lead to emergence of extended, transient rafts. Upon homogenizing the recycling rate, the membrane microstructure becomes homogeneous, as shown in the panel on the right [Fan et al., PRE 2010].
We have also developed a comprehensive classification scheme to experimentally distinguish between competing theoretical models for raft formation [Fan et al, PRL 2010]. The method is based on quantifying the spatio-temporal fluctuations of the raft domains via speckle autocorrelation function and static structure factor; the combination of the two provides the means to identify the operative raft formation mechanism in living cells. Below, typical microstructures corresponding to the five existing theoretical models are displayed (I – critical fluctuations [Veatch et al., 2008]; II – pinning by immobile membrane proteins [Yethiraj & Weisshaar, 2007]; III – stochastic recycling above critical point [Edidin, 1999]; IV – stochastic recycling below the critical point [Fan, Sammalkorpi, and Haataja]; and V – coupling to lipid reservoir [Foret, 2005]).
The corresponding data for the temporal decay of fluctuations and static structure factor are shown below. Collecting data over a broad range of wavenumbers allows one to experimentally distinguish between the raft formation scenarios.
We have also investigated the role of membrane and exterior solvent hydrodynamics on membrane critical dynamics [Haataja, PRE (2009)] and coarsening dynamics [Fan, Han, and Haataja, J. Chem. Phys. (2010)]. The two snapshots from the left below illustrate the coupling between compositional dynamics and membrane hydrodynamic flow fields, while the snapshot on the right displays the flow fields induced within the solvent due to compositional membrane evolution; this flow field subsequently back-reacts with the membrane flow field and influences the phase separation process. In particular, we have demonstrated that the 2D nature of the membrane flow fields coupled to a 3D exterior fluid gives rise to novel coarsening behavior.
Finally, the leftmost figure displays a snapshot from a simulation where micelle formation takes place. In addition to lipids and detergents, we are interested in the behavior of synthetic molecular switches in biology. To this end, we have constructed physically-based models for the tether length dependence of signal integration proteins, shown schematically in the figure on the right [Van Valen, Haataja, and Phillips, Biophys. J. 96, 1275 (2009)].
