DLPC

Understanding the forces that stabilize membrane proteins in their native states is one of the contemporary challenges of biophysics. To date, estimates of side chain partitioning free energies from water to the lipid environment show disparate values between experimental and computational measures. Resolving the disparities is particularly important for understanding the energetic contributions of polar and charged side chains to membrane protein function because of the roles these residue types play in many cellular functions. In general, computational free energy estimates of charged side chain partitioning into bilayers are much larger than experimental measurements. However, the lack of a protein-based experimental system that uses bilayers against which to vet these computational predictions has traditionally been a significant drawback. Moon & Fleming recently published a novel hydrophobicity scale that was derived experimentally by using a host-guest strategy to measure the side chain energetic perturbation due to mutation in the context of a native membrane protein inserted into a phospholipid bilayer. These values are still approximately an order of magnitude smaller than computational estimates derived from molecular dynamics calculations from several independent groups.

It is postulated that the specific interactions between cholesterol and lipids in biological membranes are crucial in the formation of complexes leading subsequently to membrane domains (so-called rafts). These interactions are studied in molecular dynamics simulations performed on a dipalmitoylphosphatidylcholine (DPPC)-cholesterol bilayer mixture and a dilauroylphosphatidylcholine (DLPC)-cholesterol bilayer mixture, both having a cholesterol concentration of 40 mol %. Complexation of the simulated phospholipids with cholesterol is observed and visualized, exhibiting 2:1 and 1:1 stoichiometries. The most popular complex is found to be 1:1 in the case of DLPC, whereas the DPPC system carries a larger population of 2:1 complexes. This difference in the observed populations of complexes is shown to be a result of differences in packing geometry and phospholipid conformation due to the differing tail length of the two phosphatidylcholine lipids. Furthermore, aggregation of these complexes appears to form hydrogen-bonded networks in the system containing a mixture of cholesterol and DPPC. The CH...O hydrogen bond plays a crucial role in the formation of these complexes as well as the hydrogen bonded aggregates.

The ordering of liquid crystals (LCs) within micrometer-sized droplets is known to depend strongly on the presence of interfacial adsorbates, although the exact sequence of ordered equilibrium states that accompany a change in interfacial anchoring from tangential to perpendicular has not been established. In this paper, we report use of a methodology that permits the preparation of monodisperse LC droplets in aqueous phases to investigate ordering transitions in the LC droplets that accompany the adsorption of amphiphiles at the aqueous-LC droplet interface. By using an amphiphile that undergoes reversible adsorption at the aqueous-LC interface (sodium dodecylsulfate, SDS), we identified six distinct topologically ordered states of the LC droplets as a function of increasing concentration of SDS. We exploited the reversible adsorption of the SDS to LC droplets with diameters of 8.0+/-0.2 microm to confirm that these topological states are equilibrium ones. We also exposed LC droplets to a continuous gradient in concentration of SDS to document the continuous transitions between topological states and to confirm the absence of additional, intermediate topological states. The formation of the LC droplets as aqueous dispersions also enabled an investigation of ordering transitions in LC droplets driven by biomolecular interactions.