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polymer physics amphiphilic systems rod-like colloids reaction rates
alpha-synuclein clathrin

Amphiphilic Systems

Surfactant molecules dissolved in water will spontaneously self-assemble into a variety of structures: in an attempt to shield their hydrophobic tails from exposure to the water, they orient their hydrophilic heads to form a protective layer. Depending on the lipid composition and concentration, the aggregates formed are near-spherical micelles, cylindrical rods (a.k.a. wormlike micelles) or bilayers (see the picture on the right). The latter are ubiquitous in biological tissues, where they serve as the membranes surrounding and compartmentalizing living cells. image title

membrane mechanics and dynamics

In our simulations we employ fully atomistic models, as well as coarse grained models in which a lipid is reduced to a small number of hydrophilic (head) and hydrophobic (tail) particles. We studied the mechanical properties of these bilayers, i.e. the elastic modulus and the bending rigidity, and showed that undulations have a significant effect on the apparent elasticities reported in the simulation literature [1,2].
By exposing the membrane to shear flows with perpendicular and parallel orientations, we were able to calculate the surface shear viscosity and the intermonolayer friction coefficient, respectively [3]. The surface viscosity was found to be related to the lateral diffusion coefficient of the lipids [3], while the friction coefficient determines the relaxation rates of thermal undulations with short wave lengths [4]. We also studied the influences of lipid architecture and surface tension on the dynamics.

membrane pores

The first line tension or edge energy coefficient was obtained from the average radii of stable pores in stretched membranes, yielding values in good agreement with experimental data [5]. Interestingly, we derived that a stable pore requires a minimum elongation that scales with the number of lipids to the power -1/3, explaining the marked difference in elongations between experimental (~2%) and simulation (~35%) studies of pores. We developed a constraint technique to smoothly open and close a pore in a fully controlled and reversible fashion, allowing us to calculate the free energy profile of the pore formation process [6].
By simulating a bilayer of ceramide-2, the main lipid in the nearly impermeable upper layer of the human skin, we established that the potent penetration enhancer dimethylsulfide (DMSO) undermines the skin's barrier function by inducing a phase transition in the membrane [7], as well as drastically lowering the line tension of pores [8].

worm-like micelles

two entangled worm-like micelles By tuning the parameters of a coarse grained model, we performed the first simulation of a stable wormlike micelle [9], yielding a bending rigidity and persistence length at the lower end of the experimental range. An interesting buckling transition is observed, in worms and bilayers alike, if the compressive stress exceeds Euler's force.
When a solution of wormlike micelles is sheared, the worms easily get hooked behind one-another and a tension builds up in the worms until they yield. We simulated entanglements of two worms, see picture, and observed that the tension eventually leads -- contrary to previous conceptions -- to the merging of the worms and the formation of two Y-junctions [10]. The entangled worms never broke up. One readily envisages how the merging of worms, and the easy dynamics of junctions, explains the shear thinning flow behaviour observed in many of these systems.

selected publications

1 The bending rigidity of an amphiphilic bilayer from equilibrium and nonequilibrium molecular dynamics
W.K. den Otter and W.J. Briels
J. Chem. Phys. 118, 4712 (2003)
2 Area compressibility and buckling of amphiphilic bilayers in molecular dynamics simulations
W.K. den Otter
J. Chem. Phys. 123, 214906 (2005)
3 Surface viscosity, diffusion and intermonolayer friction: simulating sheared amphiphilic bilayers
S.A. Shkulipa, W.K. den Otter and W.J. Briels
Biophys. J. 89, 823 (2005)
4 Thermal undulations of lipid bilayers relax by intermonolayer friction at sub-micrometer length scales
S.A. Shkulipa, W.K. den Otter and W.J. Briels
Phys. Rev. Lett. 96, 178302 (2006)
5 Simulations of stable pores in membranes - system size dependence and line tension
T.V. Tolpekina, W.K. den Otter and W.J. Briels
J. Chem. Phys. 121, 8014 (2004).
6 Nucleation free energy of pore formation in an amphiphilic bilayer studied by molecular dynamics simulations
T.V. Tolpekina, W.K. den Otter and W.J. Briels
J. Chem. Phys. 121, 12060 (15 December 2004).
7 The permeability enhancing mechanism of DMSO in ceramide bilayers simulated by molecular dynamics
R. Notman, W.K. den Otter, M.G. Noro, W.J. Briels and J. Anwar
Biophys. J. 93, 2056 (2007)
8 Simulations of skin barrier function: free energies of hydrophobic and hydrophilic transmembrane pores in ceramide bilayers
R. Notman, J. Anwar, W.J. Briels, M.G. Noro and W.K. den Otter
Biophys. J. 95, 4763 (2008)
9 Buckling and persistence length of an amphiphilic worm from molecular dynamics simulations
W.K. den Otter, S.A. Shkulipa and W.J. Briels
J. Chem. Phys. 119, 2363 (2003)
10 Simulations of elementary processes in entangle wormlike micelles under tension: a kinetic pathway to Y-junctions and shear induced structures
W.J. Briels, P. Mulder and W.K. den Otter
J. Phys.: Condens. Matter 16, S3965 (2004)