Motivation

A typical membrane cartoon (from a textbook, for instance) includes most of the following elements: multiple species of lipid trans-membrane and extrinsic proteins cytoskeleton and pinning sites cholesterol carbohydrate attached to lipids and proteins Solvent and individual atoms are excluded not just because the cartoonist found their illustration too time-consuming, but because the brain (like a computer) can only process so many particles at once.  The picture presents its particular message effectively without atomic detail. Simplified models are justified partly by the hypothesis that the same can hold for simulations. Despite the frequency with which the cartoons described above are found in textbooks, simulations at these resolutions are rarely conducted. The mesoscopic regime is tricky because it requires a particle based method with lots of particles.  Sophisticated force fields developed for atomistic dynamics are not applicable, and complicating the membrane composition can increase both development and computation time. Our "cartoon-resolution" model is both simple and general, allowing for several species within the same model framework.  Simulations are  motivated by a  number of questions regarding the behavior of heterogenous membranes over long length scales.

Model

Each "lipid" in our model is represented by a soft, rigid rod (spherocylinder). The amphiphiles interact through an excluded volume interaction, an attraction between "hydrophobic" ends, and an orientation-dependent "alignment" interaction. By varying the three energetic parameters, a number of structures can be formed. Mixtures are easily represented by rods of different sizes without making any other changes to the model. Alterations in shape and the location of "hydrophobic sites" can produce a number of protein-like molecules with minimal effort.

Publications

Composition dependence of bilayer elasticity, G. Brannigan and F.L.H. Brown, in Press,  Journal of Chemical Physics (2004). The role of molecular shape in bilayer elasticity and phase behavior , G. Brannigan, A.C. Tamboli, and F. L. H. Brown, Journal of Chemical Physics , 121, 3259-3271 (2004). Solvent-free simulations of fluid membrane bilayers, G. Brannigan and F. L. H. Brown, Journal of Chemical Physics , 120,1059-1071 (2004).