Calculating the bulk modulus for a lipid bilayer with nonequilibrium molecular dynamics simulation.

Nonequilibrium molecular dynamics (NEMD) computer simulations are used to calculated the bulk modulus for a dimyristoylphosphatidylcholine bilayer. A methodology is developed whereby NEMD can be effectively used to calculate material properties for complex systems that undergo long time-scale conformational changes. It is found that the bulk modulus upon expansion from a zero stress state agrees well with experimental estimates. However, it is also found that the modulus upon contraction from a zero stress state is larger. From a molecular perspective, it is possible to explain this phenomena by examining the molecular origins of the pressure response. The finding that the two moduli are not equal upon compression and expansion is in apparent contradiction to osmotic stress experiments where the area modulus was found to be the same upon expansion and contraction. This issue is addressed

Interfacing molecular dynamics and macro-scale simulations for lipid bilayer vesicles.

A continuum-level model for a giant unilamellar vesicle (GUV) is bridged to a corresponding atomistic model of a dimyristoylphosphatidylcholine (DMPC) bilayer at various cholesterol concentrations via computation of the bulk modulus. The bulk modulus and other microscopically determined parameters are passed to a continuum-level model operating in time- and length-scales orders of magnitude beyond that which is accessible by atomistic-level simulation. The continuum-level simulation method used is the material point method (MPM), and the particular variation used here takes advantage of the spherical nature of many GUVs. An osmotic pressure gradient due to a solvent concentration change is incorporated into the continuum-level simulation, resulting in osmotic swelling of the vesicle. The model is then extended to treat mixtures of DMPC and cholesterol, where small domains of different composition are considered