Multiscale simulations of soft matter: systematic structure-based coarse-graining approach

Abstract: The soft matter field considers a wide class of objects such as liquids, polymers, gels, colloids, liquid crystals and biological macromolecules, which have complex internal structure and conformational flexibility leading to phenomena and properties having multiple spacial and time scales. Existing computer simulation methods are able to cover these scales, but with different resolutions, and ability to link them together performing a multiscale simulation is highly desirable.The present work addresses systematic multiscaling approach for soft matter studies, using structure-based coarse-graining (CG) methods such as iterative Boltzmann inversion and inverse Monte Carlo. A new software package MagiC implementing these methods is introduced. The software developed for the purpose of effective CG potential derivation is applied for ionic water solution and for water solution of DMPC lipids. A thermodynamic transferability of the obtained potentials is studied.The effective inter-ionic solvent mediated potentials derived for NaCl successfully reproduce structural properties obtained in explicit solvent simulation, which indicates the perspectives of using the structure-based coarse-graining for studies of ion-DNA and other polyelectrolytes systems. The potentials have temperature dependence, dominated mostly by the electrostatic long-range part which can be described by temperature dependent effective dielectric permittivity, leaving the short-range part of the potential thermodynamically transferable.For CG simulations of lipids a 10-bead water-free model of dimyristoylphosphatidylcholine is introduced. Four atomistic reference systems, having different lipid/water ratio are used to derive the effective bead-bead potentials, which are used for subsequent coarse-grained simulations of lipid bilayer. A significant influence of lipid/water ratio in the reference system on the properties of the simulated bilayers is noted, however it can be softened by additional angle-bending interactions. At the same time the obtained bilayers have stable structure with correct density profiles. The model provides acceptable agreement between properties of coarse-grained and atomistic bilayer, liquid crystal - gel phase transition with temperature change, as well as realistic self-aggregation behavior, which results in formation of bilayer, bicell or vesicle from a dispersed lipid solution in a large-scale simulation.