Computing free-energy landscapes of co-operative structure changes in soft, biological matter

Using computer simulation and self-consistent field theory of coarse-grained models for lipid membranes and diblock copolymers, we study the free-energy landscape of collective phenomena that alter the topology of lipid membranes. These basic processes – pore formation, fusion and fission – often involve time scales of tens of nanometers and milliseconds that are large for atomistic simulation. Frequently, they involve transition states with high curvatures that are difficult to describe by Helfrich-like models. Coarse-grained models can access the relevant time and length scales, allow for a systematic exploration of parameters like the lipid architecture or membrane tension, and they are well suited to study collective phenomena that alter the topology of membranes.

The talk will discuss different computational techniques – Wang-Landau sampling, field-theoretic umbrella sampling, and the string method – to investigate metastable intermediates (like the stalk in the course of membrane fusion) and transition states of pore formation, membrane fusion and fission. Using coarse-grained models, we explore the universal aspects of topology-altering processes in membranes and comment on the extent, to which coarse-grained model capture specific effects of protein-mediated processes.