Coil-Globule-Coil Transition of PNIPAm in Aqueous Methanol: Coupling All-Atom Simulations to Semi-Grand Canonical Coarse-Grained Reservoir

Conformational transitions of (bio)macromolecules in aqueous mixtures are intimately linked to local concentration fluctuations of different solvent components. Though computer simulations are ideally suited to investigate such phenomena, in conventional setups the excess of one cosolvent close to the solute leads to depletion elsewhere, requiring very large simulation domains to avoid system size effects. We, here, propose an approach to overcome this depletion effect, which combines the adaptive resolution scheme (AdResS) with a Metropolis particle exchange criterion. In AdResS, a small all-atom region, containing the solute, is coupled to a coarse-grained reservoir, where the particle exchange is performed. The particle exchange would be almost impossible had they been performed in an all-atom setup of a dense molecular liquid. As a first application of the method, we study the concentration driven reentrant collapse and swelling transition of poly(N-isopropylacrylamide) (PNIPAm) in aqueous methanol and demonstrate the role of the delicate interplay of the different intermolecular interactions.