Binary phases of aliphatic N-oxides and water:: Force field development and molecular dynamics simulation

Aliphatic N-oxides as cosolvents with water play an important role in stabilizing and destabilizing the structure of biopolymers such as cellulose and proteins. To allow for detailed microscopic investigations, an empirical force field to be used in molecular simulations is developed for two N-oxide species, N,N,N-trimethylamine-N-oxide (TMAO) and N-methylmorpholine-N-oxide (NMMO). The intra- and intermolecular force field is parametrized mainly on the basis of quantum-chemical calculations and is tested against available experimental spectroscopic, crystallographic, and liquid state data. Special emphasis is put on the identification of transferable potential terms in order to guide future parametrization of other species. By construction, the force field is compatible with widely used potential functions for proteins and carbohydrates. With the resulting parameter set, molecular dynamics simulations are carried out on binary mixtures of water and N-oxides, revealing structural features and the influence of intramolecular N-oxide flexibility. Limitations and possible extensions of the presented models are also discussed.