High density water clusters observed at high concentrations of the macromolecular crowder PEG400

What happens to water in crowded environments? A detailed understanding of how water is affected by the presence of a crowding agent is still lacking. In the present work, we focus on macromolecular crowding. In particular, we study aqueous solutions of a macromolecular crowder with many industrial applications, namely, poly-(ethylene) glycol with a mass weight of 400 g/mol (PEG400), at compositions ranging from infinite dilution to polymer weight fractions of 0.95 by means of molecular dynamics simulations and 1H DOSY-NMR experiments. Our data show that water density is severely affected by the presence of macromolecular chains at all concentrations, and is, on average, always higher than the bulk water density as a result of the superimposition of the hydration shells of the polymer chains. Moreover, the combined computational-experimental approach concurs well with the following scenario: water still forms clusters even within the solutions at the highest concentration, rather than saturating all available hydrophilic sites of the polymeric chains, indicating a clear predilection for water-water interactions.