On the coupling of protein and water dynamics in confinement: Spatially resolved molecular dynamics simulation studies

To ascertain protein-water couplings in confinement, we exploit that molecular dynamics simulations enable full control and systematic variation of the model system as well as componentwise and spatially resolved analyses. We use an elastin-like molecule and vary the thickness of the hydration shell by producing confinements of desired shape and size. Moreover, we tune the rigidity of the confinement. The approach reveals prominent dynamical couplings at the protein-water interface in combination with strong spatial variation of both protein and water dynamics. Explicitly, changes in the mobility of the hydration shell in response to altered confinement conditions are accompanied by changes in the mobility of the protein surface, leading to a relation of respective correlation times. Moreover, with increasing distance to the protein-water interface, water dynamics speeds up and protein dynamics slow down, where, however, internal and global protein motions show quantitative differences. These findings indicate that an understanding of biological functions requires consideration of mutual dependencies of protein and water dynamics, strong mobility gradients across the protein molecule and the hydration shell, and differences between internal and tumbling motions. In addition, we observe that the effect of the confining walls is very strong but short ranged. As a consequence, protein dynamics is significantly altered under severe confinement conditions, whereas bulk-like behavior is recovered for confinement sizes larger than ∼4.3 nm, corresponding to hydration levels above ∼1.5 g/g.