A statistical mechanics definition of the hydration shell of a membrane in an open system Molecular Dynamics Simulation.

Defining and quantifying the solvation shell of a biological macromolecule poses a major challenge, and even with state of the art equipment and methods, the question of the extent of hydration can vary depending on the quantity of interest, emphasizing the need for an unambiguous definition. The adaptive resolution simulation (AdResS) technique offers the unique ability to estimate the extent of the hydration shell around a biomolecule through the use of an open boundary atomistic system embedded within a thermodynamic reservoir of non-interacting, structureless particles. Here, we apply the latest AdResS scheme to the study of a pure DPPC membrane solvated in liquid water and determine that the mandatory solvation shell of a DPPC membrane extends to ~1.5 nm away from the DPPC phosphate headgroups, well beyond distances predicted from radial distribution functions.