Thermal breaking of spanning water networks in the hydration shell of proteins.

The presence of a spanning hydrogen-bonded network of water at the surface of biomolecules is important for their conformational stability, dynamics, and function. We have studied by computer simulations the clustering and percolation of water in the hydration shell of a small elastinlike peptide (ELP) and the medium-size protein staphylococcal nuclease (SNase), in aqueous solution. We have found that in both systems a spanning network of hydration water exists at low temperatures and breaks up with increasing temperature via a quasi-two-dimensional percolation transition. The thermal breaking of the spanning water network occurs at biologically relevant temperatures, in the temperature range, which is close to the temperature of the "inverse temperature transition" of ELP and the unfolding temperature of SNase, respectively.