  The formation of a spanning hydrogen-bonded network of hydration water is
found to occur via a 2D percolation transition in various systems: smooth
hydrophilic surfaces, the surface of a single protein molecule, protein powder
and diluted peptide solution. The average number of water-water hydrogen bonds
nH at the percolation threshold varies from 2.0 to 2.3, depending on
temperature, system size and surface properties. Calculation of nH allows an
easy estimation of the percolation threshold of hydration water in various
systems, including biomolecules.
