Bulk and Surface Water Molecules

Thinking about the hydration of protein complexes is simplified by dividing water molecules into four classes bulk water molecules that are not directly in contact with the biomolecules, surface water hydrogen bonded to the protein or ligand, surface water associated with apolar biomolecular groups, and buried water molecules that have no direct connection to the bulk solvent. 

Water at apolar surfaces has rather distinctive thermodynamic properties, in particular an unusually low entropy and high heat capacity. Displacement of water from apolar surfaces to bulk during protein folding and ligand binding dominates observed heat-capacity changes of the whole system. Consequently, apolar surface hydration has received much more attention than that of polar surfaces. Water 

Surface waters are displaced on formation of the protein-ligand complex and thus provide a favorable entropic contribution to the free energy of complex formation. In particular, displacement of the water found interacting on apolar surfaces makes a large contribution to the AG and provides the driving force for many interactions (the hydrophobic effect). 

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