Hydration

The total hydration of a protein (or nucleic acid) is the effective amount of water immobilized by ) bound to) a protein molecule or other macromolecule. This consists of a summation of interactions of water molecules with individual sites on the accessible surface of a protein molecule. Such interactions vary from (1) very strong ones, such as water molecules trapped within cavities on a protein molecule and involved in the actual folded structure of the protein, (2) weak interactions, such as water molecules that hydrate charged and other polar groups on the surface of a protein molecule; and (3) very weak interactions that comprise water molecules whose rotation or translation is momentarily perturbed by the proximity of a protein molecule (by weak attraction or repulsion). This last type of interaction is primarily entropic in nature and reflected by a very small value of the free energy. The summation of all these interactions manifests itself as effective binding (ie, immobilization of the water by the protein) that may have a small fractional value at each protein surface site, but which sums up to some whole numbers over the entire molecule.

Total hydration is very difficult to measure, and its values may be a function of the techniques used. X-ray crystallography detects the more strongly interacting water molecules. The NMR technique that detects water molecules whose freezing is perturbed by the presence of the protein (1) and vapor pressure osmometry (2) give similar values of total hydration. These values are of similar magnitude to those obtained from equilibrium dialysis in those cases in which there is total exclusion of ligand from the protein surface. Furthermore, a measurement of site occupancy by the ligand by a contact detecting technique (total binding), when combined with equilibrium dialysis, yields the total hydration (see Eq. 1 of Binding). See also Preferential Hydration.

References

1. I. D. Kuntz and W. Kauzmann (1974) Adv. Protein Chem. 28, 339–345. 

2. H. B. Bull and K. Breese (1974) Arch. Biochem. Biophys. 161, 665–670.