Dielectric properties of water of adsorption on protein crystals

The dielectric properties of dry and wet protein crystals were studied in a wide frequency range. The dry protein crystals did not have appreciable dielectric constant until the water content was about 15–20%. However, further increase of water of hydration (and/or adsorption) suddenly increased the dielectric constant of the crystals, and it reached a very large value which cannot be explained merely by the orientation of water of hydration (and/or adsorption). However the increase of dielectric constant leveled off rapidly, and further addition of water did not have significant effect. Thus the dielectric constant–water content plot followed a curve which was similar to the Langmuir adsorption isotherm. This fact strongly indicates that the dielectric constant of hydrated crystals arises, for the most part, from the polarizability of the first hydration layer of the protein molecules, which supposedly have an icelike structure.