Dielectric studies of the binding of water to lysozyme

Dielectric dispersion measurements as a function of hydration are reported for lysozyme powder. The dispersion that occurs in the frequency range 10 kHz to 10 GHz can be analysed in terms of bound water molecules that form single or multiple hydrogen bonds, and the numbers found in these two categories agree well with recent X-ray data for lysozyme crystals. The dielectric data also indicate that at 20% ($\text{w}\text{w}$) hydration the bound water acts as a plasticizer to increase the vibrational freedom of the protein structure, and that this may be of relevance to the fact that the onset of enzymatic activity occurs at this hydration level. Also, a sudden transition in the polarizability of the protein-water system is found to occur at 7% ($\text{w}\text{w}$) hydration.