Protonic conductivity of hydrated lysozyme powders at megahertz frequencies

Dielectric losses were measured for lysozyme powders of varied hydration level by a dielectric-gravimetric technique in the frequency range of 10 kHz to 10 MHz. The relaxation showed an isotope effect and pH dependence, indicating that the inferred conductivity is protonic. The transport process is likely restricted to the surface of individual macromolecules and involves shifting of protons between ionizable side chain groups of the protein. The time constant of the relaxation shows cooperativity in its seventh-order dependence on bound protons. The process develops in the hydration region critical for the onset of the catalytic properties of the enzyme. The binding of a substrate increases the relaxation time by a factor of 2. These observations suggest that the megahertz dispersion reflects behavior at the protein surface, specifically the cooperative channeling of proton flow through the active site, that may be of particular significance for the enzymatic and other functional properties of proteins.