Proton Conductivity in Hydrated Proteins. Evidence for Percolation

Abstract

Proton conductivity in hydrated lysozyme powders displays a 2-dimensional percolation character above a critical moisture threshold, which coincides with the onset of catalytic activity. Here we shall present the behavior of this protein at low temperature, and the generalization to a complex system of proteins. Dielectric data on hydrated protein powders have been obtained in the 10 KHz – 1 MHz range from room down to liquid nitrogen temperatures.

At the lowest temperature investigated the conductivity is electronic, while with increasing temperature a protonic contribution becomes superimposed and grows to reach an Arrhenius law only at room temperature. As shown by deuterium substitution, in the intermediate temperature range the conductivity is likely due to proton tunneling. Quite similar results have been obtained in powdered samples of corn embryos, an intact system of several enzymes imbedded in membranes.