Protons in hydrated protein powders

Abstract

Previous work from this laboratory has shown that hydrated lysozyme powders exhibit a dielectric behaviour, due to proton conductivity, explainable within the frame of percolation theory. Long range proton displacement appears only above the critical hydration for percolation, when the 2-dimensional motion takes place on fluctuating clusters of hydrogen-bonded water molecules adsorbed on the protein surface. The emergence of biological function, enzyme catalysis, was found to coincide with the critical hydration for percolation.

More recentently, we have evaluated the protonic conductivity of hydrated lysozyme powders, from room down to liquid N₂ temperature. In the high temperature limit a classical iso-topic effect can be detected, and the conductivity follows the familiar Arrhenius law for thermally activated hopping. In the low temperature region the conductivity shows a temperature dependance in agreement with prediction by the theory of dissipative quantum tunneling. Below room temperature the static dielectric constant, and the dielectric relaxation time for charge transport showed an increase likely to be identified with the formation of a polaronic-solitonic species as predicted by the theory of proton transport in water chains, a species which displays a larger effective mass and a larger dipole moment than the usual hydrated protonic defects.

The purpose of this paper is twofold. In the first section we present a tutorial report of some previous experimental results on proton displacement in slightly hydrated biological systems at room temperature, to show that in these systems the emergence of biological function coincides with the onset of percolative pathways in the water molecules network adsorbed on the surface of biomolecules. In the second section, we report on preliminary data on the dielectric relaxation of hydrated lysozyme below room temperature, to suggest that protons move along percolative water pathways on the protein surface according to a polaronic-solitonic model recently proposed by theory.