Percolation and Dissipative Quantum Tunneling of Protons in Hydrated Protein Powders

Abstract

Previous work from this laboratory has shown that lysozyme powders exhibit dielectric behavior due to proton conductivity assisted by water molecules adsorbed on surface (Careri et al., 1985), and that this behavior can be described in the frame of percolation theory (Careri et al., 1986; Careri et al., 1988). This statistical-physical model, has been shown to be applicable to a wide range of processes where spatially random events and topological disorder are of intrinsic importance. A typical physical application of the percolation theory^1,2 is to the electrical conductivity of a network of conducting and non-conducting elements. One of the most appealing aspects of the percolation process is the presence of a sharp transition, where long-range connectivity among the elements of a system suddenly appear at a critical concentration of the carriers. A similar 2-dimensional protonic percolation has been detected in powdered samples of purple membrane of Halobacterium Halobium (Rupley et al., 1988). In both cases the emergence of biological function, respectively enzyme catalysis and photoresponse, has been found to coincide with the critical hydration for protonic percolation h_c. More recently the above room temperature studies have been extended to samples of viable biological systems to confirm the close connection between protonic percolation threshold and the onset of biological function in nearly anhydrous biosystems (see Table 1).