Dielectric saturation of water in a membrane protein channel

Water molecules in confined geometries like nanopores and biological ion channels exhibit structural and dynamical properties very different from those found in free solution. Protein channels that open aqueous pores through biological membranes provide a complex spatial and electrostatic environment that decreases the translational and rotational mobility of water molecules, thus altering the effective dielectric constant of the pore water. By using the Booth equation, we study the effect of the large electric field created by ionizable residues of an hour-glass shaped channel, the bacterial porin OmpF, on the pore water dielectric constant, epsilon(w). We find a space-dependent significant reduction (down to 20) of epsilon(w) that may explain some ad hoc assumptions about the dielectric constant of the protein and the water pore made to reconcile model calculations with measurements of permeation properties and pK(a)'s of protein residues. The electric potential calculations based on the OmpF protein atomic structure and the Booth field-dependent dielectric constant show that protein dielectric constants ca. 10 yield good agreement with molecular dynamics simulations as well as permeation experiments.