Electrodiffusion in ionic channels of biological membranes

An important class of biological molecules - proteins called ionic channels conduct ions (like Na+, K+, Ca++ and Cl-) through a narrow tunnel of fixed charge. Ionic channels are the main pathway by which substances move into cells and son are of great biological and medical importance: asubstantial fraction of all drugs used by physicians act on channels. ionic channels can be modified by the powerful techniques of of molecular biology. Charged groups can be engineered (i.e. replaced one at a time) and the location of every atom can be determined. channels can be studied in the tradition of electrochemistry. If coupled to the Poisson equation, the drift diffusion equations (i.e. Nernst-Planck equations) form an adequate model of the current through 6 different channel proteins with quite different characteristics in 10 solutions over +/-150 mV. In this theory the channel is represented as a distribution of fixed charge, and the ion as a mobile charge with a diffusion coefficient. The theory predicts the electric field (i.e. potential profile) and resulting current produced by the fixed charge and other charges in the system. In this theory, the shape of the electric field is found to be a sensitive function of ionic conditions and the potential difference across the channel, in contrast to traditional theories that assume potential profiles (or rate constants) independent of experimental conditions. Traditional theories fail to fit data, probably for because they assume the shape of the electric field. The Poisson-Nernst-Planck (PNP) theory is nearly idemntical to the drift diffusion equations used to analyze the flow of quasi-particles in semiconductors, implying that - given appropriate geometry and profiles of fixed charge - ionic channels can perform many of the useful functions of transistors, acting as resistors, voltage amplifiers, current amplifiers, or logic elements. Channels form a useful system for electrochemistry since they are biologically and clinically important, they follow the simple rules of electrodiffusion, and they promise to be of considerable use in technology (C) 2000 Elsevier Science B.V. All rights reserved.