The surface compartment model: a theory of ion transport focused on ionic processes in the electrical double layers at membrane protein surfaces.

This paper is a review of the surface compartment model of ion flow across the channels of natural membranes. The model emphasizes the role of electrical double layers in ion transport and is derived from first principles. When the surface compartment model is applied to the membrane of an excitable cell, (e.g., the squid axon), one can calculate voltage clamp currents that are similar to those observed in the sodium and potassium channels of excitable membranes. The difference in the selectivity of the two types of ion channel appears to be determined by the difference in gating current, and is in line with measurements on the sodium and potassium channels of squid axon. These results indicate that there is a kinetic basis for the selectivity of voltage gated channels and suggest that other types of channels may operate by related mechanisms. The focus of the surface compartment model on charged surfaces has led to a description of the channel opening/closing process in terms of surface free energy, assuming an analogy to the aggregation/disaggregation reactions in oligomeric proteins. The opening of voltage gated oligomeric channels can be formulated in terms of variations in the surface free energy that are triggered by changes in the surface charge density. On this basis, it is possible to introduce gating phenomena into the surface compartment model and to couple the channel processes with charge movements.