Access permeability of ionic channels. Dependence on aqueous jump distance.

The access diffusion permeability of pores with diameters comparable to the aqueous jump distance is characterized using a rate theory analysis for the aqueous diffusion process. It is found that this process gives rise to two permeability terms, one associated with bulk diffusion and the other a jump from the aqueous solution into a position where it has access to the channel. The latter term dominates for small channel diameters and vice versa for large channel diameters. The properties of access diffusion with respect to concentration polarization is shown to be different in the two limits of large and small values of the channel radius. A necessary criterion for bulk access diffusion to be rate limiting is given in terms of measured channel conductance G, aqueous jump distance lambda and aqueous resistivity rho, G greater than pi lambda/rho, which does not require a knowledge of channel geometry.