PERMEATION THROUGH AN OPEN-CHANNEL - POISSON-NERNST-PLANCK THEORY OF A SYNTHETIC IONIC CHANNEL

The synthetic channel [acetyl-(LeuSerSerLeuLeuSerLeu)(3)-CONH2](6) (po re diameter similar to 8 Angstrom, length similar to 30 Angstrom) is a bundle of six alpha-helices with blocked termini. This simple channel has complex properties, which are difficult to explain, even qualitat ively, by traditional theories: its single-channel currents rectify in symmetrical solutions and its selectivity (defined by reversal potent ial) is a sensitive function of bathing solution. These complex proper ties can be fit quantitatively if the channel has fixed charge at its ends, forming a kind of macrodipole, bracketing a central charged regi on, and the shielding of the fixed charges is described by the Poisson -Nernst-Planck (PNP) equations. PNP fits current voltage relations mea sured in 15 solutions with an r.m.s. error of 3.6% using four adjustab le parameters: the diffusion coefficients in the channel's pore D-K = 2.1 x 10(-6) and D-Cl = 2.6 x 10(-7) cm(2)/s; and the fixed charge at the ends of the channel of +/-0.12e (with unequal densities 0.71 M = 0 .021e/Angstrom on the N-side and -1.9 M = -0.058e/Angstrom on the C-si de). The fixed charge in the central region is 0.31e (with density P-2 = 0.47 M = 0.014e/Angstrom). In contrast to traditional theories, PNP computes the electric field in the open channel from all of the charg es in the system, by a rapid and accurate numerical procedure. In esse nce, PNP is a theory of the shielding of fixed (i.e., permanent) charg e of the channel by mobile charge and by the ionic atmosphere in and n ear the channel's pore. The theory fits a wide range of data because t he ionic contents and potential profile in the channel change signific antly with experimental conditions, as they must, if the channel simul taneously satisfies the Poisson and Nernst-Planck equations and bounda ry conditions. Qualitatively speaking, the theory shows that small cha nges in the ionic atmosphere of the channel (i.e., shielding) make big changes in the potential profile and even bigger changes in flux, bec ause potential is a sensitive function of charge and shielding, and fl ux is an exponential function of potential.