Cation diffusion selectivity in a pore model. The phosphatidylcholine/water lamellar phase.

The diffusion coefficients D (cm2/s), of four monovalent cations K+, Na+, Rb+ and Cs+ and of Ca2+ have been measured in phosphatidylcholine/water lamellar phase as a function of phase hydration and temperature and in the presence of divalent cations. Diffusion rates vary strongly with phase hydration, between 10(-7) and 10(-6) cm2/s for monovalent and 10(-8) and 10(-7) for Ca2+. The activation energies obtained are relatively small (5--10 kcal/mol). As the phase water content increases, a series of diffusion sequences is obtained, corresponding to the sequences predicted by Eisenman's theory of alkali ion equilibrium selectivity. This diffusional selectivity, which depends exclusively upon non-equilibrium parameters (mobility) within the hydrophilic path is discussed in respect to current theories of pore selectivity.