brane (12). The experimental results were qualitatively ex-The leucine permeability coefficients, P, through a cation explained using a theoretical model which incorporates the change membrane were measured as a function of HCl concentraequilibrium dissociation equations for the fraction of amino tion, C HCl , from 0 to 10 01 mol/L for three different interfacial acid forms at different pH values, a rate equation for the conditions and were compared with the results for glycine. Leucine membrane interfacial transport based on the interfacial transport phenomena remarkably depended on the interfacial conchemical reaction, and the Nernst-Planck flux equations for dition between the membrane and the external solution. The lowest the ion transport through the bulk of the membrane. The P (2.33 1 10 08 cm 2 /s) was obtained at C HCl Γ
0 for both sides of results appeared to indicate that the rate-limiting step in the the membrane, and generally P increases with an increase in C HCl .
amino acid transport process could be the rate at which
In case of the glycine, the lowest P (1.85 1 10 07 cm 2 /s) had been obtained if C HCl on the glycine source side was 10 01 mol/L and glycine exited the membrane and thus contributed to a better that on the opposite side was 0. The highest P (4.34 1 10 07 cm 2 / understanding of amino acid permeation across charged s) was obtained if C HCl on the leucine source side was 0 and that membranes. on the opposite side was 10 01 mol/L as well as for glycine (4.16 In this paper, the transport phenomena of leucine across 1 10 06 cm 2 /s). The P values of glycine had been explained by the a cation exchange membrane are examined as a function of transport mechanism based on the interfacial chemical reaction HCl concentration for three different interfacial conditions (protonation, deprotonation, or ion-exchange) and the ionic transand compared with the results for glycine. The pK a of leucine port across a charged membrane. The P values of leucine were (Γ
2.36) is quite similar to that of glycine (Γ
2.35). On also explained qualitatively by the same mechanism as glycine; the other hand, leucine solubility in water is lower and the however, they are remakably affected by the physicochemical molecular size is larger than those of glycine. It is expected properties of the membrane, such as hydrophobicity and molecular that the leucine transport phenomena are different from those size.