Modeling of Amino Acid Electrodiffusion through Fixed Charge Membranes

Abstract

We study theoretically the electrodiffusion of amino acids through fixed charge membranes, calculating the ionic fractions of the amino acid in the membrane as well as its total flux as a function of the relevant experimental parameters (amino acid concentration, salt concentration, and pH of the external solution; membrane fixed charge concentration; and amino acid membrane/solution partition coefficients) under different experimental conditions (symport vs antiport transport, uphill transport, etc.). The theoretical approach employed is based on the Nernst–Planck flux equations in the (Goldman) constant electric field assumption and considers all the species present in the system (cationic, anionic, and zwitterionic forms of the amino acid, hydrogen and hydroxide ions, and salt ions). The results show that many of the experimental trends observed in the amino acid transport through fixed charge membranes can be explained qualitatively when the ionic nature of both the amino acid and the membrane fixed groups is incorporated in the theoretical model.