Transport of monocarboxylic acids with different acyl chain length through the planar bilayer lipid membrane

Electrically silent hydrogen ion flux across the planar bilayer lipid membrane (BLM) induced by the addition of monocarboxylic acids al one side of the BLM was studied by measuring the pH changes in the unstirred layers near the BLM surface. It was shown that the mechanism of the acid transport changed qualitatively upon the increase in the length of the hydrophobic chain. At pH<pKa, the total transport was limited by the diffusion of the anionic form of the short-chain acid across the unstirred layers, whereas at the alkaline pH (pH>> pKa), the transport was limited by the diffusion of the neutral form across the membrane. However, in the case of long-chain acids (more than 8 carbon atoms), the transport was limited by the diffusion of the anionic form withing the whole range of pH studied. In the latter case, pH changes in the unstirred layers did not depend on the presence of cholesterol in the membrane and on the width of the unstirred layer. It was proposed that the peculiarities of the long-chain acid-induced proton transport can be explained by the formation of micelles of the acid in the bathing solutions.