Phase transition and ion permeability of phosphatidylcholine monolayers at the polarized oil/water interface

Abstract

The monolayer characteristics and the ion permeability of saturated monolayers of six l-α-phosphatidylcholines, dilauroyl (DLPC), dimiristoyl (DMPC), dipalmitoyl (DPPC), distearoyl (DSPC), diarachidoyl (DAPC), and dibehenoylphosphatidylcholine (DBPC), have been studied by measuring ac impedance at the polarized nitrobenzene/water interface. The DLPC and DMPC monolayers are in a liquid-expanded state between 5 and 30°C, whereas the DSPC, DAPC, and DBPC monolayers are in a liquid-condensed state throughout the same temperature range. The DPPC monolayer exhibits a temperature-induced phase transition at 13°C. The monolayers in the liquid-condensed state reduce the rate of ion transfer for both tetramethylammonium ion (TMA +) and tetraethylammonium ion (TEA +). In contrast, the monolayers in the liquid-expanded state unexpectedly accelerate the transfer of both TMA + and TEA + ions across the interface. The results indicate that a phosphatidylcholine monolayer in the liquid-condensed state exerts a hydrodynamic friction on transferring ions, whereas a monolayer in the liquid-expanded state itself is transparent to the ion transfer.