The equilibria of lipid-K+ ions in monolayer at the air/water interface

Abstract

The effect of K+ ion interaction with monolayers of phosphatidylcholine (lecithin, PC) or cholesterol (Ch) was investigated at the air/water interface. We present surface tension measurements of lipid monolayers obtained using a Langmuir method as a function of K+ ion concentration. Measurements were carried out at 22°C using a Teflon trough and a Nima 9000 tensiometer. Interactions between lecithin and K+ ions or Ch and K+ ions result in significant deviations from the additivity rule. An equilibrium theory to describe the behavior of monolayer components at the air/water interface was developed in order to obtain the stability constants and area occupied by one molecule of lipid-K+ ion complex (LK+). The stability constants for lecithin-K+ ion (PCK+) complex, KPCK+ = 3.26 × 102 dm3 mol-1 and for cholesterol-K+ ion (ChK +) complex, KChK+ = 1.00 × 103 dm 3 mol-1 were calculated by inserting the experimental data. The value of area occupied by one PCK+ complex is 60 Å2molecule-1, while the area occupied by one ChK+ complex is 40.9 Å2molecule-1. The complex formation energy (Gibbs free energy) values for the PCK+ and ChK+ complexes are -14.18 ± 0.71 and -16.92 ± 0.85 kJ mol-1, respectively. © 2011 The Author(s).