The Equilibria of Lipid–K+ Ions in Monolayer at the Air/Water Interface

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, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$K_{{{\text{PCK}}^{ + } }} = { 3}. 2 6\times 10^{ 2} {\text{dm}}^{ 3} \,{\text{mol}}^{ - 1}$$\end{document}, and for cholesterol–K+ ion (ChK+) complex, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$K_{{{\text{ChK}}^{ + } }} = { 1}.00 \times 10^{ 3} {\text{dm}}^{ 3} \,{\text{mol}}^{ - 1}$$\end{document}, were calculated by inserting the experimental data. The value of area occupied by one PCK+ complex is 60 Å2 molecule−1, while the area occupied by one ChK+ complex is 40.9 Å2 molecule−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

SINGLE-TAIL PHOSPHATES CONTAINING BRANCHED ALKYL CHAINS - SYNTHESIS AND AGGREGATION IN WATER OF A NOVEL CLASS OF VESICLE-FORMING SURFACTANTS

Four disodium monoalkyl phosphates containing branched undecyl and dodecyl groups have been synthesized. The aggregation properties of these surfactants were characterized by transmission electron microscopy of uranyl acetate-stained samples and freeze-fracture replicas. As predicted from a packing parameter analysis, the branching of the alkyl substituents induces aggregation into bilayers. Thermally stable unilamellar vesicles with diameters between 25 and 60 nm were prepared by vigorous stirring in water at room temperature. Lowering of the pH leads to partial protonation of the phosphate headgroup and to less electrostatic repulsion between headgroups and bilayers. This results in larger vesicle diameters (50-100 nm), a higher tendency to aggregate, and reduced thermal stability. Phase penetration experiments show a lamellar boundary phase upon dissolving the crystalline surfactant in water and growth of myelin structures upon increasing the temperature or decreasing the pH. Upon addition of Ca2+, the monoalkyl phosphate vesicles aggregate at [Ca2+] > 0.5 mM and undergo bilayer fusion, leading to stable vesicles (diameters between 100 and 250 nm) above a Ca2+ threshold concentration of 1.0 mM

SINGLE-TAIL PHOSPHATES CONTAINING BRANCHED ALKYL CHAINS - SYNTHESIS AND AGGREGATION IN WATER OF A NOVEL CLASS OF VESICLE-FORMING SURFACTANTS

Four disodium monoalkyl phosphates containing branched undecyl and dodecyl groups have been synthesized. The aggregation properties of these surfactants were characterized by transmission electron microscopy of uranyl acetate-stained samples and freeze-fracture replicas. As predicted from a packing parameter analysis, the branching of the alkyl substituents induces aggregation into bilayers. Thermally stable unilamellar vesicles with diameters between 25 and 60 nm were prepared by vigorous stirring in water at room temperature. Lowering of the pH leads to partial protonation of the phosphate headgroup and to less electrostatic repulsion between headgroups and bilayers. This results in larger vesicle diameters (50-100 nm), a higher tendency to aggregate, and reduced thermal stability. Phase penetration experiments show a lamellar boundary phase upon dissolving the crystalline surfactant in water and growth of myelin structures upon increasing the temperature or decreasing the pH. Upon addition of Ca2+, the monoalkyl phosphate vesicles aggregate at [Ca2+] > 0.5 mM and undergo bilayer fusion, leading to stable vesicles (diameters between 100 and 250 nm) above a Ca2+ threshold concentration of 1.0 mM