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

Hydrogen bonding strength of interfacial water determined with surface sum-frequency generation

We demonstrate that marked variations exist in hydrogen bonding interactions of interfacial water at different aqueous interfaces. The average hydrogen bond strength and its distribution are inferred from surface sum-frequency generation (SFG) spectra through the center frequency and width, respectively, of the O-D stretch vibration of isotopically diluted HDO in H2O. The use of partially deuterated water prevents complications due to intramolecular vibrational coupling, which we show gives rise to features in the SFG spectra that are unrelated to hydrogen bonding interactions. At the water-air interface, the SFG spectrum in the hydrogen-bonded region strongly resembles the bulk Raman spectrum, indicating that, at this interface, the interfacial hydrogen bonding properties are very similar to those in bulk water. In contrast, for silica-water and lipid-water interfaces, interfacial hydrogen bonding is substantially stronger, with a larger degree of heterogeneity

Calcium-induced phospholipid ordering depends on surface pressure.

The effect of sodium and calcium ions on zwitterionic and anionic phospholipids monolayers is investigated using vibrational sum-frequency generation in conjunction with surface pressure measurements and fluorescence microscopy. Sodium ions only subtly affect the monolayer structure, while the effect of calcium is large and depends strongly on the surface pressure. At low surface pressures (5 mN/m), the presence on Ca2+ results in the unexpected appearance of ordered domains. For pressures between 5 and 25 mN/m, Ca2+ ions induce disorder in the monolayer. For pressures exceeding 25 mN/m, calcium cations expand the monolayer, while simultaneously ordering the lipid chains. Interestingly, effects are similar for both zwitterionic lipids and negatively charged lipids. In both vibrational sum-frequency generation and surface tension measurements, the molecular signature of the association of Ca2+ with the lipids is evident from Ca2+-induced changes in the signals corresponding to area changes of 4 Å2/lipid‐precisely the surface area of a Ca2+ ion, with evidence for a change in lipid Ca2+ complexation at high pressures