Three Dimensional Nano "Langmuir Trough" for Lipid Studies

Yixing Chen, Kailash Chandra Jena, Halil Ibrahim Okur, Sylvie Roke
Amer Chemical Soc, 2015
Journal paper

Abstract

A three-dimensional-phospholipid monolayer with tunable molecular structure was created on the surface of oil nanodroplets from a mixture of phospholipids, oil, and water. This simple nanoemulsion preparation technique generates an in situ prepared membrane model system with controllable molecular surface properties that resembles a lipid droplet. The molecular interfacial structure of such a nanoscopic system composed of hexadecane, 1,2-dihexadecanoyl-sn-glycero-3-phosphocholine (DPPC), and water was determined using vibrational sum frequency scattering and second harmonic scattering techniques. The droplet surface structure of DPPC can be tuned from a tightly packed liquid condensed phase like monolayer to a more dilute one that resembles the liquid condensed/liquid expanded coexistence phase by varying the DPPC/oil/water ratio. The tunability of the chemical structure, the high surface-to-volume ratio, and the small sample volume make this system an ideal model membrane for biochemical research.

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https://infoscience.epfl.ch/record/212094?ln=en

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EPFL2017

Aqueous interfaces are omnipresent in nature. Some of them are visible: droplets in clouds, aerosols, and surfaces of oceans, lakes, and rivers. Many more surfaces are hidden: water in contact with cell membranes, proteins, and the surface of droplets in emulsions. The hydrophobic interaction plays a key role in many biological and industrial processes; hence understanding of water structure at the hydrophobic interfaces is needed. To elucidate molecular-level water structure at these interfaces and to learn about the molecular-level mechanisms behind hydrophobicity we use sum frequency and second harmonic scattering spectroscopy. We apply these techniques to probe the molecular structure (orientational order and strength of the hydrogen bonds) at the interfaces of droplet and liposomes. First, we demonstrate a way to use the focusing properties of a liquid jet for nonlinear scattering experiments. As the jet surface is continuously refreshed, reduced heating effects and cleanliness are advantages of this technique. Then, investigating water structure in water droplets in oil, we find that hydrogen bonds of water at the surface of the droplet resemble that of planar oil/water interface made of the same chemicals at 50°C lower temperature. Using water droplets dispersed in CCl4 we show that second harmonic scattering data has a high sensitivity to the ionic strength of the solution. We estimate the concentration of free charge carriers (reverse micelles and hydrated ions) and discuss changes in the surface water ordering and the surface potential, which reflect the formation of reversed micelles at the droplet interface. Finally, we study water and lipid transmembrane asymmetry using liposomes composed of neutral and charged lipids, which are a good model system of a phospholipid bilayer of a cell membrane. We calculate the distribution of the phosphate tilt angle and quantify lipid transmembrane asymmetry. Furthermore, we discuss a mechanism responsible for the observed lipid asymmetry that involves intermolecular hydrogen bonding between phosphate and amine groups of adjacent lipids.

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Freezing effects of oil-in-water emulsions studied by sum-frequency scattering spectroscopy

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Temperature-dependent sum-frequency scattering spectroscopy is used to study the properties of hexadecane and dodecane oil droplets in water. The sum-frequency scattering spectra contain vibrational bands that correspond to the symmetric and antisymmetric CH stretching vibrations of the methylene (CH2) and methyl (CH3) groups of the alkane molecules. The relative amplitudes of the vibrational bands provide information on the surface structure and the shape of the oil droplets. We study the sum-frequency scattering spectra over a temperature range of -48 to 24 degrees C, including the freezing transitions of the water matrix and the oil droplets. Hexadecane oil droplets freeze at a higher temperature than the surrounding water, whereas dodecane oil droplets freeze at a lower temperature than the surrounding water. This allows us to independently study the freezing effect of oil and water on the surface structure of the oil droplets. In both cases, freezing leads to a change in the polarization dependencies that are valid in the case of the spherical-symmetric shapes that the oil droplets assume when both water and oil are liquid. We find that the freezing of water leads to a strong distortion of the liquid dodecane surface but has little effect on the surface of already solidified hexadecane. For completely frozen emulsions a further decrease in temperature is observed to lead to a further distortion of the surface of the solid oil particles, which might be caused by increasing hardness of the ice matrix encapsulating the particles. Published by AIP Publishing.

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