We created a new platform to employ in lipid monolayer and lipid membrane studies. Namely, a 3-dimensional (3D) phospholipid monolayer with tunable molecular structure was created on the surface of oil nanodroplets from a mixture of phospholipids, oil and water. Such a simple sample preparation technique generates an in-situ prepared membrane model system with controllable molecular surface properties. 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. By varying the DPPC / oil/ water ratio, DPPC lipid structure 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. 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.
The system is very similar to lipid droplets that one finds as organels in almost every living cell.
Nanodroplets with Lipid Monolayers. Left: Schematic showing the nanodroplets in aqueous solution in a sample cuvette. The red and green waves are representing the incoming IR and vis beams. The blue waves represent the generated sum frequency scattering beam. Right: Spectra showing the molecular configuration of the interface. Adapted from: Nano Lett. (2015), 15, (8), pp5558-5563
Variations between the inner and outer leaflets of cell membranes are crucial for cell functioning and signaling, drug–membrane interactions, and the formation of lipid domains. Transmembrane asymmetry can in principle be comprised of an asymmetric charge distribution, differences in hydration, specific headgroup / H-bonding interactions, or a difference in the number of lipids per leaflet. Here, we characterize the transmembrane asymmetry of small unilamellar liposomes consisting of zwitterionic and charged lipids in aqueous solution using vibrational sum frequency scattering and second harmonic scattering, label-free methods, specifically sensitive to lipid and water asymmetries. For single component liposomes, transmembrane asymmetry is present for the charge distribution and lipid hydration, but the leaflets are not detectably asymmetric in terms of the number of lipids per leaflet, even though geometrical packing arguments would predict so. Such a lipid transmembrane asymmetry can, however, be induced in binary lipid mixtures under conditions that enable H-bonding interactions between phosphate and amine groups. In this case, the measured asymmetry consists of a different number of lipids in the outer and inner leaflet, a difference in transmembrane headgroup hydration, and a different headgroup orientation for the interacting phosphate groups.
Transmembrane Asymmetry in Liposomes. Sum Frequency Scattering spectrum of H-bonded phosphate head groups on the outer leaflet of a mixed DOPC/DPPS liposome.From: JACS (2016),138 (12), pp. 4053-4060