Zwitterionic and Charged Lipids Form Remarkably Different Structures on Nanoscale Oil Droplets in Aqueous Solution

The molecular structure of zwitterionic and charged monolayers on small oil droplets in aqueous solutions is determined using a combined second harmonic and sum frequency study. From the interfacial vibrational signature of the acyl chains and phosphate headgroups as well as the response of the hydrating water, we find that zwitterionic and charged lipids with identical acyl chains form remarkably different monolayers. Zwitterionic phospholipids form a closely packed monolayer with highly ordered acyl tails. In contrast, the charged phospholipids form a monolayer with a low number density and disordered acyl tails. The charged headgroups are oriented perpendicular to the monolayer rather than parallel, as is the case for zwitterionic lipids. These significant differences between the two types of phospholipids indicate important roles of phospholipid headgroups in the determination of properties of cellular membranes and lipid droplets. The observed behavior of charged phospholipids is different from expectations based on studies performed on extended planar interfaces, at which condensed monolayers are readily formed. The difference can be explained by nanoscale related changes in charge condensation behavior that has its origin in a different balance of interfacial intermolecular interactions

Three Dimensional Nano “Langmuir Trough” for Lipid Studies

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-<i>sn</i>-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

Lysozyme Interaction with Phospholipid Nanodroplets Probed by Sum Frequency Scattering Vibrational Spectroscopy

When a nanoparticle (NP) is introduced into a biological environment, its identity and interactions are immediately attributed to the dense layer of proteins that quickly covers the particle. The formation of this layer, dubbed the protein corona, is in general a combination of proteins interacting with the surface of the NP and a contest between other proteins for binding sites either at the surface of the NP or upon the dense layer. Despite the importance for surface engineering and drug development, the molecular mechanisms and structure behind interfacial biomolecule action have largely remained elusive. We use ultrafast sum frequency scattering (SFS) spectroscopy to determine the structure and the mode of action by which these biomolecules interact with and manipulate interfaces. The majority of work in the field of sum frequency generation has been done on flat model interfaces. This limits some important membrane properties such as membrane fluidity and dimensionalityimportant factors in biomolecule–membrane interactions. To move toward three-dimensional (3D) nanoscopic interfaces, we utilize SFS spectroscopy to interrogate the surface of 3D lipid monolayers, which can be used as a model lipid-based nanocarrier system. In this study, we have utilized SFS spectroscopy to follow the action of lysozyme. SFS spectra in the amide I region suggest that there is lysozyme at the interface and that the lysozyme induces an increased lipid monolayer order. The binding of lysozyme with the NP is demonstrated by an increase in acyl chain order determined by the ratio of the CH3 symmetric and CH2 symmetric peak amplitudes. Furthermore, the lipid headgroup orientation s-PO2– change strongly supports lysozyme insertion into the lipid layer causing lipid disruption and reorientation. Altogether, with SFS, we have made a huge stride toward understanding the binding and structure change of proteins within the protein corona

Kinetically Stable Triglyceride-Based Nanodroplets and Their Interactions with Lipid-Specific Proteins

Understanding of the interactions between proteins and natural and artificially prepared lipid membrane surfaces and embedded nonpolar cores is important in studies of physiological processes and their pathologies and is applicable to nanotechnologies. In particular, rapidly growing interest in cellular droplets defines the need for simplified biomimetic lipid model systems to overcome in vivo complexity and variability. We present a protocol for the preparation of kinetically stable nanoemulsions with nanodroplets composed of sphingomyelin (SM) and cholesterol (Chol), as amphiphilic surfactants, and trioleoylglycerol (TOG), at various molar ratios. To prepare stable SM/Chol-coated monodisperse lipid nanodroplets, we modified a reverse phase evaporation method and combined it with ultrasonication. Lipid composition, ζ-potential, gyration and hydrodynamic radius, shape, and temporal stability of the lipid nanodroplets were characterized and compared to extruded SM/Chol large unilamellar vesicles. Lipid nanodroplets and large unilamellar vesicles with theoretical SM/Chol/TOG molar ratios of 1/1/4.7 and 4/1/11.7 were further investigated for the orientational order of their interfacial water molecules using a second harmonic scattering technique, and for interactions with the SM-binding and Chol-binding pore-forming toxins equinatoxin II and perfringolysin O, respectively. The surface characteristics (ζ-potential, orientational order of interfacial water molecules) and binding of these proteins to the nanodroplet SM/Chol monolayers were similar to those for the SM/Chol bilayers of the large unilamellar vesicles and SM/Chol Langmuir monolayers, in terms of their surface structures. We propose that such SM/Chol/TOG nanoparticles with the required lipid compositions can serve as experimental models for monolayer membrane to provide a system that imitates the natural lipid droplets