3 research outputs found
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
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