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
Specific Ion Effects in Amphiphile Hydration and Interface Stabilization
Specific ion effects
can influence many processes in aqueous solutions:
protein folding, enzyme activity, self-assembly, and interface stabilization.
Ionic amphiphiles are known to stabilize the oil/water interface,
presumably by dipping their hydrophobic tails into the oil phase while
sticking their hydrophilic head groups in water. However, we find
that anionic and cationic amphiphiles adopt strikingly different structures
at liquid hydrophobic/water interfaces, linked to the different specific
interactions between water and the amphiphile head groups, both at
the interface and in the bulk. Vibrational sum frequency scattering
measurements show that dodecylsulfate (DS<sup>–</sup>) ions
do not detectably perturb the oil phase while dodecyltrimethylammonium
(DTA<sup>+</sup>) ions do. Raman solvation shell spectroscopy and
second harmonic scattering (SHS) show that the respective hydration-shells
and the interfacial water structure are also very different. Our work
suggests that specific interactions with water play a key role in
driving the anionic head group toward the water phase and the cationic
head group toward the oil phase, thus also implying a quite different
surface stabilization mechanism