5 research outputs found
Monitoring Demineralization and Subsequent Remineralization of Human Teeth at the Dentin–Enamel Junction with Atomic Force Microscopy
Using
atomic force microscopy, we monitored the nanoscale surface morphology
of human teeth at the dentin–enamel junction after performing
successive demineralization steps with an acidic soft drink. Subsequently,
we studied the remineralization process with a paste containing calcium
and phosphate ions. Repeated atomic force microscopy imaging of the
same sample areas on the sample allowed us to draw detailed conclusions
regarding the specific mechanism of the demineralization process and
the subsequent remineralization process. The about 1-μm-deep
grooves that are caused by the demineralization process were preferentially
filled with deposited nanoparticles, leading to smoother enamel and
dentine surfaces after 90 min exposure to the remineralizing agent.
The deposited material is found to homogeneously cover the enamel
and dentine surfaces in the same manner. The temporal evolution of
the surface roughness indicates that the remineralization caused by
the repair paste proceeds in two distinct successive phases
Highly Asymmetrical Glycerol Diether Bolalipids: Synthesis and Temperature-Dependent Aggregation Behavior
In the present work, we describe
the synthesis and temperature-dependent
aggregation behavior of two examples of a new class of highly asymmetrical
glycerol diether bolaphospholipids. The bolalipids contain a long
alkyl chain (C32) bound to glycerol in the <i>sn</i>-3 position,
carrying a hydroxyl group at the ω position. The C16 alkyl chain
in the <i>sn</i>-2 position either possesses a racemic methyl
branch at the 10 position of the short alkyl chain (lipid <b>II</b>) or does not (lipid <b>I</b>). The <i>sn</i>-1 position
of the glycerol is linked to a zwitterionic phosphocholine moiety.
The temperature-dependent aggregation behavior of both bolalipids
was studied using differential scanning calorimetry (DSC), Fourier-transform
infrared (FTIR) spectroscopy, and X-ray scattering. Aggregate structures
were visualized by transmission electron microscopy (TEM). We show
that both bolalipids self-assemble into large lamellar sheetlike aggregates.
Closed lipid vesicles or other aggregate structures such as tubes
or nanofibers, as usually found for diglycerol tetraether lipids,
were not observed. Within the lamellae the bolalipid molecules are
arranged in an antiparallel (interdigitated) orientation. Lipid <b>I</b>, without an additional methyl moiety in the short alkyl
chain, shows a lamellar phase with high crystallinity up to a temperature
of 34 °C, which was not observed before for other phospholipids
Controlling the Localization of Polymer-Functionalized Nanoparticles in Mixed Lipid/Polymer Membranes
Surface hydrophobicity plays a significant role in controlling the interactions between nanoparticles and lipid membranes. In principle, a nanoparticle can be encapsulated into a liposome, either being incorporated into the hydrophobic bilayer interior or trapped within the aqueous vesicle core. In this paper, we demonstrate the preparation and characterization of polymer-functionalized CdSe NPs, tuning their interaction with mixed lipid/polymer membranes from 1,2-dipalmitoyl-<i>sn</i>-glycero-3-phophocholine and PIB<sub>87</sub>-<i>b</i>-PEO<sub>17</sub> block copolymer by varying their surface hydrophobicity. It is observed that hydrophobic PIB-modified CdSe NPs can be selectively located within polymer domains in a mixed lipid/polymer monolayer at the air/water interface, changing their typical domain morphologies, while amphiphilic PIB-PEO-modified CdSe NPs showed no specific localization in phase-separated lipid/polymer films. In addition, hydrophilic water-soluble CdSe NPs can readily adsorb onto spread monolayers, showing a larger effect on the molecule packing at the air/water interface in the case of pure lipid films compared to mixed monolayers. Furthermore, the incorporation of PIB-modified CdSe NPs into hybrid lipid/polymer GUVs is demonstrated with respect to the prevailing phase state of the hybrid membrane. Monitoring fluorescent-labeled PIB-CdSe NPs embedded into phase-separated vesicles, it is demonstrated that they are enriched in one specific phase, thus probing their selective incorporation into the hydrophobic portion of PIB<sub>87</sub>-<i>b</i>-PEO<sub>17</sub> BCP-rich domains. Thus, the formation of biocompatible hybrid GUVs with selectively incorporated nanoparticles opens a new perspective for subtle engineering of membranes together with their (nano-) phase structure serving as a model system in designing functional nanomaterials for effective nanomedicine or drug delivery
A T-Shaped Amphiphilic Molecule Forms Closed Vesicles in Water and Bicelles in Mixtures with a Membrane Lipid
The T-shaped amphiphilic molecule A6/6 forms a columnar
hexagonal
liquid-crystalline phase between the crystalline and the isotropic
liquid when studied in bulk (Chen et al., 2005). Because of the hydrophilic
and flexible oligo(oxyethylene) side chain terminated by a 1-acylamino-1-deoxy-d-sorbitol moiety attached to a rigid terphenyl core with terminal
hexyloxy alkyl chains, it was expected that also formation of lyotropic
phases could be possible. We therefore studied the behavior of A6/6
in water and also in mixtures with bilayer-forming phospholipids,
such as dipalmitoyl-phosphatidylcholine (DPPC), using differential
scanning calorimetry (DSC), transmission electron microscopy (TEM),
cryo-transmission electron microscopy (cryo-TEM), dynamic light scattering
(DLS), and solid-state nuclear magnetic resonance (ssNMR). DSC showed
for the pure A6/6 suspended in water a phase transition at ca. 23
°C. TEM and cryo-TEM showed vesicular as well as layered structures
for pure A6/6 in water below and above this phase transition. By atomic
force microscopy (AFM), the thickness of the layer was found to be
5–6 nm. This leads to a model for a bilayer formed by A6/6
with the laterally attached polar side chains shielding the hydrophobic
layer built up by the terphenyl core with the terminal alkyl chains
of the molecules. For DPPC:A6/6 mixtures (10:1), the DSC curves indicated
a stabilization of the lamellar gel phase of DPPC. Negative staining
TEM and cryo-TEM images showed planar bilayers with hexagonal morphology
and diameters between 50 and 200 nm. The hydrodynamic radius of these
aggregates in water, investigated by dynamic light scattering (DLS)
as a function of time and temperature, did not change indicating a
very stable aggregate structure. The findings lead to the proposition
of a new bicellar structure formed by A6/6 with DPPC. In this model, the bilayer edges are covered by the T-shaped
amphiphilic molecules preventing very effectively the aggregation
to larger structures
Temperature-Dependent In-Plane Structure Formation of an X‑Shaped Bolapolyphile within Lipid Bilayers
Polyphilic compound B12 is an X-shaped
molecule with a stiff aromatic
core, flexible aliphatic side chains, and hydrophilic end groups.
Forming a thermotropic triangular honeycomb phase in the bulk between
177 and 182 °C but no lyotropic phases, it is designed to fit
into DPPC or DMPC lipid bilayers, in which it phase separates at room
temperature, as observed in giant unilamellar vesicles (GUVs) by fluorescence
microscopy. TEM investigations of bilayer aggregates support the incorporation
of B12 into intact membranes. The temperature-dependent behavior of
the mixed samples was followed by differential scanning calorimetry
(DSC), FT-IR spectroscopy, fluorescence spectroscopy, and X-ray scattering.
DSC results support in-membrane phase separation, where a reduced
main transition and new B12-related transitions indicate the incorporation
of lipids into the B12-rich phase. The phase separation was confirmed
by X-ray scattering, where two different lamellar repeat distances
are visible over a wide temperature range. Polarized ATR-FTIR and
fluorescence anisotropy experiments support the transmembrane orientation
of B12, and FT-IR spectra further prove a stepwise “melting”
of the lipid chains. The data suggest that in the B12-rich domains
the DPPC chains are still rigid and the B12 molecules interact with
each other via π–π interactions. All results obtained
at temperatures above 75 °C confirm the formation of a single,
homogeneously mixed phase with freely mobile B12 molecules