5 research outputs found

    Monitoring Demineralization and Subsequent Remineralization of Human Teeth at the Dentin–Enamel Junction with Atomic Force Microscopy

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    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

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    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

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    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

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    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

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    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
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