Sebastian Tiemeyer

Elucidating the mechanism of lipid membrane-induced IAPP fibrillogenesis and its inhibition by the red wine compound resveratrol: a synchrotron X-ray reflectivity study.

The islet amyloid polypeptide (IAPP) or amylin is a pancreatic hormone and crucially involved in the pathogenesis of type-II diabetes mellitus (T2DM). Aggregation and amyloid formation of IAPP is considered as the primary culprit for pancreatic beta-cell loss in T2DM patients. In this study, first X-ray reflectivity (XRR) measurements on IAPP at lipid interfaces have been carried out, providing a molecular level characterization of the first steps of the lipid-induced fibrillation process of IAPP, which is initiated by lipid-induced nucleation, oligomerization, followed by detachment of larger IAPP aggregate structures from the lipid membrane, and terminated by the formation of mature fibrils in the bulk solution. The adsorption process of IAPP at lipid interfaces in the absence and presence of negatively charged lipid has also been studied by complementary ATR-FTIR spectroscopic measurements. The morphological properties were followed by atomic force microscopy (AFM). Moreover, we show that the polyphenolic red wine compound resveratrol is able to inhibit IAPP aggregation also in the presence of aggregation-fostering negatively charged lipid interfaces, revealing its potential as a drug candidate for T2DM.

Intercalation in Layered Metal–Organic Frameworks: Reversible Inclusion of an Extended π-System

We report the synthesis of layered [Zn(2)(bdc)(2)(H(2)O)(2)] and [Cu(2)(bdc)(2)(H(2)O)(2)] (bdc = benzdicarboxylate) metal-organic frameworks (MOF) carried out using the liquid-phase epitaxy approach employing self-assembled monolayer (SAM) modified Au-substrates. We obtain Cu and Zn MOF-2 structures, which have not yet been obtained using conventional, solvothermal synthesis methods. The 2D Cu(2+) dimer paddle wheel planes characteristic for the MOF are found to be strictly planar, with the planes oriented perpendicular to the substrate. Intercalation of an organic dye, DXP, leads to a reversible tilting of the planes, demonstrating the huge potential of these surface-anchored MOFs for the intercalation of large, planar molecules.

Stress‐Induced Stabilization of Crystals in Shape Memory Natural Rubber

In contrast to all known shape memory polymers, the melting temperature of crystals in shape memory natural rubber (SMNR) can be greatly manipulated by the application of external mechanical stress. As shown previously, stress perpendicular to the prior programming direction decreases the melting temperature by up to 40 K. In this study, we investigated the influence of mechanical stress parallel to prior stretching direction during programming on the stability of the elongation-stabilizing crystals. It was found that parallel stress stabilizes the crystals, which is indicated by linear increase of the trigger temperature by up to 17 K. The crystal melting temperature can be increased up to 126.5 °C under constrained conditions as shown by X-ray diffraction measurements.

The Effect of Ionic Strength, Temperature, and Pressure on the Interaction Potential of Dense Protein Solutions: From Nonlinear Pressure Response to Protein Crystallization

Understanding the intermolecular interaction potential, V(r), of proteins under the influence of temperature, pressure, and salt concentration is essential for understanding protein aggregation, crystallization, and protein phase behavior in general. Here, we report small-angle x-ray scattering studies on dense lysozyme solutions of high ionic strength as a function of temperature and pressure. We show that the interaction potential changes in a nonlinear fashion over a wide range of temperatures, salt, and protein concentrations. Neither temperature nor protein and salt concentration lead to marked changes in the pressure dependence of V(r), indicating that changes of the water structure dominate the pressure dependence of the intermolecular forces. Furthermore, by analysis of the temperature, pressure, and ionic strength dependence of the normalized second virial coefficient, b2, we show that the interaction can be fine-tuned by pressure, which can be used to optimize b2 values for controlled protein crystallization.

Temperature-Driven Adsorption and Desorption of Proteins at Solid−Liquid Interfaces

The heat-induced desorption and adsorption of the proteins lysozyme, ribonuclease A, bovine serum albumin, and fibronectin at protein layers was investigated in two different environments: pure buffer and protein solution. Using two different environments allows us to distinguish between thermodynamic and kinetic mechanisms in the adsorption process. We observed a desorption in buffer and an adsorption in protein solution, depending upon protein properties, such as size, stability, and charge. We conclude that the desorption in buffer is mainly influenced by the mobility of the proteins at the interface, while the adsorption in protein solution is driven by conformational changes and, thereby, a gain in entropy. These results are relevant for controlling biofilm formation at solid-liquid interfaces.

Sticking polydisperse hydrophobic magnetite nanoparticles to lipid membranes.

The formation of a layer of hydrophobic magnetite (Fe(3)O(4)) nanoparticles stabilized by lauric acid is analyzed by in situ X-ray reflectivity measurements. The data analysis shows that the nanoparticles partially disperse their hydrophobic coating. Consequently, a Langmuir layer was formed by lauric acid molecules that can be compressed into an untilted condensed phase. A majority of the nanoparticles are attached to the Langmuir film integrating lauric acid residue on their surface into the Langmuir film. Hence, the particles at the liquid-gas interface can be identified as so-called Janus beads, which are amphiphilic solids having two sides with different functionality.

Adsorption of nanoparticles at the solid-liquid interface.

The adsorption of differently charged nanoparticles at liquid-solid interfaces was investigated by in situ X-ray reflectivity measurements. The layer formation of positively charged maghemite (γ-Fe(2)O(3)) nanoparticles at the aqueous solution-SiO(2) interface was observed while negatively charged gold nanoparticles show no adsorption at this interface. Thus, the electrostatic interaction between the particles and the charged surface was determined as the driving force for the adsorption process. The data analysis shows that a logarithmic particle size distribution describes the density profile of the thin adsorbed maghemite layer. The size distribution in the nanoparticle solution determined by small angle X-ray scattering shows an average particle size which is similar to that found for the adsorbed film. The formed magehemite film exhibits a rather high stability.

Polaron-induced lattice distortion of (In,Ga)As/GaAs quantum dots by optically excited carriers

We report on a high resolution x-ray diffraction study unveiling the effect of carriers optically injected into (In,Ga)As quantum dots on the surrounding GaAs crystal matrix. We find a tetragonal lattice expansion with enhanced elongation along the [001] crystal axis that is superimposed on an isotropic lattice extension. The isotropic contribution arises from excitation induced lattice heating as confirmed by temperature dependent reference studies. The tetragonal expansion on the femtometer scale is tentatively attributed to polaron formation by carriers trapped in the quantum dots.