Metin Tolan

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.

Microscopic structure of water at elevated pressures and temperatures

We report on the microscopic structure of water at sub- and supercritical conditions studied using X-ray Raman spectroscopy, ab initio molecular dynamics simulations, and density functional theory. Systematic changes in the X-ray Raman spectra with increasing pressure and temperature are observed. Throughout the studied thermodynamic range, the experimental spectra can be interpreted with a structural model obtained from the molecular dynamics simulations. A spatial statistical analysis using Ripley’s K-function shows that this model is homogeneous on the nanometer length scale. According to the simulations, distortions of the hydrogen-bond network increase dramatically when temperature and pressure increase to the supercritical regime. In particular, the average number of hydrogen bonds per molecule decreases to ≈0.6 at 600 °C and p = 134 MPa.

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.

Effect of Osmolytes on Pressure‐Induced Unfolding of Proteins: A High‐Pressure SAXS Study

Herein, we explore the effect of different types of osmolytes on the high-pressure stability and tertiary structure of a well-characterized monomeric protein, staphylococcal nuclease (SNase). Changes in the denaturation pressure and the radius of gyration are obtained in the presence of different concentrations of trimethylamine N-oxide (TMAO), glycerol and urea. To reveal structural changes in the protein upon compression at various osmolyte conditions, small-angle X-ray scattering (SAXS) experiments were carried out. To this end, a new high-pressure cell suitable for high-precision SAXS studies at synchrotron sources was built, which allows one to carry out scattering experiments up to maximum pressures of about 7 kbar. Our data clearly indicate that the osmolytes that stabilize proteins against temperature-induced unfolding drastically increase their pressure stability and that the elliptically shaped curve of the pressure-temperature-stability diagram of proteins is shifted to higher temperatures and pressures with increasing osmolyte concentration. A drastic stabilization is observed for the osmolyte TMAO, which exhibits not only a significant stabilization against temperature-induced unfolding, but also a particularly strong stabilization of the protein against pressure. In fact, such findings are in accordance with in vivo studies (for example P. J. Yancey, J. Exp. Biol. 2005, 208, 2819-2830), where unusually high TMAO concentrations in some deep-sea animals were found. Conversely, chaotropic agents such as urea have a strong destabilizing effect on both the temperature and pressure stability of the protein. Our data also indicate that sufficiently high TMAO concentrations might be able to largely offset the destabilizing effect of urea. The different scenarios observed are discussed in the context of recent experimental and theoretical studies.

Analysis of x-ray reflectivity data from low-contrast polymer bilayer systems using a Fourier method.

X-ray reflectivity data of polymer bilayer systems have been analyzed using a Fourier method which takes into account different limits of integration in q-space. It is demonstrated that the interfacial parameters can be determined with high accuracy although the difference in the electron density (the contrast) of the two polymers is extremely small. This method is not restricted to soft-matter thin films. It can be applied to any reflectivity data from low-contrast layer systems.

Exploring the Piezophilic Behavior of Natural Cosolvent Mixtures

Proteins are only marginally stable and are hence very sensitive to environmental conditions, such as high and low temperatures or high hydrostatic pressures. In nature, living organisms are able to compensate for extreme environmental conditions and hence rescue proteins from denaturation by using osmolytes. Organic osmolytes are accumulated under anhydrobiotic, thermal, and pressure stresses. Among those osmolytes are amino acids, polyols and sugars (e.g., glycerol and trehalose), methylamines such as trimethylamine-Noxide (TMAO), and urea. TMAO has been found to enhance protein folding and ligand binding most efficiently. On the other hand, urea, a highly concentrated waste product in mammalian kidneys, is a perturbant. It is also a major organic osmolyte in marine elasmobranch fishes. Interestingly, TMAO has been found to counteract perturbations imposed by urea and hydrostatic pressure in deep-sea animals, most effectively at a 2:1 urea:TMAO ratio. In the deep sea, hydrostatic pressures up to the 1 kbar (100 MPa) range prevail, and living organisms have to cope with such extreme environmental conditions. High hydrostatic pressure generally destabilizes the protein structure, inhibits polymerization of proteins and ligand binding. Interestingly, TMAO has been shown to largely offset these pressure effects. In fact, it was found that the amount of TMAO in the cells of a series of marine organisms increases linearly with the depth of the ocean. For that reason, TMAO is thought to serve as pressure counteractant. The term “piezolyte” has been coined for such kind of cosolute. About the underlying mechanism of stabilization by TMAO at ambient pressure conditions several experimental and theoretical (molecular dynamics simulations) articles have been published in recent years. TMAO is largely excluded from the protein surface and enhances the water structure causing greater organization through more and stronger hydrogen bonding among water molecules. However, the mechanism of this “chemical chaperon” at high hydrostatic pressure (HHP) conditions is still unclear. To yield a deeper understanding of this phenomenon, we determined the intermolecular interaction of dense protein solutions in the absence and presence of cosolvent mixtures of TMAO and urea also under HHP conditions. Small-angle Xray scattering (SAXS) experiments on dense lysozyme solutions have been carried out in the pressure range from 1 bar up to 4 kbar. The SAXS technique accurately monitors structural alterations of the protein solution and yields quantitative information on the state-dependent protein– protein interaction potential. As lysozyme is a highly stable protein, pressure-induced effects will only be attributed to changes in the protein–protein interaction of the native protein and how this is influenced by osmolytes. No pressureinduced unfolding of the protein occurs in the pressure range covered. Complementary thermodynamic data, that is, the temperature of unfolding and the volume change upon unfolding of the protein, were obtained by differential scanning (DSC) and pressure perturbation calorimetry (PPC), respectively. To verify that the protein is folded at all solution conditions studied, SAXS measurements on diluted lysozyme solutions (cP= 10 mgmL ) were carried out in the whole pressure range covered. For diluted protein solutions, the scattering intensity I(q) is proportional to the form factor P(q) (q= (4p/l)sin(V/2) is the wave vector transfer, l the wavelength of the X-rays, and V the scattering angle), which depends on the structure and size of the protein. For dilute lysozyme solutions, the radius of gyration of the particle, Rg, could be determined. We found a constant Rg value of (15.1 0.4) up to 4 kbar, indicating the absence of unfolding even at the highest pressure applied. In the case of concentrated protein solutions, the interaction between the particles gives rise to an additional scattering contribution. This SAXS signal can be described as the product of the form factor and an effective structure factor, which is related to the intermolecular structure factor S(q). To relate the structure factor to the protein–protein interaction potential, statistical mechanical model approaches have to be employed. Here, the mean-spherical approximation (MSA) in combination with the DLVO (Derjaguin– Landau–Verwey–Overbeek) potential V(r) has been used. The pair potential V(r) is given as the sum of a hard sphere potential VHS(r), a repulsive screened Coulomb-like potential VSC(r) and an attractive Yukawian potential VY(r), which is frequently used to describe protein–protein interactions (for details, see the Supporting Information). [*] Y. Zhai, Prof. Dr. R. Winter Fakult t Chemie, TU Dortmund Physikalische Chemie—Biophysikalische Chemie Otto-Hahn Str. 6, 44227 Dortmund (Germany) E-mail: roland.winter@tu-dortmund.de

The carbon dioxide-water interface at conditions of gas hydrate formation.

The structure of the carbon dioxide-water interface was analyzed by X-ray diffraction and reflectivity at temperature and pressure conditions which allow the formation of gas hydrate. The water-gaseous CO2 and the water-liquid CO2 interface were examined. The two interfaces show a very different behavior with respect to the formation of gas hydrate. While the liquid-gas interface exhibits the formation of thin liquid CO2 layers on the water surface, the formation of small clusters of gas hydrate was observed at the liquid-liquid interface. The data obtained from both interfaces points to a gas hydrate formation process which may be explained by the so-called local structuring hypothesis.

The new diffractometer for surface X-ray diffraction at beamline BL9 of DELTA.

The experimental endstation of the hard X-ray beamline BL9 of the Dortmund Electron Accelerator is equipped with a Huber six-circle diffractometer. It is dedicated to grazing-incidence X-ray diffraction and X-ray reflectivity experiments on solid surfaces and thin films as well as to powder diffraction measurements. A new set-up for grazing-incidence X-ray scattering of liquids has been built up using a silicon mirror to reflect the incident X-ray to the liquid surface at angles of incidence around the critical angle of total reflection of the sample. X-ray reflectivity measurements of a polymer film and grazing-incidence X-ray diffraction measurements of an epitaxically grown Gd40Y60 film, an oxidized surface of Fe-15at.%Al alloy and aqueous salt solutions are presented and discussed.

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 small-angle and wide-angle X-ray scattering set-up at beamline BL9 of DELTA.

The multi-purpose experimental endstation of beamline BL9 at the Dortmund Electron Accelerator (DELTA) is dedicated to diffraction experiments in grazing-incidence geometry, reflectivity and powder diffraction measurements. Moreover, fluorescence analysis and inelastic X-ray scattering experiments can be performed. Recently, a new set-up for small-angle and wide-angle X-ray scattering utilizing detection by means of an image-plate scanner was installed and is described in detail here. First small-angle X-ray scattering experiments on aqueous solutions of lysozyme with different cosolvents and of staphylococcal nuclease are discussed. The application of the set-up for texture analysis is emphasized and a study of the crystallographic texture of natural bio-nanocomposites, using lobster and crab cuticles as model materials, is presented.