Reliable plate–plate MRF magnetorheometry based on validated radial magnetic flux density profile simulations

The apparent shear stress from plate–plate magnetorheometry, using the commercial magnetocell MRD180/1T (Physica/Anton Paar) in standard configuration, is distinctly overestimated. The effect is due to a flux density maximum near the sample rim and radial migration of iron particles towards the rim. Radial magnetic flux density profiles were investigated by finite element simulations using the Maxwell^®2D code and by direct Hall probe measurements. The reliability of the finite element method results, both for the empty magnetocell and with magnetorheological fluid (MRF) in the measuring gap, allows conclusions on the true flux density within the MRF, which cannot be accessed by Hall probe measurements. If the MRF sits on top of the bottom yoke (standard configuration), the flux density maximum reaches twice the plateau value (0.74 T for 3 A coil current and 0.3 mm gap height of the investigated MRF). This yields a higher effective flux density and causes radial iron particle migration, resulting in an additional magnetic flux increase near the rim due to augmented MRF magnetisation. As a result, the rotor torque at constant rotary speed increases with time. Reliable results are achieved by a modification of the magnetocell, such that the MRF sits on a non-magnetic Hall disc of 1.5 mm thickness, allowing an online flux density measurement by a FW Bell Hall probe. In this configuration, the radial flux density profile near the rim remains sufficiently flat and no iron particle migration is detected.     

The role of disaccharides for protein–protein interactions – a SANS study

ABSTRACT

The disaccharide trehalose has shown outstanding anti-aggregation properties for proteins, which are highly important for the possibility to treat neurodegenerative diseases, such as Alzheimer’s and Huntington’s disease. However, the role and mechanism of trehalose for such stabilising effects are still largely unknown, partly because a direct structural picture of how trehalose organises around proteins in an aqueous system is missing. Here we compare small-angle neutron scattering (SANS) data on myoglobin in aqueous solutions of either sucrose or trehalose, in order to investigate their effect on protein–protein interactions. We find that both trehalose and sucrose induces a well-defined protein–protein distance, which could explain why these inhibit protein–protein interactions and associated protein aggregation. It does not however explain the superior anti-aggregation effect of trehalose and suggests that the local solvent structures are highly important for explaining the protein stabilisation mechanism. In a broader perspective, these findings are important for understanding the role of sugars in biological stabilisation, and could provide a structural explanation for why trehalose is a promising candidate for the treatment of neurodegenerative and other protein aggregation related diseases.     

Mechanisms of protein oligomerization: inhibitor of functional amyloids templates α-synuclein fibrillation

Small organic molecules that inhibit functional bacterial amyloid fibers, curli, are promising new antibiotics. Here we investigated the mechanism by which the ring-fused 2-pyridone FN075 inhibits fibrillation of the curli protein CsgA. Using a variety of biophysical techniques, we found that FN075 promotes CsgA to form off-pathway, non-amyloidogenic oligomeric species. In light of the generic properties of amyloids, we tested whether FN075 would also affect the fibrillation reaction of human α-synuclein, an amyloid-forming protein involved in Parkinson's disease. Surprisingly, FN075 stimulates α-synuclein amyloid fiber formation as measured by thioflavin T emission, electron microscopy (EM), and atomic force microscopy (AFM). NMR data on (15)N-labeled α-synuclein show that upon FN075 addition, α-synuclein oligomers with 7 nm radius form in which the C-terminal 40 residues remain disordered and solvent exposed. The polypeptides in these oligomers contain β-like secondary structure, and the oligomers are detectable by AFM, EM, and size-exclusion chromatography (SEC). Taken together, FN075 triggers oligomer formation of both proteins: in the case of CsgA, the oligomers do not proceed to fibers, whereas for α-synuclein, the oligomers are poised to rapidly form fibers. We conclude that there is a fine balance between small-molecule inhibition and templation that depends on protein chemistry.     

Anomalous dependence of particle size on supersaturation in the preparation of iron nanoparticles from iron pentacarbonyl

Anisotropic colloidal particles consisting of different compositions and geometry are useful for various applications. These include optical biosensing, antireflective coatings and electronic displays. In this work we demonstrate a simple and cost-effective method for fabricating anisotropic colloidal particles bearing a snowman-like shape. This is achieved by first settling the positively-charged polystyrene latex (PSL) colloids and negatively-charged silica colloids in deionized water onto a glass substrate, forming heterodoublets. The temperature is then raised above the glass transition temperature of the polymer. As a result, the silica particle spontaneously rises to the top of the PSL particle forming a snowman like structure. We have extended this method to different sizes and shown that the structure of the hybrid particles can be tuned by adjusting the size ratio between the silica and the PSL colloids. The surface coverage of the PSL, and hence of the snowman particles, on the glass substrate can also be varied by changing the ionic strength of the solution during the adhesion of PSL to the glass.     

Rigorous definition of oxidation states of ions in solids

We present justification and a rigorous procedure for electron partitioning among atoms in extended systems. The method is based on wave-function topology and the modern theory of polarization, rather than charge density partitioning or wave-function projection, and, as such, reformulates the concept of oxidation state without assuming real-space charge transfer between atoms. This formulation provides rigorous electrostatics of finite-extent solids, including films and nanowires.     

The probe rules in single particle tracking

Single particle tracking (SPT) enables light microscopy at a sub-diffraction limited spatial resolution by a combination of imaging at low molecular labeling densities and computational image processing. SPT and related single molecule imaging techniques have found a rapidly expanded use within the life sciences. This expanded use is due to an increased demand and requisite for developing a comprehensive understanding of the spatial dynamics of bio-molecular interactions at a spatial scale that is equivalent to the size of the molecules themselves, as well as by the emergence of new imaging techniques and probes that have made historically very demanding and specialized bio-imaging techniques more easily accessible and achievable. SPT has in particular found extensive use for analyzing the molecular organization of biological membranes. From these and other studies using complementary techniques it has been determined that the organization of native plasma membranes is heterogeneous over a very large range of spatial and temporal scales. The observed heterogeneities in the organization have the practical consequence that the SPT results in investigations of native plasma membranes are time dependent. Furthermore, because the accessible time dynamics, and also the spatial resolution, in an SPT experiment is mainly dependent on the luminous brightness and photostability of the particular SPT probe that is used, available SPT results are ultimately dependent on the SPT probes. The focus of this review is on the impact that the SPT probe has on the experimental results in SPT.     

A noncommutative catenoid

A noncommutative algebra corresponding to the classical catenoid is introduced together with a differential calculus of derivations. We prove that there exists a unique metric and torsion-free connection that is compatible with the complex structure, and the curvature is explicitly calculated. A noncommutative analogue of the fact that the catenoid is a minimal surface is studied by constructing a Laplace operator from the connection and showing that the embedding coordinates are harmonic. Furthermore, an integral is defined and the total curvature is computed. Finally, classes of left and right modules are introduced together with constant curvature connections, and bimodule compatibility conditions are discussed in detail.     

Assessing the interaction between Helicobacter pylori and human neutrophils by freeze-fracture replica labeling

We recently introduced a freeze-fracture replica labeling method adapted to studies of bacterial envelopes. This report describes a further development of this detergent-digested freeze-fracture replica labeling technique, thus more exactly the conception of this explicit methodology for visualization of bacteria-host cell interactions. Our experimental model employs human neutrophils and the gastric pathogenic bacterium Helicobacter pylori. The phagocytic process performed by the neutrophils represents a crucial element of the host defense system against invading microorganisms, and by so doing, it allows direct observation of the interplay between bacteria and host cells at an ultrastructural level. The here launched methodology can be used as a tool to investigate the events taking place between pathogenic microbes and phagocytes, as well as for pinpoint targeting of other cell-cell communications in the field of cell biology.     

Intracellular Photophysics of an Osmium Complex bearing an Oligothiophene Extended Ligand

This contribution describes the excited-state properties of an Osmium-complex when taken up into human cells. The complex 1 [Os(bpy)₂(IP-4T)](PF₆)₂ with bpy=2,2′-bipyridine and IP-4T=2-{5′-[3′,4′-diethyl-(2,2′-bithien-5-yl)]-3,4-diethyl-2,2′-bithiophene}imidazo[4,5-f][1,10]phenanthroline) can be discussed as a candidate for photodynamic therapy in the biological red/NIR window. The complex is taken up by MCF7 cells and localizes rather homogeneously within in the cytoplasm. To detail the sub-ns photophysics of 1 , comparative transient absorption measurements were carried out in different solvents to derive a model of the photoinduced processes. Key to rationalize the excited-state relaxation is a long-lived ³ILCT state associated with the oligothiophene chain. This model was then tested with the complex internalized into MCF7 cells, since the intracellular environment has long been suspected to take big influence on the excited state properties. In our study of 1 in cells, we were able to show that, though the overall model remained the same, the excited-state dynamics are affected strongly by the intracellular environment. Our study represents the first in depth correlation towards ex-vivo and in vivo ultrafast spectroscopy for a possible photodrug.