Daniel Clemens

Multiple Scale Reorganization of Electrostatic Complexes of Poly(styrenesulfonate) and Lysozyme

We report on a SANS investigation into the potential for these structural reorganization of complexes composed of lysozyme and small PSS chains of opposite charge if the physicochemical conditions of the solutions are changed after their formation. Mixtures of solutions of lysozyme and PSS with high matter content and with an introduced charge ratio [-]/[+](intro) close to the electrostatic stoichiometry lead to suspensions that are macroscopically stable. They are composed at local scale of dense globular primary complexes of radius approximately 100 A; at a higher scale they are organized fractally with a dimension 2.1. We first show that the dilution of the solution of complexes, all other physicochemical parameters remaining constant, induces a macroscopic destabilization of the solutions but does not modify the structure of the complexes at submicronic scales. This suggests that the colloidal stability of the complexes can be explained by the interlocking of the fractal aggregates in a network at high concentration: dilution does not break the local aggregate structure, but it does destroy the network. We show, second, that the addition of salt does not change the almost frozen inner structure of the cores of the primary complexes, although it does encourage growth of the complexes; these coalesce into larger complexes as salt has partially screened the electrostatic repulsions between two primary complexes. These larger primary complexes remain aggregated with a fractal dimension of 2.1. Third, we show that the addition of PSS chains up to [-]/[+](intro) approximately 20, after the formation of the primary complex with a [-]/[+](intro) close to 1, only slightly changes the inner structure of the primary complexes. Moreover, in contrast to the synthesis achieved in the one-step mixing procedure where the proteins are unfolded for a range of [-]/[+](intro), the native conformation of the proteins is preserved inside the frozen core.

A new time-of-flight small-angle scattering instrument at the Helmholtz-Zentrum Berlin: V16/VSANS

Small-angle scattering methods have become routine techniques for the structural characterization of macromolecules and macromolecular assemblies like polymers, (block) copolymers or micelles in the spatial range from a few to hundreds of nanometres. Neutrons are valuable scattering probes, because they offer freedom with respect to scattering length density contrast and isotopic labelling of samples. In order to gain maximum benefit from the allotted experiment time, the instrumental setup must be optimized in terms of statistics of scattered intensity, resolution and accessible range in momentum transfer Q. The new small-angle neutron scattering instrument V16/VSANS at the Helmholtz-Zentrum in Berlin, Germany, augments neutron guide collimation and pinhole optics with time-of-flight data recording and flexible chopper configuration. Thus, the available Q range and the respective instrumental resolution in the intermediate and high momentum transfer regions can be adjusted and balanced to the individual experimental requirements. This renders V16/VSANS a flexible and versatile instrument for soft-matter research.

Structure, roughness and magnetization behaviour of triode sputtered Co11Fe89-Si multilayers

A study of monochromator-type neutron mirrors, produced under different sputtering conditions by a triode sputtering system is presented, and the effect on the magnetic behaviour and the film structure is revealed. Special attention was paid to the dependence of the interface roughness in respect to the deposition parameters. A structural characterization of the specimen was performed by x-ray reflectometry and diffraction, supported by transmission electron microscopy and electron diffraction studies, made on ultramicrotom cuts oriented perpendicular to the surface. SQUID magnetometry and polarized neutron reflectometry in combination with a simulation of the reflection data on the Co11Fe89-Si system was used to examine the magnetization profile and its evanescence near the chemical interface. Mosbauer spectroscopy clarified the model of interpretation.

Polarizing Ti1-uXu/FexCoyVz supermirrors

Abstract Polarizing multilayers and supermirrors composed of Ti1−uXu and FexCoyVz have been produced using DC-magnetron sputtering. The structure of the samples has been characterized by neutron and X-ray diffraction. The results show that the smoothness of the layers can be improved by adding controlled amounts of alloying materials, an effect that has already been observed for Ni/Ti supermirrors. Flipping ratios of more than 40 and reflectivities of more than 85% at twice the critical angle of Ni are obtained, depending on the concentrations of the materials, u, x, y, z. The sputtering process can be optimized in such a way that the magnetization within the plane is anisotropic, thus allowing a saturation of the samples in low fields.

Polyelectrolyte multilayer hollow capsules studied by small-angle neutron scattering (SANS)

Silica-templated polyelectrolyte (PE) hollow capsules with various template diameters and PE layer numbers were fabricated by using the layer-by-layer technique and subsequent core dissolution in hydrofluoric acid. The properties of the resulting freestanding polyelectrolyte multilayers (PEM) were characterized in an aqueous environment by means of SANS and compared with scanning force microscopy (SFM) data. The thickness of the capsule wall was found to be about 25% thicker and with correspondingly higher water content than the template-supported PE layers. The wall thickness increase as a result of core dissolution was anisotropic. The average single layer thickness of hollow capsules was independent of the surface curvature and decreased slightly with increasing PE layer number. SANS was used to determine whether the capsules were empty or contained the rest of the core. An annealing at 70 °C for 4 h induced capsule shrinking by about 20% at an ionic strength of 1 mol L−1. Furthermore, the capsule wall upon annealing increased in thickness by about 38%. These changes corresponded to a wall densification of about 13%.

Upgrade Program for the Cold Neutron Instrumentation of the Helmholtz-Zentrum Berlin

The Helmholtz-Zentrum Berlin (HZB, former Hahn-Meitner-Institut) operates more than 20 instruments at the medium-flux research reactor BER II, a modern neutron source which was completely renewed during the years 1985–1992. The creation of the Berlin Neutron Scattering Center (BENSC) in 1993 marked the beginning of an extended user program, offering access to a great variety of new instruments with some unique features suited for research in many fields of science. In the course of time the Helmholtz-Zentrum Berlin has established itself as a major neutron science facility with an exceptionally high demand from international, in particular European users.

Cyclic trimer of human cystatin C, an amyloidogenic protein - molecular dynamics and experimental studies

Human cystatin C (HCC) is a cysteine protease inhibitor that takes a series of oligomeric forms in solution (e.g., dimers, trimers, tetramers, decamers, dodecamers, and other higher oligomers). The best-known form of cystatin C is the dimer, which arises as a result of a domain swapping mechanism. The formation of the HCC oligomeric forms, which is most likely due to this domain swapping mechanism, is associated with the aggregation of HCC into amyloid fibrils and deposits. To investigate the structure of a specific HCC oligomer, we developed a covalently stabilized trimer of HCC. An atomic model of this HCC trimer was proposed on the basis of molecular docking and molecular dynamics simulations. The most stable model of the HCC trimer obtained from the molecular dynamics simulations is characterized by a well-preserved secondary structure. The molecular size and structural parameters of the HCC trimer in solution were also confirmed by Small Angle Neutron Scattering and Nuclear Magnetic Resonance Diffuso...

New polarizing guide for neutron wavelengths above 2.5 Å

We present a new polarizing system built for the relocated wide angle Neutron Spin Echo instrument SPAN. The new instruments at the second Guide Hall of BENSC and the relocation of SPAN to this hall of BENSC required a new beam extraction system and a new polarizer for SPAN, which replaced the old beam splitter produced in 1994 with FeCo-Si supermirrors with m=2. The new polarizer uses Fe-Si supermirrors, which do not run the risk to become activated as the old FeCo-Si supermirrors and was designed to deliver a polarized beam for wavelengths above 2.5 A. The final polarizing cavity has a length of 9 m with a cross section of 60 mm × 100 mm. Si wafers coated on both sides with m=2.5 Fe-Si polarizing supermirrors are glued into the guide at an angle of 0.38° to the walls. The guide was installed during the second half year of 2006 and the first tests in early 2007 revealed excellent polarization efficiency over the whole wavelength range of the spectrometer of 2.5 A to 9 A, amounting to above 95% at 4.5 A.