Protein adhesion on dental surfaces—a combined surface analytical approach

Protein adsorption is a field of huge interest in a number of application fields. Information on protein adhesion is accessible by a variety of methods. However, the results obtained are significantly influenced by the applied technique. The objective of this work was to understand the role of adhesion forces (obtained by scanning force spectroscopy, SFS) in the process of protein adsorption and desorption. In SFS, the protein is forced to and retracted from the surface, even under unfavorable conditions, in contrast to the natural situation. Furthermore, adhesion forces are correlated with adhesion energies, neglecting the entropic part in the Gibbs enthalpy. In this context, dynamic contact angle (DCA) measurements were performed to identify the potential of this method to complement SFS data. In DCA measurements, the protein diffuses voluntarily to the surface and information on surface coverage and reversibility of adsorption is obtained, including entropic effects (conformational changes and hydrophobic effect). It could be shown that the surface coverage (by DCA) of bovine serum albumin on dental materials correlates well with the adhesion forces (by SFS) if no hydrophobic surface is involved. On those, the entropic hydrophobic effect plays a major role. As a second task, the reversibility of the protein adsorption, i.e., the voluntary desorption as studied by DCA, was compared to the adhesion forces. Here, a correlation between low adhesion forces and good reversibility could be found as long as no covalent bonds were involved. The comparative study of DCA and SFS, thus, leads to a more detailed picture of the complete adsorption/desorption cycle.     

Joint European XFEL and DESY Photon Science Users' Meeting 2015

The European XFEL and DESY Photon Science Users' Meeting 2015 broke the attendance record of the previous year. In total, more than 800 scientists from around the world came to Deutsches Elektronen-Synchrotron (DESY) in Hamburg, Germany, to participate in this three-day event, which took place on January 28–30, 2015. In particular, the latest news about the construction of the European XFEL facility as well as the extension projects at DESY's synchrotron source PETRA III and the Free-Electron Laser FLASH attracted a lot of interest.     

Kinetic Evidence for a Non‐Langmuir‐Hinshelwood Surface Reaction: H/D Exchange over Pd Nanoparticles and Pd(111)

The mechanism of hydrogen recombination on a Pd(111) single crystal and well‐defined Pd nanoparticles is studied using pulsed multi‐molecular beam techniques and the H₂/D₂ isotope exchange reaction. The focus of this study is to obtain a microscopic understanding of the role of subsurface hydrogen in enhancing the associative desorption of molecular hydrogen. HD production from H₂ and D₂ over Pd is investigated using pulsed molecular beams, and the temperature dependence and reaction orders are obtained for the rate of HD production under various reaction conditions designed to modulate the amount of subsurface hydrogen present. The experimental data are compared to the results of kinetic modeling based on different mechanisms for hydrogen recombination. We found that under conditions where virtually no subsurface hydrogen species are present, the HD formation rate can be described exceptionally well by a classic Langmuir–Hinshelwood model. However, this model completely fails to reproduce the experimentally observed high HD formation rates and the reaction orders under reaction conditions where subsurface hydrogen is present. To analyze this phenomenon, we develop two kinetic models that account for the role of subsurface hydrogen. First, we investigate the possibility of a change in the reaction mechanism, where recombination of one subsurface and one surface hydrogen species (known as a breakthrough mechanism) becomes dominant when subsurface hydrogen is present. Second, we investigate the possibility of the modified Langmuir–Hinshelwood mechanism with subsurface hydrogen lowering the activation energy for recombination of two hydrogen species adsorbed on the surface. We show that the experimental reaction kinetics can be well described by both kinetic models based on non‐Langmuir–Hinshelwood‐type mechanisms.     

Effect of a π-donor on the radical bulk polymerization of methyl methacrylate

Bulk polymerizations of methyl methacrylate (MMA) at 60°C initiated with 2,2′-azoisobutyronitrile are influenced by the presence of an organic π-donor such as tetrathiafulvalene (TTF). Upon addition of TTF, the ratio of weight- to number-average molecular weights M̄_w/M̄_n are significantly reduced and the thermal stability of the poly(methyl methacrylate) samples is increased. Kinetic investigations indicate that TTF acts as a retarder on the polymerization mechanism.