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.     

Monodisperse foams in one to three dimensions

Most major recent advances in the physics of monodisperse foams are centred around the ability to generate them with excellent control of bubble sizes down to a few micrometers thanks to the development of appropriate micro- and millifluidic techniques. As a natural consequence, monodisperse liquid and solid foams are playing an increasingly important role in fundamental research and in the development of industrial applications. In this review, we will address the different properties of monodisperse foams, comparing them to the more standard polydisperse foams.     

Comparison of the Complexation of Open-Chain and Cyclic N2S2 Ligands with Cu+ and Cu2+

The complexation properties of the open-chain N₂S₂ ligands 1–4 are described and compared to those of analogous N₂S₂ macrocycles 5–7 . With Cu²⁺, the open-chain ligands give complexes with the stoichiometry CuL²⁺ and CuLOH⁺, the stabilities and absorption spectra of which have been determined. The ligand field exerted by these ligands is relatively constant and independent of the length of the chain. With Cu⁺, the species CuLH, CuLH²⁺, and CuL⁺ were identified and their stabilities measured. The redox potentials calculated from the equilibrium constants and measured by cyclic voltammetry agree and lie between 250 and 280 mV against SHE. The comparison between open-chain and cyclic ligands shows that (1) a macrocyclic effect is found for Cu²⁺ but not for Cu⁺, (2) the ligand-field strength is very different for the two types of ligands, and (3) the redox potentials span a larger interval for the macrocyclic than for the open-chain complexes.     

Highly structured porous solids from liquid foam templates

We demonstrate the generation of highly structured porous solids from liquid foam templates, using ordered foam layers and threads made from hydrogels. For this purpose we separate sufficiently foam generation and solidification: well known and highly controllable liquid foam structures are created, which are thereafter ‘frozen’ in situ through polymerisation and cross-linking. Being extendible to a large range of materials and length scales, such an approach opens up a plethora of opportunities in material development.     

Enantioselective Phenol Coupling by Laccases in the Biosynthesis of Fungal Dimeric Naphthopyrones

Biaryl compounds are ubiquitous metabolites that are often formed by dimerization through oxidative phenol coupling. Hindered rotation around the biaryl bond can cause axial chirality. In nature, dimerizations are catalyzed by oxidative enzymes such as laccases. This class of enzymes is known for non‐specific oxidase reactions while inherent enantioselectivity is hitherto unknown. Here, we describe four related fungal laccases that catalyze γ‐naphthopyrone dimerization in a regio‐ and atropselective manner. In vitro assays revealed that three enzymes were highly P‐selective (ee >95 %), while one enzyme showed remarkable flexibility. Its selectivity for M‐ or P‐configured dimers varied depending on the reaction conditions. For example, a lower enzyme concentration yielded primarily (P)‐ustilaginoidin A, whereas the M atropisomer was favored at higher concentration. These results demonstrate inherent enantioselectivity in an enzyme class that was previously thought to comprise only non‐selective oxidases.     

Cationic Niobium-Sandwich and Piano-Stool Complexes

The syntheses of the homoleptic bis(arene) niobium cations [Nb(arene)₂]⁺ (arene = C₆H₃Me₃, C₆H₅Me) with 16 valence electrons and heteroleptic arene-carbonyl cations [(CO)Nb(arene)₂]⁺ (arene = C₆H₃Me₃, C₆H₅Me) and [(arene)M(CO)₄]⁺ (arene = C₆H₃Me₃, C₆H₆) obeying 18 valence electrons are described. Stabilization of these complexes was achieved by using the weakly coordinating anions [Al(OR^F)₄]⁻ or [F{Al(OR^F)₃}₂]⁻ (R^F = C(CF₃)₃). The limits of two synthesis routes starting from neutral Nb(arene)₂ (arene = C₆H₃Me₃, C₆H₅Me) or [NEt₄][M(CO)₆] (M = Nb, Ta) were investigated. All compounds were analyzed by single crystal X-ray determination, vibrational and NMR spectroscopy. DFT calculations were executed to support the experimental data.     

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.