High-resolution Mn EXAFS of the oxygen-evolving complex in photosystem II: structural implications for the Mn4Ca cluster

The biological generation of oxygen by the oxygen-evolving complex in photosystem II (PS II) is one of nature's most important reactions. The recent X-ray crystal structures, while limited by resolutions of 3.2-3.5 A, have located the electron density associated with the Mn4Ca cluster within the multiprotein PS II complex. Detailed structures critically depend on input from spectroscopic techniques, such as EXAFS and EPR/ENDOR, as the XRD resolution does not allow for accurate determination of the position of Mn/Ca or the bridging and terminal ligand atoms. The number and distances of Mn-Mn/Ca/ligand interactions determined from EXAFS provide important constraints for the structure of the Mn4Ca cluster. Here, we present data from a high-resolution EXAFS method using a novel multicrystal monochromator that show three short Mn-Mn distances between 2.7 and 2.8 A and, hence, the presence of three di-mu-oxo-bridged units in the Mn4Ca cluster. This result imposes clear limitations on the proposed structures based on spectroscopic and diffraction data and provides input for refining such structures.     

Characterization of a new tailoring domain in polyketide biogenesis: the amine transferase domain of MycA in the mycosubtilin gene cluster

We report the expression and characterization of a truncated form of MycA from the Mycosubtilin gene cluster from Bacillus subtilis. The MycA fragment contains a new amino transferase (AMT) tailoring domain, allowing the first detailed study of a PLP-dependent enzyme operating in cis within the PKS and NRPS biosynthetic paradigm. As the AMT domain acts on covalently bound beta-ketothioesters, and is therefore a single-turnover system, electrospray ionization-Fourier transform mass spectrometry (ESI-FTMS) was used to observe the amine-transfer reaction both for amine donor substrate specificity and to regiospecifically determine enzyme-bound intermediates. We confirm the function of the AMT domain, dissect the mechanistic steps of amine transfer, identify the preferred amine source, and localize the beta-ketothioester substrate during amine transfer.     

Numerical study of the flow in a three-dimensional thermally driven cavity

Solutions for the fully compressible Navier–Stokes equations are presented for the flow and temperature fields in a cubic cavity with large horizontal temperature differences. The ideal-gas approximation for air is assumed and viscosity is computed using Sutherland's law. The three-dimensional case forms an extension of previous studies performed on a two-dimensional square cavity. The influence of imposed boundary conditions in the third dimension is investigated as a numerical experiment. Comparison is made between convergence rates in case of periodic and free-slip boundary conditions. Results with no-slip boundary conditions are presented as well. The effect of the Rayleigh number is studied.     

Conformational analysis of 1,2-difluoroethane, an ab-initio investigation

Contrary to expectation, the gauche conformer of 1,2-difluoroethane is more stable than the trans conformer in the gas phase. In order to understand the underlying causes of the “gauche effect”, a complete geometry relaxation was performed for the gauche and trans conformers of 1,2-difluoroethane with the 4-21G, 4-31G and 4-31G** basis sets. The 4-31G** optimized geometry of the gauche conformer compares well with the experimental values obtained from a number of electron-diffraction studies. A correction for the correlation energy, calculated by means of second-order Møller—Plesset perturbation theory with the 6-31G** basis set, proves to be essential to obtain a correct estimate of the energy difference between the gauche and trans conformers 1,2-difluoroethane.     

Photostabilizing of bisphenol A polycarbonate by using UV-absorbers and self protective block copolymers based on resorcinol polyarylate blocks

Bisphenol A polycarbonate degrades due to sunlight, humidity and oxygen. In this study two possible techniques to stabilize the polymer were compared, i.e. blending of UV-absorbers (UVAs) into the polymer or using block copolymers based on resorcinol polyarylates. Combination of different analysis techniques shows that the protection by UVAs is not as good as by the resorcinol polyarylate block copolymers. The block copolymer rearranges itself through a photo-Fries rearrangement within hours into a UV-absorbing top layer. Two different block compositions were studied, and the copolymer with the highest concentration of resorcinol polyarylate groups shows the best protection.     

The modal interpretation of quantum mechanics and its generalization to density operators

We generalize the modal interpretation of quantum mechanics so that it may be applied to composite systems represented by arbitrary density operators. We discuss the interpretation these density operators receive and relate this to the discussion about the interpretation of proper and improper mixtures in the standard interpretation.     

Comments on the wetting behavior of non-porous substrates for ceramic coated-conductor applications

This work gives an overview of the possibilities to improve the wetting behavior of precursors for coated conductors on non-porous substrates. Within this work, all coatings were performed on a metallic Ni–W/La₂Zr₂O₇/CeO₂ substrate using water-based Y, Ba, Cu containing precursors. The results described in this paper can be used for different technologies of chemical solution deposition, as there are ink jet printing, dip coating, spin coating etc. Starting from the forces involved during wetting, a separation between solid and liquid modifications was made. This study revealed that if a good cleaning procedure of the substrate, whether or not combined with a targeted modification of the precursor is applied, water-based solutions can be used without restriction towards their wetting behaviour leading to a sustainable technology within the coating industry. Within this work, special attention is given to (1) fast determination of the substrate cleaning procedure quality by the creation of wetting envelopes and (2) the use of a screening design of experiment to study the effects of intrinsic solution factors, such as precursor formulation, influencing the coating behavior. All modification discussed are expandable to all kinds of precursors and substrates.     

How thermal curing of an organic paper coating changes topography, chemistry, and wettability

Celluloses are preferred renewable substrates, but hydrophilicity and porosity disfavor their water resistance. We present here an ecofriendly application of imidized nanoparticles and a method to flexibly tune the surface wettability of papers. The soft nanostructured coating is sensitive to thermal curing, which affects both the surface chemistry and morphology. The thermal stability of the coating is first investigated with conventional and modulated differential scanning calorimetry, revealing influences of the imide content and an endotherm reaction below the glass transition temperature at 120-150 °C. The latter is studied in detail for an appropriate selection of the copolymer precursors. According to diffuse reflection infrared spectroscopy, Raman spectroscopy, and UV/vis spectroscopy, the endotherm corresponds to an imidization reaction. The morphology of the coatings is followed at various scale levels by contactless roughness measurements and atomic force microscopy. Finally, the experimental values are fitted to the parameters of the Wenzel wetting model, and so-called calibration curves for the relation between contact angles, surface roughness, and surface chemistry are presented. They allow the prediction of the water contact angle of coated papers from the hydrophilic to the hydrophobic range, with a maximum in hydrophobicity after increasing the imide content at 120-150 °C curing.     

Additive-free clicking for polymer functionalization and coupling by tetrazine-norbornene chemistry

Herein we report the use of a tetrazine-norbornene inverse electron demand Diels-Alder conjugation applied to polymer end-functionalization and polymer-polymer coupling. The reaction was found to be applicable to polymer-polymer coupling, as judged by SEC, DOSY NMR, and LCxSEC analyses, giving diblock copolymers by merely mixing the constituent homopolymers together under ambient conditions, using no catalyst, additive, or external stimulus.