Survival of the Fittest: Competitive Co‐crystal Reactions in the Ball Mill

The driving forces triggering the formation of co‐crystals under milling conditions were investigated by using a set of multicomponent competitive milling reactions. In these reactions, different active pharmaceutical ingredients were ground together with a further compound acting as coformer. The study was based on new co‐crystals including the coformer anthranilic acid. The results of the competitive milling reactions indicate that the formation of co‐crystals driven by intermolecular recognition are influenced and inhibited by kinetic aspects including the formation of intermediates and the stability of the reactants.     

Simultaneous visualization of surface topography and concentration field by means of scanning electrochemical microscopy using a single electrochemical probe and impedance spectroscopy

Scanning electrochemical microscopy visualizes concentration profiles. To determine the location of the probe relative to topographical features of the substrate, knowledge of the probe-to-sample distance at each probe position is required. The use of electrochemical impedance spectroscopy for obtaining information on the substrate-to-probe distance and on the concentration of interest using the electrochemical probe alone is suggested. By tuning the frequencies of interrogation, the probe-to-substrate distance can be derived followed by interrogation of processes that carry information on concentration at lower frequencies. These processes may include charge-transfer relaxation, diffusional relaxation at the electrode, and open-circuit potential at zero frequency. A potentiometric chloride sensing microprobe is used herein to reconstruct both topography and the concentration field at a microscopic diffusional source of chloride.     

Advances in high throughput screening of gas sensing materials

The workflow of a high throughput screening setup for the rapid identification of new and improved gas sensor materials is presented. The polyol method was applied to prepare nanoparticulate metal oxides as base materials. These materials have been modified by surface and volume doping. Using multielectrode substrates and high throughput impedance spectroscopy (HT-IS) a wide range of materials could be screened on a short time scale. Selected examples reflect the state of the art for applying HT-IS in search of new selective gas sensing materials.     

Experimental study of stable and unstable regimes of ethanol-to-acetaldehyde oxidation on a copper catalyst in a flow reactor

The oxidation of ethanol to acetaldehyde by atmospheric oxygen on a stationary copper catalyst in a flow reactor is investigated. It is shown that by operating in unstable rather than stable regimes the specific efficiency of conversion w of ethanol to the target product — acetaldehyde — can be increased by 13%. Operating in unstable regimes makes it possible, without reducing w (as compared with the most efficient stable regime) to increase the total productivity of the reactor by a factor of 2.89.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 11–17, May–June, 1989.     

Study of the stable and unstable operating regimes of a chemical reactor

The problem of the stabilization of the unstable operating regimes of a flow reactor is considered. It is shown that for the same values of the external parameters up to five steady-state regimes may exist, and that without the use of stabilizing systems it is impossible to realize not only the unstable but also some stable regimes.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 54–59, May–June, 1988.     

Facile Synthesis and Theoretical Conformation Analysis of a Triazine‐Based Double‐Decker Rotor Molecule with Three Anthracene Blades

The facile synthesis of a rotor‐shaped compound with two stacked triazine units, which are symmetrically connected by three anthracene blades through oxygen linkers, is presented. This new double‐decker, which is a potential monomer for two‐dimensional polymerization, was synthesized by using readily available, cheap building blocks, exploiting the known selectivity difference for the nucleophilic substitution of cyanuric chloride. The crystal structure of a C_3h symmetric rotor‐shaped compound with 9,10‐dihydroanthracene blades, which is a direct precursor to the targeted monomer, and the crystal structure of the new double‐decker with the desired C_3h symmetry, are also reported. The synthetic efforts were preceded by a computational analysis, which was triggered by the question of conformational stability of the potential monomer. Two stable conformers could be found, and the barrier for the transition path in the gas phase between these conformers was determined by quantum chemical calculations. Exploratory Born–Oppenheimer molecular‐dynamics simulations revealed a strong influence of solvent–solute interactions on the stability of the conformers, which resulted in an energetic preference of the C_3h symmetric conformation of the double‐decker.     

Teachers’ views on effective mathematics teaching: commentaries from a European perspective

The previous papers of this issue discuss the views teachers from the United States, Australia, Hong Kong SAR, and Mainland China have on effective mathematics teaching and learning. Similarities and differences are found and a differentiation from West to East can be worked out in the order of the regions as listed above. The picture of teachers’ views can, however, be differentiated when they are looked at from a European perspective. On the basis of the analysis of two comparative studies on teaching cultures in three European countries and a questioning of teachers, this commentary therefore locates France, Germany, and England within this framework so that the East–West-contrast is distinguished in more detail.     

Experimental realization of unstable regimes of ethanol oxidation into acetaldehyde on a copper-oxide catalyst in a chemical flow reactor

Unstable regimes of operation of a chemical reactor have been stabilized in an experiment by means of a control system with feedback. It has been shown that the concentration limits of the region of multiplicity of reactor steady states do not in general coincide with the concentration limits that can be obtained without the use of stabilizing systems.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 2, pp. 8–13, March–April, 1984.     

Proton storage site in bacteriorhodopsin: new insights from quantum mechanics/molecular mechanics simulations of microscopic pK(a) and infrared spectra

Identifying the group that acts as the proton storage/loading site is a challenging but important problem for understanding the mechanism of proton pumping in biomolecular proton pumps, such as bacteriorhodopsin (bR) and cytochrome c oxidase. Recent experimental studies of bR propelled the idea that the proton storage/release group (PRG) in bR is not an amino acid but a water cluster embedded in the protein. We argue that this idea is at odds with our knowledge of protein electrostatics, since invoking the water cluster as the PRG would require the protein to raise the pK(a) of a hydronium by almost 11 pK(a) units, which is difficult considering known cases of pK(a) shifts in proteins. Our recent quantum mechanics/molecular mechanics (QM/MM) simulations suggested an alternative "intermolecular proton bond" model in which the stored proton is shared between two conserved Glu residues (194 and 204). Here we show that this model leads to microscopic pK(a) values consistent with available experimental data and the functional requirement of a PRG. Extensive QM/MM simulations also show that, independent of a number of technical issues, such as the influence of QM region size, starting X-ray structure, and nuclear quantum effects, the "intermolecular proton bond" model is qualitatively consistent with available spectroscopic data. Potential of mean force calculations show explicitly that the stored proton strongly prefers the pair of Glu residues over the water cluster. The results and analyses help highlight the importance of considering protein electrostatics and provide arguments for why the "intermolecular proton bond" model is likely applicable to the PRG in biomolecular proton pumps in general.