A comparison of X-ray crystal structures including methyl 3,5-bis(hydroxymethyl)benzoate, its phenylethynyl extended derivative in polymorphous forms and the corresponding carboxylic acids

Considering the specific supramolecular synthon design of carboxylic acid and alcoholic hydroxyl groups in the field of crystal engineering, we compared the solid state structures of particular benzoates and corresponding acids 1–4 both in the non-spacered (1, 2) and spacered (3, 4) forms. Based on the single crystal X-ray study, there is only a slight influence of the phenylacetylene spacer with regard to the layer lattice arrangement of the benzoates while referring to the benzoic acid analogues the addition of the spacer gives rise to a modification of the lattice from a layer to a tape structure. Dependent on the crystallization conditions, two crystalline polymorphs of compound 3 (3a, 3b) were obtained and discussed regarding their structural differences.     

Pd(0)/Au(I) redox incompatibilities as revealed by Pd-catalyzed homo-coupling of arylgold(I)-complexes

A Pd(II)-catalyzed homo-coupling of Au(I)-aryls is reported. The reaction is driven by a Pd(0)/Au(I) redox reaction that generates a gold mirror and Pd(II), and illustrates one of the challenges for developing dual catalytic Au-Pd systems.     

Biosynthesis of the terpene phenalinolactone in Streptomyces sp. Tü6071: analysis of the gene cluster and generation of derivatives

Phenalinolactones are terpene glycosides with antibacterial activity. A striking structural feature is a highly oxidized gamma-butyrolactone of elusive biosynthetic origin. To investigate the genetic basis of the phenalinolactones biosynthesis, we cloned and sequenced the corresponding gene cluster from the producer strain Streptomyces sp. Tü6071. Spanning a 42 kbp region, 35 candidate genes could be assigned to putatively encode biosynthetic, regulatory, and resistance-conferring functions. Targeted gene inactivations were carried out to specifically manipulate the phenalinolactones pathway. The inactivation of a sugar methyltransferase gene and a cytochrome P450 monoxygenase gene led to the production of modified phenalinolactone derivatives. The inactivation of a Fe(II)/alpha-ketoglutarate-dependent dioxygenase gene disrupted the biosynthetic pathway within gamma-butyrolactone formation. The structure elucidation of the accumulating intermediate indicated that pyruvate is the biosynthetic precursor of the gamma butyrolactone moiety.     

Variation in the isotopic composition of zinc in the natural environment and the use of zinc isotopes in biogeosciences: a review

Zinc (Zn) is a trace element that is, as a building block in various enzymes, of vital importance for all living organisms. Zn concentrations are widely determined in dietary, biological and environmental studies. Recent papers report on the first efforts to use stable Zn isotopes in environmental studies, and initial results point to significant Zn isotope fractionation during various biological and chemical processes, and thus highlight their potential as valuable biogeochemical tracers. In this article, we discuss the state-of-the-art analytical methods for isotopic analysis of Zn and the procedures used to obtain accurate Zn isotope ratio results. We then review recent applications of Zn isotope measurements in environmental and life sciences, emphasizing the mechanisms and causes responsible for observed natural variation in the isotopic composition of Zn. We first discuss the Zn isotope variability in extraterrestrial and geological samples. We then focus on biological processes inducing Zn isotope fractionation in plants, animals and humans, and we assess the potential of Zn isotope ratio determination for elucidating sources of atmospheric particles and contamination. Finally, we discuss possible impediments and limitations of the application of Zn isotopes in (geo-) environmental studies and provide an outlook regarding future directions of Zn isotope research.     

Development of mullite and mullite/Al2O3 precursor sols for electrophoretic deposition of oxidation protection coatings

Mullite and mullite/Al₂O₃ precursor sols have been developed for the deposition of oxidation barrier coatings on carbon fibre reinforced composites using a combination of sol–gel synthesis and electrophoresis.The sols were synthesised by controlled hydrolysis and condensation of TEOS (tetraethoxysilane) and Al(OBu^s)₃ (aluminium tri-sec-butylate). The main objective was the definition of synthesis conditions which yield sols suitable for the electrophoretic deposition (EPD). Measurements of the Electrokinetic Sonic Amplitude were used to investigate the electrokinetic properties of the sols in the as-prepared state and depending on the later addition of H₂O. ²⁹Si CP/MAS NMR spectra of dried precursor samples were recorded to study the homogeneity of Al/Si mixture. The progress of crystallisation with increasing temperature of heat treatment was examined by XRD. Oxidation protection coatings on C/C–SiC composites were prepared by EPD.Whereas a low H₂O to TEOS ratio during the sol synthesis was advantageous for a low mullite formation temperature, a high H₂O to TEOS ratio was necessary to enable the EPD. The synthesis of a sol with a low H₂O to TEOS ratio in the first step and the later modification of this sol by the addition of water was a successful method to combine the required electrokinetic properties and mullitisation temperatures below 1200 °C.     

Zeit- und temperaturaufgelöste Röntgenpulverdiffraktometrie: Die Reaktion von (NH4)2SnF6 mit Ammoniak

Time and Temperature Resolved in situ X-Ray Powder Diffractometry. The Reaction of (NH₄)₂SnF₆ with Ammonia The thermal decomposition of (NH₄)₂SnF₆ under an atmosphere of ammonia is reported. The complicated reaction paths were illucidated by time and temperature resolved in situ x-ray powder diffractometry. It is shown that this technique is a powerful tool to observe structural changes during reaction. It offers also a valuable access to thermodynamic and kinetic data for solid state and gas phase reactions. (NH₄)₂SnF₆ decomposes under ammonia below room temperature to NH₄F and amorphous SnF₄ · x NH₃. At a temperature of 80°C an intermediate product, (NH₄)₄SnF₈, is formed, which decomposes at 140°C into (NH₄)₂SnF₆ and NH₄F. At 250°C (NH₄)[Sn(NH₃)F₅] and Sn(NH₃)₂F₄ are formed. The latter crystallises C-centered monoclinic with lattice constants a = 844.1(5) pm, b = 630.5(3) pm, c = 520.2(3) pm and b? = 114.02(7)°. At 330°C a further decomposition yields SnF₂(NH₂)₂ with a C-centered monoclinic cell and lattice constants a = 1 069(7), b = 325.3(2), c = 504.8(3) pm and b? = 105.83(7)°. Finally above 500°C tin metal is formed.     

Excited quartet and doublet states in the complex of tetraphenylporphine zinc (II) and a nitroxide radical in solution: X- and W-band time-resolved EPR studies

The photoexcited quartet (Q₁) and doublet (D₁) states of the complex of tetra-phenyl-porphine zinc (II) and a nitroxide radical have been studied in toluene solution by X-(9.4 GHz) and W-band (94 GHz) time-resolved electron paramagnetic resonance spectroscopy. The spin-polarized signals of the Q₁ and D₁ states are observed and assigned by analysis of theg-values. Line broadening and spin polarization mechanisms in this system are discussed.     

Metabolic and Isotopic Correlations between D-Glucose,L-Ascorbic Acid and L-Tartaric Acid

Abstract

L-Ascorbic acid in fruit juices is enriched in ¹³C by 4.8 δ-units relative to its precursor glucose, and this enrichment is mainly located in position C-1. On the other hand, biotechnologically produced L-ascorbic acid preserves the pattern as expected from glucose. In grapes L-tartaric acid is synthesized from glucose via L-ascorbic acid and it is one of its main products. It preserves C-atoms 1 to 4 from glucose and the measured δ¹³C-value is identical to that calculated from this precursor.

The findings are interpreted as overlapping influences of isotope discriminations after metabolic branching and relative turnover rates leading to isotopic balances. The results can also be used to distinguish between identical products by different synthetic processes.     

Strong correlations in the He ground state momentum wave function observed in the fully differential momentum distributions for the p + He transfer ionization process

The four-particle process of proton-helium transfer ionization has been studied using cold target recoil ion momentum spectroscopy to measure the momenta of all three particles in the final state. Most of the electrons are emitted in the H0 scattering plane and in the backward direction. The final state momentum distributions show discrete structures very different from those expected for uncorrelated capture and ionization. The measured momentum pattern is interpreted to be due to a new transfer ionization reaction channel which results from strong correlations in the initial He ground state momentum wave function.     

Anomalous elastic response of silicon to uniaxial shock compression on nanosecond time scales

We have used x-ray diffraction with subnanosecond temporal resolution to measure the lattice parameters of orthogonal planes in shock compressed single crystals of silicon (Si) and copper (Cu). Despite uniaxial compression along the (400) direction of Si reducing the lattice spacing by nearly 11%, no observable changes occur in planes with normals orthogonal to the shock propagation direction. In contrast, shocked Cu shows prompt hydrostaticlike compression. These results are consistent with simple estimates of plastic strain rates based on dislocation velocity data.