Shimalactone Biosynthesis Involves Spontaneous Double Bicyclo‐Ring Formation with 8π‐6π Electrocyclization

Shimalactones A and B are neuritogenic polyketides possessing characteristic oxabicyclo[2.2.1]heptane and bicyclo[4.2.0]octadiene ring systems that are produced by the marine fungus Emericella variecolor GF10. We identified a candidate biosynthetic gene cluster and conducted heterologous expression analysis. Expression of ShmA polyketide synthase in Aspergillus oryzae resulted in the production of preshimalactone. Aspergillus oryzae and Saccharomyces cerevisiae transformants expressing ShmA and ShmB produced shimalactones A and B, thus suggesting that the double bicyclo‐ring formation reactions proceed non‐enzymatically from preshimalactone epoxide. DFT calculations strongly support the idea that oxabicyclo‐ring formation and 8π‐6π electrocyclization proceed spontaneously after opening of the preshimalactone epoxide ring through protonation. We confirmed the formation of preshimalactone epoxide in vitro, followed by its non‐enzymatic conversion to shimalactones in the dark.     

Effect of quenching rate and annealing on ESR of Cu2+ and γ-induced Cd+ in low alkali borate glasses

A new ESR hyperfine structure (hfs) of Cu²⁺ was detected in moderately (cooling rate ≈ 3 K/s) and rapidly (10³–10⁵K/s) quenched borate glasses with Na₂O content smaller than 5 mol% in addition to the already reported three patterns, spectra I, II and III, for the moderately quenched borate glasses with 5 ? [Na₂O]?13, 20 ? [Na₂O] ?37 and 55 ? [Na₂O] ? 75, respectively. The ESR parameters of this spectrum were much the same as those of II, but a significant difference was observed between thermal stabilities of these spectra. The newly observed spectrum was named spectrum II′. A superposition of spectra II′ and I was seen in low alkali borate glasses ([Na₂O] ? 5 mol%). The relative intensity of II′ to I increased remarkably with increasing quenching rate of the melts and with decreasing alkali content under the same quenching condition. In addition, spectrum II′ was found to relax into I upon thermal annealing of the glasses. Similar changes were observed also in ESR of the γ-induced Cd⁺ with 5s¹ electronic configuration. A tentative model to account for the spectral changes was proposed on the basis of the characteristics of B—O bonding.     

Production of spin polarized 12N through heavy ion reactions

The structural and magnetic properties of Ho substituted BiFeO₃ (BHFO) have been investigated using ⁵⁷Fe Mössbauer spectroscopy and X-Ray diffraction (XRD) as a function of temperature. The Mössbauer spectrum obtained at room temperature for the as-synthesized BHFO sample exhibits broadened features due to the hyperfine field distributions related to the local variation of the neighbourhood of Fe and the magnetic hyperfine splitting patterns are indicative of magnetic ordering, mostly probably screwed or slightly antiferromagnetic. The spectrum was fitted with two superimposed asymmetric sextets, with similar hyperfine magnetic fields of B_hf1 = 48.0(1) T and B_hf2 = 49.0(1) T, corresponding to rhombohedral BFO. The hyperfine fields of the magnetic components decreased systematically with increasing temperature to a ‘field distribution’ just below the Néel temperature, T_N ~ 600 K. At temperatures above 600 K, the spectral line associated with the Bi₂₅FeO₄₀ impurity phase dominates the spectra. This phase is confirmed by XRD measurements. From the temperature dependence of the site populations of the spectral components an average Debye temperature of θ _D = 240(80) K has been estimated.     

Chemical shifts of metallic and non-metallic Al–Re–Si approximant crystals studied by EELS and SXES

Chemical shifts of approximant crystals of 1/0-Al₁₂Re (1/0-metallic), 1/1-Al₇₃Re₁₅Si₁₂ (1/1-metallic) and 1/1-Al₇₃Re₁₇Si₁₀ (1/1-non-metallic) were examined by using electron energy-loss spectroscopy (EELS) and soft-X-ray emission spectroscopy (SXES). Al L-shell excitation EELS spectra of these alloys showed an apparent chemical shift only for the 1/1-non-metallic alloy to the larger binding energy side by 0.2?eV. Al-Kα, Re-Mα and Si-Kα emission SXES spectra also showed a shift to the larger binding energy side only for 1/1-non-metallic alloy. 1/0-metallic and 1/1-metallic alloys did not show any chemical shift in EELS and SXES experiments. Chemical shifts were observed only in larger binding energy side compared with pure materials. This implies the decrease of valence charge at constituent atomic sites of 1/1-non-metallic alloy compared with 1/0-metallic, 1/1-metallic and pure materials. The decreased charges should distribute intermetallic sites, which should be related to a formation of covalent bonding among Al atomic sites reported by maximum-entropy method (MEM)/Rietveld analysis on this material. This relation between chemical shift and covalent bonding nature of this approximant alloy may support the presence of covalent bonding in Al-based quasicrystals.     

Study on unsteady molten insulation volume change during flame spreading over wire insulation in microgravity

Tests with flames spreading over wire insulation in microgravity were performed at varying external opposed flow conditions to examine the influence of flow velocity in the time dependent volume change of molten insulation. In the experiments, low density polyethylene insulated Nickel–chrome wire specimens were used and the oxygen concentration was fixed at 30% (N₂ balanced). The results show that the time dependent changes in molten insulation volume are related to the opposed flow velocity. Further, as opposed flow velocity increases, the volume change rate decreases monotonically. By subtracting the volume change rate from the volume supply rate from the solid part to the molten part, which is calculated by multiplying the rate of spreading of molten insulation at the leading edge by the cross sectional area of the insulation, a pyrolysis volume rate for the polyethylene was established. The pyrolysis volume rate is defined as the amount of consumed molten insulation volume per unit time. After these calculations, it was found that the pyrolysis volume rate increases monotonically with increases in the opposed flow velocity. Further, numerical calculations of time dependent volume change in the molten insulation at different flow velocities were made. The numerical results show good agreement with the experimental results of the molten insulation volume change during the 0–4.5 s of microgravity measured here. By using the numerical calculations for this initial short period, the time dependent volume change in molten insulation during longer-term microgravity is predicted. The calculated results show that the volume finally reaches a steady state value in flow velocities of 10–250 mm/s investigated here. These results provide insight into the mechanism of flame spreading over wire insulation, especially the unsteadiness of the flame in flame spreading events.     

Synthetic studies on thiostrepton family of peptide antibiotics: synthesis of the pentapeptide segment containing dihydroxyisoleucine, thiazoline and dehydroamino acid

The dihydroxyisoleucine-, thiazoline- and dehydroamino acid-containing pentapeptide of the thiostrepton family of peptide antibiotics was synthesized, which featured the β-lactone opening by phenylselenylation, the vinylzinc addition to the chiral sulfinimine, the Wipf oxazoline-thiazoline conversion method and the oxidative syn-elimination of the phenylseleno group.     

The Source of “Fairy Rings”: 2‐Azahypoxanthine and its Metabolite Found in a Novel Purine Metabolic Pathway in Plants

Rings or arcs of fungus‐stimulated plant growth occur worldwide; these are commonly referred to as “fairy rings”. In 2010, we discovered 2‐azahypoxanthine (AHX), a compound responsible for the fairy‐ring phenomenon caused by fungus; AHX stimulated the growth of all the plants tested. Herein, we reveal the isolation and structure determination of a common metabolite of AHX in plants, 2‐aza‐8‐oxohypoxanthine (AOH). AHX is chemically synthesized from 5‐aminoimidazole‐4‐carboxamide (AICA), and AHX can be converted into AOH by xanthine oxidase. AICA is one of the members of the purine metabolic pathway in animals, plants, and microorganisms. However, further metabolism of AICA remains elusive. Based on these results and facts, we hypothesized that plants themselves produce AHX and AOH through a pathway similar to the chemical synthesis. Herein, we demonstrate the existence of endogenous AHX and AOH and a novel purine pathway to produce them in plants.     

Redox‐Dependent Transformation of a Hydrazinobuckybowl between Curved and Planar Geometries

A red‐fluorescent heterobuckybowl with an embedded hydrazine structure was synthesized from a cyclobiphenothiazine derivative via a strained cyclobicarbazole. The hydrazinobuckybowl was found to possess bowl and twist structures in the neutral state, a shallow bowl structure in the monocation state, and a planar structure in the dication state by means of X‐ray crystallographic analysis, DFT calculations, and a comparison of experimental and calculated ¹³C NMR chemical shifts. The hydrazinobuckybowl is the first buckybowl that changes its geometry between curved bowl/twist structures and a planar structure depending on the oxidation state. The drastic geometrical change was possible as a result of the presence of two heteroatoms in the bowl skeleton and the multiple reversible redox reactions of the compound. Owing to the two kinds of bowl and twist conformations, the bowl‐inversion dynamics of the hydrazinobuckybowl were found to follow a triple‐well potential model.