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41.
Vicenistatin, an antitumor antibiotic isolated from Streptomyces halstedii, is a unique 20-membered macrocyclic lactam with a novel aminosugar vicenisamine. The vicenistatin biosynthetic gene cluster (vin) spanning approximately 64 kbp was cloned and sequenced. The cluster contains putative genes for the aglycon biosynthesis including four modular polyketide synthases (PKSs), glutamate mutase, acyl CoA-ligase, and AMP-ligase. Also found in the cluster are genes of NDP-hexose 4,6-dehydratase and aminotransferase for vicenisamine biosynthesis. For the functional confirmation of the cluster, a putative glycosyltransferase gene product, VinC, was heterologously expressed, and the vicenisamine transfer reaction to the aglycon was chemically proved. A unique feature of the vicenistatin PKS is that the loading module contains only an acyl carrier protein domain, in contrast to other known PKS-loading modules containing certain activation domains. Activation of the starter acyl group by separate polypeptides is postulated as well. 相似文献
42.
The growth mechanism of water clusters in carbon nanopores is clearly elucidated by in situ small-angle X-ray scattering (SAXS) studies and grand canonical Monte Carlo (GCMC) simulations at 293-313 K. Water molecules are isolated from each other in hydrophobic nanopores below relative pressures (P/P(0)) of 0.5. Water molecules associate with each other to form clusters of about 0.6 nm in size at P/P(0)=0.6, accompanied by a remarkable aggregation of these clusters. The complete filling of carbon nanopores finishes at about P/P(0)=0.8. The correlation length analysis of SAXS profiles leads to the proposal of a growth mechanism for these water clusters and the presence of the critical cluster size of 0.6 nm leads to extremely stable clusters of water molecules in hydrophobic nanopores. Once a cluster of the critical size is formed in hydrophobic nanopores, the predominant water adsorption begins to fill carbon nanopores. 相似文献
43.
Naoki Haga Hiroaki Takayanagi Haruo Ogura Yasunao Kuriyama Katsumi Tokumaru 《Photochemistry and photobiology》1995,61(6):557-562
The mechanism of photocycloaddition of 2′-deoxyuridine (1a) and thymidine (1b) to 2,3-dimethyl-2-butene (Bu) in acetonitrile by UV irradiation has been studied. The reciprocal quantum yield for the cycloaddition increased linearly with reciprocal concentrations of Bu in acetonitrile to give limiting quantum yields at infinite concentration of Bu as 0.030 and 0.0096 for 1a and 1b , respectively. This shows that the cycloaddition proceeds in a two-step mechanism between the triplet state of 1 and Bu through biradical intermediates. Addition of cis-1,3-pentadiene quenched the reaction obeying the Stern–Volmer equation. The above quenching experiments and laser transient spectroscopy revealed that the triplet state of 1a reacts with Bu with much larger rate constant (1.3–1.6 × 109 M?1 s?1) than that of 1b (4–5 × 107 M?1 s?1) reflecting larger steric hindrance exerted in the reaction of 1b than that of 1a . 相似文献
44.
The enzymatic resolution products [(1R,4aR,8aR)-1,2,3,4,4a,5,6,7,8,8a-decahydro-5,5,8a-trimethyl-2-oxo-trans-naphthalene-1-methanol-2-ethylene acetal (8aR)-7 (98% ee) and {acetate of (1S,4aS,8aS)-1,2,3,4,4a,5,6,7,8,8a-decahydro-5,5,8a-trimethyl-2-oxo-trans-naphthalene-1-methanol-2-ethylene acetal} (8aS)-9 (>99% ee)] obtained by the lipase-catalyzed enantioselective acetylation of (±)-7 in the presence of vinyl acetate as an acyl donor were converted to the ,β-unsaturated ketones (8aR)-6 and (8aS)-6, respectively. Concise syntheses of (+)-totarol 1, (+)-podototarin 2 and (+)-sempervirol 3 were achieved based on Michael reactions between (8aS)-6 and the appropriate β-keto ester followed by aldol condensation. The first chiral syntheses of (+)-jolkinolides E 4 and D 5 were achieved from (5R,10R,12R)-12-hydroxypodocarpa-8(14)-en-13-one 15 derived from (8aR)-6. 相似文献
45.
Ryugo S. HAYANO Masaharu TSUBOKURA Makoto MIYAZAKI Hideo SATOU Katsumi SATO Shin MASAKI Yu SAKUMA 《Proceedings of the Japan Academy. Series B, Physical and biological sciences》2013,89(4):157-163
The Fukushima Dai-ichi NPP accident contaminated the soil of densely-populated regions in Fukushima Prefecture with radioactive cesium, which poses significant risks of internal and external exposure to the residents. If we apply the knowledge of post-Chernobyl accident studies, internal exposures in excess of a few mSv/y would be expected to be frequent in Fukushima.Extensive whole-body-counter surveys (n = 32,811) carried out at the Hirata Central Hospital between October, 2011 and November, 2012, however show that the internal exposure levels of residents are much lower than estimated. In particular, the first sampling-bias-free assessment of the internal exposure of children in the town of Miharu, Fukushima, shows that the 137Cs body burdens of all children (n = 1,383, ages 6–15, covering 95% of children enrolled in town-operated schools) were below the detection limit of 300 Bq/body in the fall of 2012. These results are not conclusive for the prefecture as a whole, but are consistent with results obtained from other municipalities in the prefecture, and with prefectural data. 相似文献
46.
Kentaro Mishiba Dr. Yuya Tanaka Prof. Dr. Munetaka Akita 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(17):5432-5438
Herein, we report the synthesis and electrochemical and photophysical properties of aromatic hydrocarbons having one or two dimesitylborylethynyl peripherals. The mono- ( 1 ) and diboryl compounds ( 2 ), readily prepared by nucleophilic substitution reaction, are fairly stable to air and moisture in the solid state. The inserted ethynediyl (C≡C) spacer cancels the steric hindrance between the bulky dimesitylboryl groups and aromatic rings, leading to effective π conjugation over the B−C≡C−Ar linkages, as revealed by cyclic voltammetry. Despite the small structural differences, the photophysical properties of the benzene, naphthalene, and anthracene derivatives are different. Virtually no emission was observed from the benzene derivatives, whereas the anthracene derivatives emitted with high quantum yields both in solution and in the solid state. Notably, the naphthalene derivatives showed aggregation-induced emission behavior. Unlike the common sterically congested triarylborane derivatives reported so far, the anthracene derivatives showed π–π*-type absorption and emission bands, which derive from efficient intramolecular orbital interactions between the boron centers and anthracene moieties, as supported by DFT calculations. As a result, the dimesitylborylethynyl substituents effectively lower the LUMO levels of the aromatic hydrocarbon parts, whereas the HOMO levels are almost unaffected, thereby leading to materials with controllable HOMO–LUMO gaps. 相似文献
47.
Atsushi Yashiro Dr. Yuya Tanaka Prof. Tomofumi Tada Prof. Shintaro Fujii Prof. Tomoaki Nishino Prof. Munetaka Akita 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(37):9666-9673
In this work, the design, synthesis, and single-molecule conductance of ethynyl- and butadiynyl-ruthenium molecular wires with thioether anchor groups [RS=n-C6H13S, p-tert-Bu−C6H4S), trans-{RS−(C≡C)n}2Ru(dppe)2 (n=1 ( 1R ), 2 ( 2R ); dppe: 1,2-bis(diphenylphosphino)ethane) and trans-(n-C6H13S−C≡C)2Ru{P(OMe)3}4 3hex ] are reported. Scanning tunneling microscope break-junction study has revealed conductance of the organometallic molecular wires with the thioacetylene backbones higher than that of the related organometallic wires having arylethynylruthenium linkages with the sulfur anchor groups, trans-{p-MeS−C6H4-(C≡C)n}2Ru(phosphine)4 4 n (n=1, 2) and trans-(Th−C≡C)2Ru(phosphine)4 5 (Th=3-thienyl). It should be noted that the molecular junctions constructed from the butadiynyl wire 2R , trans-{ Au −RS−(C≡C)2}2Ru(dppe)2 ( Au : gold metal electrode), show conductance comparable to that of the covalently linked polyynyl wire with the similar molecular length, trans-{ Au −(C≡C)3}2Ru(dppe)2 63 . The DFT non-equilibrium Green's function (NEGF) study supports the highly conducting nature of the thioacetylene molecular wires through HOMO orbitals. 相似文献
48.
Katsumi Katoh Seiichiro Nagayama Eiko Higashi Katsuyuki Nakano 《Journal of Thermal Analysis and Calorimetry》2013,113(3):1239-1244
In the hydrosilylation of 1,6-divinyl(perfluorohexane) (FDV) with trichlorosilane (TCS) in the presence of catalytic chloroplatinic acid (Pt-Cat) under an air atmosphere (0.99 MPa), a runaway reaction accompanied by a severe pressure release occurred when Fe2O3 was present as an impurity in the system. In this study, we investigated the mechanism of action of Fe2O3 on this hydrosilylation by monitoring the thermal behavior of TCS/FDV/Pt-Cat/Fe2O3 mixtures with various compositions, using an accelerating rate calorimeter (ARC). In the case of TSC/FDV/Pt-Cat, a typical hydrosilylation composition in the industrial process, heat release, possibly due to hydrosilylation, began at 90 °C. On the other hand, for TCS/FDV/Pt-Cat/Fe2O, the heat release due to hydrosilylation was hardly observed, but abrupt heat and pressure releases occurred at higher temperatures (>170 °C). Like TCS/FDV/Pt-Cat/Fe2O3, TCS/FDV, which contain neither Pt-Cat nor Fe2O3, released heat and pressure at high temperatures (>210 °C), while the heat and pressure release rates were comparatively low. From these results, the runaway reaction may occur when hydrosilylation is prevented, and Fe2O3 behaves as a negative catalyst for hydrosilylation. In the FT-IR spectrum of TCS/FDV/Pt-Cat/Fe2O3 after heating, an absorption peak at approximately 1,710 cm?1, which may be attributed to a carbonyl group, was observed. Thus, it is considered that the runaway reaction observed during the hydrosilylation results from the action of Fe2O3 as a negative catalyst for hydrosilylation as well as as an oxidation catalyst for the by-product generated from the reaction between TCS and FDV. 相似文献
49.
Katsumi Katoh Nobuyuki Saeki Eiko Higashi Yuki Hirose Masahiko Sugimoto Katsuyuki Nakano 《Journal of Thermal Analysis and Calorimetry》2013,113(3):1275-1279
As a fundamental study on the development of dismantlable adhesives containing chemically reactive materials, the thermal behavior and dismantlability of an epoxy adhesive containing one of the twenty-seven inorganic salts (chlorides, perchlorates, and nitrates) were observed. In the thermal behavior measured by the differential scanning calorimetry, epoxy adhesives with inorganic salts containing iron, copper, zinc, and aluminum cations released heats of reaction at lower temperatures than the adhesive alone or the adhesives with other inorganic salts. Since such inorganic salts were considered to be effective candidates as fillers in dismantlable adhesives, the adhesion strengths of their mixtures with the adhesive were observed after heat aging at 270 °C for 30 min. The results showed that both chloride and perchlorate salts specifically decreased the adhesion strength after heating. On the other hand, the effect of nitrate salts on the decrease in adhesion strength was low in comparison with the chloride and perchlorate salts. 相似文献
50.
Reductive dehalogenation of aryl halides was achieved using hydrogen and a palladium catalyst. By using deuterium gas, the deuterated arenes were readily prepared. 相似文献