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971.
972.
The natural product lovastatin and its semisynthetic, more effective derivative, simvastatin, are important drugs for the treatment of hypercholesterolemia. Here, we report the biochemical characterization of a dedicated acyltransferase, LovD, encoded in the lovastatin biosynthetic pathway. We demonstrate that LovD has broad substrate specificity towards the acyl carrier, the acyl substrate, and the decalin acyl acceptor. LovD can efficiently catalyze the acyl transfer from coenzyme A thioesters or N-acetylcysteamine (SNAC) thioesters to monacolin J. When alpha-dimethylbutyryl-SNAC was used as the acyl donor, LovD was able to convert monacolin J and 6-hydroxyl-6-desmethylmonacolin J into simvastatin and huvastatin, respectively. Using the Escherichia coli LovD overexpression strain as a whole-cell biocatalyst, preparative amounts of simvastatin were synthesized in a single fermentation step. Our results demonstrate LovD is an attractive enzyme for engineered biosynthesis of pharmaceutically important cholesterol-lowering drugs. 相似文献
973.
The quenching rate constants of the excited triplet state of Rose Bengal (RB) by oxygen (k(obs)) were measured in ethylene glycol (EG) at different temperatures using nanosecond laser flash photolysis. Although a plot of the quenching rate constant k(obs) for RB triplet state vs oxygen concentration is linear at 20 degrees C, the oxygen dependence of k(obs) does not exhibit linearity but upward curvature at high temperatures from 130 to 140 degrees C. The upward curvature at high temperatures is not well-described by a kinetic scheme first postulated by Gijzeman et al., which is characterized by exciplex formation and a unimolecular dissociation of the exciplex to products, but instead by a more comprehensive mechanism involving a bimolecular dissociation in addition to a unimolecular one. The measurements of the oxygen dependence of k(obs) for RB triplet state at different temperatures yielded a reaction enthalpy for the exciplex formation of 150 kJ mol(-1). Due to the large exothermic reaction enthalpy, equilibrium was obtained for the exciplex at 20 degrees C even at low oxygen concentration and the bimolecular quenching by oxygen became the major dissociation process. The equilibrium attainment and bimolecular dissociation provide a linear oxygen dependence of k(obs) to all outward appearances. Therefore, linearity does not always mean that exciplex dissociation proceeds solely through a unimolecular mechanism. 相似文献
974.
A novel rhodium-catalyzed hydrophosphination of alkynes using a silylphosphine as a phosphino group source is described. A variety of alkynes, both terminal and internal ones with aryl, alkyl, and carboxyl groups, gave the corresponding alkenylphosphines in a highly regioselective and syn-selective manner. Alkenes with an electron-withdrawing group also gave the corresponding adducts in good yields. 相似文献
975.
We developed a sensitive analytical method and an efficient clean-up method to quantify 3,6-dinitrobenzo[e]pyrene (3,6-DNBeP) in surface soil and airborne particles. After purification using a silica gel column and two reversed-phase columns, 3,6-DNBeP was reduced to 3,6-diaminobenzo[e]pyrene by a catalyst column and analyzed by high-performance liquid chromatography (HPLC) with a fluorescence detector. 3,6-DNBeP was detected in all of the soil samples and airborne particles examined. The concentration of 3,6-DNBeP in surface soil and airborne particles was determined in the ranges of 347-5007 pg/g of soil and 137-1238 fg/m3, respectively. 相似文献
976.
The total evaporation technique of thermal ionization mass spectrometry was applied to the isotopic analysis of chromium. High measurement reproducibility of the chromium isotope ratios was verified (2 S.E. < 0.05% (53Cr/52Cr)), while a clear mass fractionation effect was observed by using conventional measurement technique. The chromium isotope ratios analyzed by the total evaporation method were not affected by the sample amount on the rhenium filament (50-500 ng Cr). The isotopic analysis under the coexistence of zinc was also performed, and its effect to the chromium isotope ratios was confirmed. 相似文献
977.
Ohkoshi S Ikeda S Hozumi T Kashiwagi T Hashimoto K 《Journal of the American Chemical Society》2006,128(16):5320-5321
A three-dimensional magnetic material [{CoII(pyrimidine)(H2O)}2{CoII(H2O)2}{WV(CN)8}2](pyrimidine)2. 2H2O is prepared. This compound exhibits a charge-transfer-induced spin transition with a large thermal hysteresis loop of 90 K. Irradiating with light causes the low-temperature phase to exhibit a spontaneous magnetization with a Curie temperature of 40 K and a magnetic hysteresis loop with a coercive field of 12 000 G, which is the highest value reported for a photomagnet. The observed photoinduced magnetization is due to the charge-transfer phase transition from the {CoIIhs(S = 3/2)}{CoIIIls(S = 0)}2-NC-{WIV(S = 0)}2 phase to the {CoIIhs(S = 3/2)}3-NC-{WV(S = 1/2)}2 phase by the irradiation. 相似文献
978.
Maeda Y Kanda M Hashimoto M Hasegawa T Kimura S Lian Y Wakahara T Akasaka T Kazaoui S Minami N Okazaki T Hayamizu Y Hata K Lu J Nagase S 《Journal of the American Chemical Society》2006,128(37):12239-12242
The dispersion of small-diameter single-walled carbon nanotubes (SWNTs) produced by the CoMoCAT method in tetrahydrofuran (THF) with the use of amine was studied. The absorption, photoluminescence, and Raman spectroscopies showed that the dispersion and centrifugation process leads to an effective separation of metallic SWNTs from semiconducting SWNTs. Since this method is simple and convenient, it is highly applicable to an industrial utilization for widespread applications of SWNTs. 相似文献
979.
Onuma K Watanabe A Kanzaki N Kubota T 《The journal of physical chemistry. B》2006,110(49):24876-24883
The intermolecular interaction and association dynamics of the Ynd1p protein were investigated using dynamic and time-resolved static light scattering measurements. The mutual diffusion coefficients of wild- and mutant-type (a single amino acid substitution) Ynd1p monomer were measured in 50 mM 2-(N-morpholino)ethanesulfonic acid (MES) buffer with 5 mM MnCl2 and 7.5% (v/v) ethylene glycol. Both translational diffusion coefficients at a zero protein concentration were (40.3 +/- 0.2) x 10(-12) m2/s at 20 degrees C and a pH of 7.0, so the hydrodynamic radius of the monomers was 4.1 +/- 0.1 nm. The measured intermolecular interaction between monomers, however, showed that the mutant-type Ynd1p had a stronger attractive force. Time-resolved static light scattering measurements showed that the association of mutant-type Ynd1p yielded a larger number of aggregates than that of wild-type Ynd1p. The time dependence of aggregate gyration radius differed between the two types. Fractal dimension analysis using scattering intensity data suggested that the inner structure of the aggregates changed from loose to rigid with time. Although this phenomenon is common for wild and mutant types, the differences in the number of aggregates yielded in the initial stages and in the intermolecular interaction affected the quality of the final grown crystals. That is, single crystals of Ynd1p grew in the mutant-type protein solution and polycrystals of Ynd1p grew in the wild-type protein solution. 相似文献
980.
Nakamura T Vinodkumar AM Sugimoto T Aoi N Baba H Bazin D Fukuda N Gomi T Hasegawa H Imai N Ishihara M Kobayashi T Kondo Y Kubo T Miura M Motobayashi T Otsu H Saito A Sakurai H Shimoura S Watanabe K Watanabe YX Yakushiji T Yanagisawa Y Yoneda K 《Physical review letters》2006,96(25):252502
An exclusive measurement has been made of the Coulomb dissociation of the two-neutron halo nucleus 11Li at 70 MeV/nucleon at RIKEN. Strong low-energy (soft) E1 excitation is observed, peaked at about Ex = 0.6 MeV with B(E1) = 1.42(18) e2fm2 for Erel < or = 3 MeV, which was largely missed in previous measurements. This excitation represents the strongest E1 transition ever observed at such low excitation energies. The spectrum is reproduced well by a three-body model with a strong two-neutron correlation, which is further supported by the E1 non-energy-weighted cluster sum rule. 相似文献