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Yeonjin Ko Mark W. Ruszczycky Sei‐Hyun Choi Prof. Dr. Hung‐wen Liu 《Angewandte Chemie (International ed. in English)》2015,54(3):860-863
DesII is a radical S‐adenosylmethionine (SAM) enzyme that catalyzes the C4‐deamination of TDP‐4‐amino‐4,6‐dideoxyglucose through a C3 radical intermediate. However, if the C4 amino group is replaced with a hydroxy group (to give TDP‐quinovose), the hydroxy group at C3 is oxidized to a ketone with no C4‐dehydration. It is hypothesized that hyperconjugation between the C4 C? N/O bond and the partially filled p orbital at C3 of the radical intermediate modulates the degree to which elimination competes with dehydrogenation. To investigate this hypothesis, the reaction of DesII with the C4‐epimer of TDP‐quinovose (TDP‐fucose) was examined. The reaction primarily results in the formation of TDP‐6‐deoxygulose and likely regeneration of TDP‐fucose. The remainder of the substrate radical partitions roughly equally between C3‐dehydrogenation and C4‐dehydration. Thus, changing the stereochemistry at C4 permits a more balanced competition between elimination and dehydrogenation. 相似文献
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Zero-valent palladium precatalysts containing rigid bidentate bis(arylimino)acenaphthene ligands (shown schematically) facilitate the highly stereoselective homogeneous catalytic hydrogenation of alkynes to (Z)-alkenes. Internal, terminal, aryl-substituted, and cyclic alkynes are suitable substrates, as are some enynes, which are chemoselectively hydrogenated to dienes. E=CO(2)Me; R(1), R(2)=4-OCH(3), 4-CH(3), 2,6-(CH(3))(2). 相似文献
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The sensitivity of all ion trap mass spectrometry (ITMS) methods is dependent on the trapping efficiency of the instrument. For ITMS instruments utilizing external ion sources, such as laser desorption, trapping efficiency is known to depend on the phase and amplitude of the radio frequency (RF) potential applied to the ring electrode at the time of ion introduction. It is remarkable that, in a considerable body of literature, no consensus exists regarding the effects of these parameters on the efficacy of trapping externally generated ions. In this paper, a summary of the literature is presented in order to highlight significant discrepancies. New laser desorption ion trap mass spectrometry (LD-ITMS) data are also presented, from which conclusions are drawn in our effort to clarify some of the confusion. Copyright 1999 John Wiley & Sons, Ltd. 相似文献
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The design, fabrication, and testing of photoelastic models of double-lap, multiple-pin connectors are discussed. Interest
is in the stresses in the inner laps. These stresses are determined by constructing models with photoelastic inner laps and
transparent-acrylic outer laps. The connectors have two pins, in tandem, parallel to the load direction. A photoelastic-isotropic
point is shown to permit the evaluation of load sharing between the two pins. A numerical scheme, utilizing the isochromatic-
and isoclinic-photoelastic data and a finite-difference representation of the planestress equilibrium equations, is used to
compute the stresses around the two pins. Representative stress distributions and stress-concentration factors are shown. 相似文献
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Choi SH Ruszczycky MW Zhang H Liu HW 《Chemical communications (Cambridge, England)》2011,47(36):10130-10132
UDP-2F-glucuronic acid was synthesized and analyzed as a mechanistic probe to investigate the ring contraction step catalyzed by UDP-d-apiose/UDP-d-xylose synthase (AXS). 相似文献
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Ruszczycky MW Choi SH Mansoorabadi SO Liu HW 《Journal of the American Chemical Society》2011,133(19):7292-7295
DesII, a radical S-adenosyl-l-methionine (SAM) enzyme from Streptomyces venezuelae, catalyzes the deamination of TDP-4-amino-4,6-dideoxy-D-glucose to TDP-3-keto-4,6-dideoxy-D-glucose in the desosamine biosynthetic pathway. DesII can also catalyze the dehydrogenation of TDP-D-quinovose to the corresponding 3-keto sugar. Similar to other radical SAM enzymes, DesII catalysis has been proposed to proceed via a radical mechanism. This hypothesis is now confirmed by EPR spectroscopy with the detection of a TDP-D-quinovose radical intermediate having a g-value of 2.0025 with hyperfine coupling to two spin 1/2 nuclei, each with a splitting constant of 33.6 G. A significant decrease in the EPR line width is observed when the radical is generated in reactions conducted in D(2)O versus H(2)O. These results are consistent with a C3 α-hydroxyalkyl radical in which the p-orbital harboring the unpaired electron spin at C3 is periplanar with the C-H bonds at both C2 and C4. 相似文献
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