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  • Authors: Nicholas H. Williams; and Anne-Marie Lebuis; Jik Chin;

    Hydrolysis of four substituted phenyl phosphate monoesters, each coordinated to a dinuclear Co(III) complex, was studied ([Co2(tacn)2(OH)2{O3P(OAr)}]2+; tacn = 1,4,7-triazacyclononane; substituent ...

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  • Authors: Pavel I. Kitov; Hiroki Shimizu; Steven W. Homans; David R. Bundle;

    A series of bivalent ligands for a Shiga-like toxin have been synthesized, their experimentally determined inhibitory activities were compared with a simplified thermodynamic model, and computer simulations were used to predict the optimal tether length in bivalent ligands. The design of the inhibitors exploits the proximity of the C-2' hydroxyl groups of two P(k)-trisaccharides when bound to two different, neighboring carbohydrate recognizing binding sites located on the surface of Shiga-like toxin. NMR studies of the complex between the toxin and bivalent ligands show that site 2 and site 1 of a single B subunit are simultaneously occupied by a tethered P(k)-trisaccharide dimer. A simplified thermodynamic treatment provides the intrinsic affinities and binding energies for the intermolecular and intramolecular association events and permits the deconvolution of the contributions to the relative binding energies for the set of bivalent ligands. Conformational analysis based on MD simulations for bivalent galabioside dimers containing different tethers demonstrated that the calculated local concentrations of the pendant ligand at the second binding site correlate with the experimentally determined relative affinity values of the respective bivalent ligands, thereby providing a predictive method to optimize tether length.

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  • Authors: Aaron C. Kinsman; Michael A. Kerr;

    The total synthesis of (+)-hapalindole Q has been achieved. The key step is a Diels-Alder reaction mediated by MacMillan's organocatalyst to provide the critical intermediate with high enantioselectivity (93% ee). This step establishes the proper arrangement of the required four contiguous stereocenters, including the quaternary center, and represents the first successful application of an enantioselective organomediated Diels-Alder reaction in total synthesis.

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  • Authors: Katerina Krumova; Lana E. Greene; Gonzalo Cosa;

    We report here the preparation of a lipophilic fluorogenic antioxidant (Mito-Bodipy-TOH) that targets the inner mitochondrial lipid membrane (IMM) and is sensitive to the presence of lipid peroxyl radicals, effective chain carriers in the lipid chain autoxidation. Mito-Bodipy-TOH enables monitoring of the antioxidant status, i.e., the antioxidant load and ability to prevent lipid chain autoxidation, within the inner mitochondrial membrane of live cells. The new probe consists of 3 segments: a receptor, a reporter, and a mitochondria-targeting element, constructed, respectively, from an α-tocopherol-like chromanol moiety, a BODIPY fluorophore, and a triphenylphosphonium cation (TPP). The chromanol moiety ensures reactivity akin to that of α-tocopherol, the most potent naturally occurring lipid soluble antioxidant, while the BODIPY fluorophore and TPP ensure partitioning within the inner mitochondrial membrane. Mechanistic studies conducted either in homogeneous solution or in liposomes and in the presence of free radical initiators show that the antioxidant activity of Mito-Bodipy-TOH is on par with that of α-tocopherol. Studies conducted on live fibroblast cells further show the antioxidant depletion in the presence of methyl viologen (paraquat), a known agent of oxidative stress and source of superoxide radical anion (and indirectly, a causative of lipid peroxidation) within the mitochondria matrix. We recorded a ca. 8-fold emission enhancement with Mito-Bodipy-TOH in cells stressed with methyl viologen, whereas no enhancement was observed in control studies with untreated cells. Our findings underscore the potential of the new fluorogenic antioxidant Mito-Bodipy-TOH to study the chemical link between antioxidant load, lipid peroxidation and mitochondrial physiology.

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  • Authors: Anna Lee; Neil Coombs; Ilya Gourevich; Eugenia Kumacheva; +1 Authors

    We report the solution-phase formation of ordered lamellar nanocrystal (NC) arrays. These semiconductor lamellae exhibit structural integrity and temporal stability, without recourse to chemical cross-linking. They are many micrometers in diameter yet only two or three nanocrystal layers in thickness. We show that these structures can integrate a cargo of water-soluble ions, molecules, metal nanoparticles, or biomolecules. Encapsulation of gold nanoparticles is found to cause photoluminescence enhancement of the host CdSe nanocrystals. These easily prepared nanocrystal "envelopes" promise properties modulated through the integration of a vast array of water-soluble species. Our approach marks a step toward ordered, compartmentalized, NC-based complexes with controlled architectures.

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  • Authors: Eric Fillion; Ashraf Wilsily;

    The asymmetric synthesis of all-carbon benzylic quaternary stereocenters has been successfully achieved through copper-catalyzed addition of dialkylzinc reagents to 5-(1-arylalkylidene) and 5-(dihydroindenylidene) Meldrum's acids in the presence of phosphoramidite ligand. The resulting benzyl-substituted Meldrum's acids and 1,1-disubstituted indanes were obtained in good yields and up to 99% ee. The significance of substituting the position para, meta, and ortho to the electrophilic benzylic center was highlighted. A benzyl Meldrum's acid product was further transformed to a 3,3-disubstituted 1-indanone and a beta,beta-disubstituted pentanoic acid.

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  • image/svg+xml art designer at PLoS, modified by Wikipedia users Nina, Beao, JakobVoss, and AnonMoos Open Access logo, converted into svg, designed by PLoS. This version with transparent background. http://commons.wikimedia.org/wiki/File:Open_Access_logo_PLoS_white.svg art designer at PLoS, modified by Wikipedia users Nina, Beao, JakobVoss, and AnonMoos http://www.plos.org/
    Authors: Oliver Schalk; Andrey E. Boguslavskiy; Albert Stolow; Michael S. Schuurman;

    The influence of through-bond interactions on nonadiabatic excited-state dynamics is investigated by time-resolved photoelectron spectroscopy (TRPES) and ab initio computation. We compare the dynamics of cyclohexa-1,4-diene, which exhibits a through-bond interaction known as homoconjugation (the electronic correlation between nonconjugated double bonds), with the nonconjugated cyclohexene. Each molecule was initially excited to a 3s Rydberg state using a 200 nm femtosecond pump pulse. The TRPES spectra of these molecules display similar structure and time constants on a subpicosecond time scale. Our ab initio calculations show that similar sets of conical intersections (a [1,2]- and [1,3]-hydrogen shift, as well as carbon–carbon bond cleavage) are energetically accessible to both molecules and that the geometry and orbital composition at the minimum energy crossing points to the ground state are directly analogous. These experimental and computational results suggest that the excited-state dynamics of cyclohexa-1,4-diene become localized at a single double bond and that the effects of through-bond interaction, dominant in the absorption spectrum, are absent in the excited-state dynamics. The notion of excited-state dynamics being localized at specific sites within the nuclear framework is analogous to the localization of light absorption by a subsystem within the molecule, designated a chromophore. We propose the utility of the analogous concept, denoted here as a dynamophore.

    image/svg+xml art designer at PLoS, modified by Wikipedia users Nina, Beao, JakobVoss, and AnonMoos Open Access logo, converted into svg, designed by PLoS. This version with transparent background. http://commons.wikimedia.org/wiki/File:Open_Access_logo_PLoS_white.svg art designer at PLoS, modified by Wikipedia users Nina, Beao, JakobVoss, and AnonMoos http://www.plos.org/ Journal of the Ameri...arrow_drop_down
    image/svg+xml art designer at PLoS, modified by Wikipedia users Nina, Beao, JakobVoss, and AnonMoos Open Access logo, converted into svg, designed by PLoS. This version with transparent background. http://commons.wikimedia.org/wiki/File:Open_Access_logo_PLoS_white.svg art designer at PLoS, modified by Wikipedia users Nina, Beao, JakobVoss, and AnonMoos http://www.plos.org/
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      image/svg+xml art designer at PLoS, modified by Wikipedia users Nina, Beao, JakobVoss, and AnonMoos Open Access logo, converted into svg, designed by PLoS. This version with transparent background. http://commons.wikimedia.org/wiki/File:Open_Access_logo_PLoS_white.svg art designer at PLoS, modified by Wikipedia users Nina, Beao, JakobVoss, and AnonMoos http://www.plos.org/ Journal of the Ameri...arrow_drop_down
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  • Authors: Jefferson Y. Chan; April Lu; Andrew J. Bennet;

    A panel of seven isotopically substituted sialoside natural substrate analogues based on the core structure 7-(5-acetamido-3,5-dideoxy-d-glycero-α-d-galacto-non-2-ulopyranosylonic acid)-(2→6)-β-D-galactopyranosyloxy)-8-fluoro-4-methylcoumarin (1, Neu5Acα2,6GalβFMU) have been synthesized and used to probe the rate-limiting step for turnover by the M. viridifaciens sialidase. The derived kinetic isotope effects (KIEs) on k(cat) for the ring oxygen ((18)V), leaving group oxygen ((18)V), anomeric carbon ((13)V), C3-carbon ((13)V), C3-R deuterium ((D)V(R)), C3-S deuterium ((D)V(S)), and C3-dideuterium ((D)(2)V) are 0.986 ± 0.003, 1.003 ± 0.005, 1.021 ± 0.006, 1.001 ± 0.008, 1.029 ± 0.007, 0.891 ± 0.008, and 0.890 ± 0.006, respectively. The solvent deuterium KIE ((D(2)O)V) for the sialidase-catalyzed hydrolysis of 1 is 1.585 ± 0.004. In addition, a linear proton inventory was measured for the rate of hydrolysis, under saturating condition, as a function of n, the fraction of deuterium in the solvent. These KIEs are compatible with rate-determining cleavage of the enzymatic tyrosinyl β-sialoside intermediate. Moreover, the secondary deuterium KIEs are consistent with the accumulating Michaelis complex in which the sialosyl ring of the carbohydrate substrate is in a (6)S(2) skew boat conformation. These KIE measurements are also consistent with the rate-determining deglycosylation reaction occurring via an exploded transition state in which synchronous charge delocalization is occurring onto the ring oxygen atom. Finally, the proton inventory and the magnitude of the solvent KIE are consistent with deglycosylation involving general acid-catalyzed protonation of the departing tyrosine residue rather than general base-assisted attack of the nucleophilic water.

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  • image/svg+xml Jakob Voss, based on art designer at PLoS, modified by Wikipedia users Nina and Beao Closed Access logo, derived from PLoS Open Access logo. This version with transparent background. http://commons.wikimedia.org/wiki/File:Closed_Access_logo_transparent.svg Jakob Voss, based on art designer at PLoS, modified by Wikipedia users Nina and Beao
    Authors: Robert W. Redmond; Juan C. Scaiano; Linda J. Johnston;

    Two-color, two-laser flash photolysis has been employed to carry out a structure-reactivity investigation onexcited diphenylketyl radicals. The effects of varying substitution at the 4 and 4' positions on the probabilities of radiative, nonradiative, and photobleaching processes have been elucidated in a quantitative manner. Rate constants and quantum yields have been measured for all of the above processes, allowing the complete relaxation schemes for these radicals to be obtained

    image/svg+xml Jakob Voss, based on art designer at PLoS, modified by Wikipedia users Nina and Beao Closed Access logo, derived from PLoS Open Access logo. This version with transparent background. http://commons.wikimedia.org/wiki/File:Closed_Access_logo_transparent.svg Jakob Voss, based on art designer at PLoS, modified by Wikipedia users Nina and Beao NRC Publications Arc...arrow_drop_down
    image/svg+xml Jakob Voss, based on art designer at PLoS, modified by Wikipedia users Nina and Beao Closed Access logo, derived from PLoS Open Access logo. This version with transparent background. http://commons.wikimedia.org/wiki/File:Closed_Access_logo_transparent.svg Jakob Voss, based on art designer at PLoS, modified by Wikipedia users Nina and Beao
    NRC Publications Archive
    Other literature type . 1992
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      Other literature type . 1992
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  • Authors: Seung Seo Lee; Shukun Yu; Stephen G. Withers;

    alpha-Glucan lyase (EC 4.2.2.13, GLase) cleaves alpha-1,4-glucosidic bonds via an elimination reaction to produce 1,5-d-anhydrofructose. GLase was inactivated by the mechanism based alpha-glucosidase inactivator, 5-fluoro-beta-l-idosyl fluoride. The trapped glycosyl-enzyme intermediate was isolated and the nucleophilic amino acid residue (Asp 553) identified is equivalent to the residue so identified in sequence-related alpha-glucosidases. This intermediate undergoes a syn-elimination reaction to release the product. Further evidence against a direct trans-elimination mechanism was provided by the absence of a primary kinetic isotope effect on the substrate substituted with deuterium at the C2 position.

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  • Authors: Nicholas H. Williams; and Anne-Marie Lebuis; Jik Chin;

    Hydrolysis of four substituted phenyl phosphate monoesters, each coordinated to a dinuclear Co(III) complex, was studied ([Co2(tacn)2(OH)2{O3P(OAr)}]2+; tacn = 1,4,7-triazacyclononane; substituent ...

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  • Authors: Pavel I. Kitov; Hiroki Shimizu; Steven W. Homans; David R. Bundle;

    A series of bivalent ligands for a Shiga-like toxin have been synthesized, their experimentally determined inhibitory activities were compared with a simplified thermodynamic model, and computer simulations were used to predict the optimal tether length in bivalent ligands. The design of the inhibitors exploits the proximity of the C-2' hydroxyl groups of two P(k)-trisaccharides when bound to two different, neighboring carbohydrate recognizing binding sites located on the surface of Shiga-like toxin. NMR studies of the complex between the toxin and bivalent ligands show that site 2 and site 1 of a single B subunit are simultaneously occupied by a tethered P(k)-trisaccharide dimer. A simplified thermodynamic treatment provides the intrinsic affinities and binding energies for the intermolecular and intramolecular association events and permits the deconvolution of the contributions to the relative binding energies for the set of bivalent ligands. Conformational analysis based on MD simulations for bivalent galabioside dimers containing different tethers demonstrated that the calculated local concentrations of the pendant ligand at the second binding site correlate with the experimentally determined relative affinity values of the respective bivalent ligands, thereby providing a predictive method to optimize tether length.

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  • Authors: Aaron C. Kinsman; Michael A. Kerr;

    The total synthesis of (+)-hapalindole Q has been achieved. The key step is a Diels-Alder reaction mediated by MacMillan's organocatalyst to provide the critical intermediate with high enantioselectivity (93% ee). This step establishes the proper arrangement of the required four contiguous stereocenters, including the quaternary center, and represents the first successful application of an enantioselective organomediated Diels-Alder reaction in total synthesis.

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  • Authors: Katerina Krumova; Lana E. Greene; Gonzalo Cosa;

    We report here the preparation of a lipophilic fluorogenic antioxidant (Mito-Bodipy-TOH) that targets the inner mitochondrial lipid membrane (IMM) and is sensitive to the presence of lipid peroxyl radicals, effective chain carriers in the lipid chain autoxidation. Mito-Bodipy-TOH enables monitoring of the antioxidant status, i.e., the antioxidant load and ability to prevent lipid chain autoxidation, within the inner mitochondrial membrane of live cells. The new probe consists of 3 segments: a receptor, a reporter, and a mitochondria-targeting element, constructed, respectively, from an α-tocopherol-like chromanol moiety, a BODIPY fluorophore, and a triphenylphosphonium cation (TPP). The chromanol moiety ensures reactivity akin to that of α-tocopherol, the most potent naturally occurring lipid soluble antioxidant, while the BODIPY fluorophore and TPP ensure partitioning within the inner mitochondrial membrane. Mechanistic studies conducted either in homogeneous solution or in liposomes and in the presence of free radical initiators show that the antioxidant activity of Mito-Bodipy-TOH is on par with that of α-tocopherol. Studies conducted on live fibroblast cells further show the antioxidant depletion in the presence of methyl viologen (paraquat), a known agent of oxidative stress and source of superoxide radical anion (and indirectly, a causative of lipid peroxidation) within the mitochondria matrix. We recorded a ca. 8-fold emission enhancement with Mito-Bodipy-TOH in cells stressed with methyl viologen, whereas no enhancement was observed in control studies with untreated cells. Our findings underscore the potential of the new fluorogenic antioxidant Mito-Bodipy-TOH to study the chemical link between antioxidant load, lipid peroxidation and mitochondrial physiology.

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  • Authors: Anna Lee; Neil Coombs; Ilya Gourevich; Eugenia Kumacheva; +1 Authors

    We report the solution-phase formation of ordered lamellar nanocrystal (NC) arrays. These semiconductor lamellae exhibit structural integrity and temporal stability, without recourse to chemical cross-linking. They are many micrometers in diameter yet only two or three nanocrystal layers in thickness. We show that these structures can integrate a cargo of water-soluble ions, molecules, metal nanoparticles, or biomolecules. Encapsulation of gold nanoparticles is found to cause photoluminescence enhancement of the host CdSe nanocrystals. These easily prepared nanocrystal "envelopes" promise properties modulated through the integration of a vast array of water-soluble species. Our approach marks a step toward ordered, compartmentalized, NC-based complexes with controlled architectures.

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  • Authors: Eric Fillion; Ashraf Wilsily;