Two-step Reaction Sequence Research Articles

Folding Mechanism of an Ankyrin Repeat Protein: Scaffold and Active Site Formation of Human CDK Inhibitor p19 INK4d

The p19 INK4d protein consists of five ankyrin repeats (ANK) and controls the human cell cycle by inhibiting the cyclin D-dependent kinases (CDK) 4 and 6. We investigated the folding of p19 INK4d by urea-induced unfolding transitions, kinetic analyses of unfolding and refolding, including double-mixing experiments and a special assay for folding intermediates. Folding is a sequential two-step reaction via a hyperfluorescent on-pathway intermediate. This intermediate is present under all conditions, during unfolding, refolding and at equilibrium. The folding mechanism was confirmed by a quantitative global fit of a consistent set of equilibrium and kinetic data revealing the thermodynamics and intrinsic folding rates of the different states. Surprisingly, the N ↔ I transition is much faster compared to the I ↔ U transition. The urea-dependence of the intrinsic folding rates causes population of the intermediate at equilibrium close to the transition midpoint. NMR detected hydrogen/deuterium exchange and the analysis of truncated variants showed that the C-terminal repeats ANK3-5 are already folded in the on-pathway intermediate, whereas the N-terminal repeats 1 and 2 are not folded. We suggest that during refolding, repeats ANK3–ANK5 first form the scaffold for the subsequent assembly of repeats ANK1 and ANK2. The binding function of p19 INK4d resides in the latter repeats. We propose that the graded stability and the facile unfolding of repeats 1 and 2 is a prerequisite for the down-regulation of the inhibitory activity of p19 INK4d during the cell-cycle.

Formation of Nanostructured Zeolite Particle Surfaces via a Halide/Grignard Route

A two-step reaction sequence, dealumination via thionyl chloride followed by reaction with a Grignard reagent, has created a nanoscale morphology that appears as significantly roughened outer surfaces on zeolite 4A particles. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) measurements revealed that such surface nanostructures are composed of Mg(OH)2 crystals. The formation mechanism of this specific surface morphology was explored. It was discovered by solid-state 29Si and 27Al NMR and XPS surface analysis that thionyl chloride partially removes aluminum from the zeolite 4A framework and yields NaCl and AlCl3. Precipitation of these extracted inorganic salts on the surfaces of zeolite particles occur. Subsequently, methylmagnesium bromide is reacted with 2-propanol in a quenching process and generates Mg(OH)2. The previously deposited NaCl and AlCl3 nanoparticles on zeolite surfaces are believed to function as heterogeneous nuclei for the growth of Mg(OH)2 crystals and thereby creates ...

Photosystem II: Structure and mechanism of the water:plastoquinone oxidoreductase

This mini-review briefly summarizes our current knowledge on the reaction pattern of light-driven water splitting and the structure of Photosystem II that acts as a water:plastoquinone oxidoreductase. The overall process comprises three types of reaction sequences: (a) light-induced charge separation leading to formation of the radical ion pair P680+*QA(-*) ; (b) reduction of plastoquinone to plastoquinol at the QB site via a two-step reaction sequence with QA(-*) as reductant and (c) oxidative water splitting into O2 and four protons at a manganese-containing catalytic site via a four-step sequence driven by P680+* as oxidant and a redox active tyrosine YZ acting as mediator. Based on recent progress in X-ray diffraction crystallographic structure analysis the array of the cofactors within the protein matrix is discussed in relation to the functional pattern. Special emphasis is paid on the structure of the catalytic sites of PQH2 formation (QB-site) and oxidative water splitting (Mn4OxCa cluster). The energetics and kinetics of the reactions taking place at these sites are presented only in a very concise manner with reference to recent up-to-date reviews. It is illustrated that several questions on the mechanism of oxidative water splitting and the structure of the catalytic sites are far from being satisfactorily answered.

Molybdenum and tungsten complexes of the neutral tripod ligands HC(pz) 3 and MeC(CH 2PPh 2) 3

Starting from [M(CO) 6], seven-coordinated complexes of tungsten and molybdenum containing the facially coordinating ligands HC(pz) 3 ( 1) and MeC(CH 2PPh 2) 3 ( 2) were obtained in a two-step reaction sequence. The complexes have a 4:3 piano stool geometry with almost perfect C S symmetry in the crystal. In solution, they show the typical fluxional behavior for seven-coordinated complexes even at low temperature. Complete oxidative decarbonylation occurs when [HC(pz) 3Mo(CO) 3] ( 4) or [MeC(CH 2PPh 2) 3Mo(CO) 3] ( 6) are treated with an excess of I 2 or Br 2.

Facile synthesis of primary amides and ketoamides via a palladium-catalysed carbonylation–deprotection reaction sequence

Various primary amides and ketoamides have been obtained in good yields in a two-step reaction sequence. The first step involves the synthesis of aryl/alkenyl N- tert-butyl amides and aryl N- tert-butyl ketoamides from the corresponding iodides via palladium-catalysed carbonylation in the presence of t-BuNH 2 as the nucleophile. Carbonylation was followed by selective cleavage of the t-Bu group using TBDMSOTf as the reagent.

Trinuclear π-conjugated chromophores formed by a neutral ferrocenyl group and a cationic mixed ruthenium sandwich linked through an unsymmetrical Schiff-base complex spacer

The synthesis of neutral dinuclear unsymmetrical Schiff base complexes 1 and 2 was carried out in an one-pot two-step reaction sequence that involves the reaction of 1-ferrocenyl-1,3-butanedione with an excess of ethylenediamine in refluxing CH2Cl2, to generate the intermediate “half-unit” Fc–C(O)CH2C(CH3)NCH2CH2NH2, Fc = (η5-Cp)Fe(η5-C5H4), Cp = C5H5), followed by its in situ templated reaction with salicylaldehyde and nickel(II) or copper(II) acetate in refluxing CH2Cl2–EtOH (3 : 1) mixture. On the other hand, ionic trinuclear unsymmetrical complexes [3]+PF6− and [4]+PF6− were synthesized by reacting dinuclear precursors 1 and 2, respectively, with [(η5-Cp*)Ru(CH3CN)3]+PF6−, Cp* = C5Me5, in refluxing CH2Cl2. Both complexes contain the neutral electron-releasing ferrocenyl group and the cationic electron-withdrawal sandwich, [(η5-Cp*)Ru(η6-arene)]+, connected by a tetradentate {M(ONNO)} chelate (M = Ni, Cu). The new unsymmetrical organometallic Schiff base compounds were characterized by elemental analysis, IR, UV-Vis, 1H NMR and EPR spectroscopy. In addition, the Ni(II) containing complexes 1 and [3]+PF6− were authenticated by X-ray diffraction analysis. Solvatochromic and electrochemical properties were also studied. The organometallic–inorganic hybrid unsymmetrical Schiff base complexes [3]+PF6− and [4]+PF6− can be defined as type I non-rod-shaped dipolar chromophores.

Synthesis of fused imidazole rings by sequential van Leusen/C–H bond activation

A concise route to access fused imidazole rings employing the van Leusen three-component reaction followed by a Pd/Cu catalyzed intramolecular C-arylation is reported. The reaction was found to be general and the products were formed in moderate to excellent yields using the two-step reaction sequence.

Biochemical Characterization of VlmL, a Seryl-tRNA Synthetase Encoded by the Valanimycin Biosynthetic Gene Cluster

Previous studies have shown that the valanimycin producer Streptomyces viridifaciens contains two genes encoding proteins that are similar to seryl-tRNA synthetases (SerRSs). One of these proteins (SvsR) is presumed to function in protein biosynthesis, because it exhibits a high degree of similarity to the single SerRS of Streptomyces coelicolor. The second protein (VlmL), which exhibits a low similarity to the S. coelicolor SerRS, is hypothesized to play a role in valanimycin biosynthesis, because the vlmL gene resides within the valanimycin biosynthetic gene cluster. To investigate the role of VlmL in valanimycin biosynthesis, VlmL and SvsR have been overproduced in soluble form in Escherichia coli, and the biochemical properties of both proteins have been analyzed and compared. Both proteins were found to catalyze a serine-dependent exchange of 32P-labeled pyrophosphate into ATP and to aminoacylate total E. coli tRNA with L-serine. Kinetic parameters for the two enzymes show that SvsR is catalytically more efficient than VlmL. The results of these experiments suggest that the role of VlmL in valanimycin biosynthesis is to produce seryl-tRNA, which is then utilized for a subsequent step in the biosynthetic pathway. Orthologs of VlmL were identified in two other actinomycetes species that also contain orthologs of the S. coelicolor SerRS. The significance of these findings is herein discussed.

Covalent Attachment of Acetylene and Methylacetylene Functionality to Si(111) Surfaces: Scaffolds for Organic Surface Functionalization while Retaining Si−C Passivation of Si(111) Surface Sites

Si(111) surfaces have been functionalized with Si-CC-R species, where R = H or -CH3, using a two-step reaction sequence involving chlorination of H-Si(111) followed by treatment with Na-CC-H or CH3-CC-Na reagents. The resulting surfaces showed no detectable oxidation as evidenced by X-ray photoelectron spectroscopic (XPS) data in the Si 2p region, electrochemical measurements of Si-H oxidation, or infrared spectroscopy. The Si-CC-R-terminated surfaces exhibited a characteristic CC stretch in the infrared at 2179 cm-1, which was strongly polarized perpendicular to the Si(111) surface plane. XPS measurements in the C 1s region showed a low binding energy peak indicative of Si-C bonding, with a coverage that was, within experimental error, identical to that of the CH3-terminated Si(111) surface, which has been shown to fully terminate the Si atop sites on an unreconstructed Si(111) surface. The Si-CC-H-terminated surfaces were further functionalized by exposure to n-C4H9Li followed by exposure to para Br-C6H5-CF3, allowing for introduction of para -C6H5CF3 groups while maintaining the desirable chemical and electrical properties that accompany complete Si-C termination of the atop sites on the Si(111) surface.

New, Diastereoselective Synthesis of 1‐Alkyl‐5‐alkylidene‐3‐methylidenepyrrolidin‐2‐ones.

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New, diastereoselective synthesis of 1-alkyl-5-alkylidene-3-methylidenepyrrolidin-2-ones

The diastereoselective synthesis of 1-alkyl-5-alkylidene-3-methylidenepyrrolidin-2-ones was readily accomplished in a two-step reaction sequence consisting of the reaction of 2-diethoxyphosphoryl-4-oxoalkanoates with amines followed by Horner–Wadsworth–Emmons olefination of formaldehyde using the intermediate 1-alkyl-5-alkylidene-3-diethoxyphosphorylpyrrolidin-2-ones.

Modeling extinction in forced opposed-flow smolder

We address a discrepancy between smolder extinction theory and experimental work regarding whether extinction can occur under oxygen-depleted conditions. Single heterogeneous (solid/gas) reaction models that model the complex reaction process of smoldering with a single reaction predict extinction for large head winds only when oxygen remains (fuel-limited smolder). Experimental evidence, however, shows that oxygen concentrations are essentially zero for smolder waves close to the extinction limit. We consider a two-step sequential reaction scheme in which the solid fuel is converted to char, which also reacts with oxygen to produce ash. This generic scheme leads to extinction with oxygen depletion. The model suggests that oxygen depletion is caused by the second reaction while extinction is driven by the first. Solid fuel must be completely converted to char for extinction to occur while oxygen consumption in the secondary reaction induces oxygen depletion. Spatial separation of the localized reactions is predicted to increase with head wind as a precursor to extinction.

Synthesis ofN-(3-[18F]Fluoropropyl)-2β-carbomethoxy-3β-(4-iodophenyl)nortropane ([18F]FP-β-CIT)

N-(3-[18F]fluoropropyl)-2β-carbomethoxy-3β-(4-iodophenyl)nortropane ([18F]FP-β-CIT) was synthesized in a two-step reaction sequence. In the first reaction, 1-bromo-3-(nitrobenzene-4-sulfonyloxy)-propane was fluorinated with no-carrier-added fluorine-18. The resulting product, 1-bromo-3-[18F]-fluoropropane, was distilled into a cooled reaction vessel containing 2β-carbomethoxy-3β-(4-iodophenyl)-nortropane, diisopropylethylamine and potassium iodide. After 30 min, the reaction mixture was subjected to a preparative HPLC purification. The product, [18F]FP-β-CIT, was isolated from the HPLC eluent with solid-phase extraction and formulated to yield an isotonic, pyrogen-free and sterile solution of [18F]FP-β-CIT. The overall decay-corrected radiochemical yield was 25 ± 5%. Radiochemical purity was > 98% and the specific activity was 94 ± 50 GBq/µmol at the end of synthesis. Copyright © 2006 John Wiley & Sons, Ltd.

Furan [3 + 2] Dipolar Cycloadditions Promoted by a π-Basic Tungsten Metal Fragment

Reactions are reported for the TpW(NO)(PMe3) complex of either 2-methylfuran or 2,5-dimethylfuran with various dipolarophiles (N-methylmaleimide, N-phenylmaleimide, acrylonitrile). The resulting 7-oxanorbornene complexes are thought to be formed from a two-step reaction sequence. The first step involves the metal shifting the coordination site from C4−C5 to C3−C4, an action that renders the furan similar to a carbonyl ylide. This is followed by a 1,3-dipolar cycloaddition. In the case of the maleimides, a high preference (>10:1) for exo stereochemistry is observed. For acrylonitrile, the preference for exo is only modest (2:1). Attempts to liberate the intact 7-oxanorbornene were unsuccessful. Two crystal structures are presented that support the stereochemical assignments of the complexed cycloadducts.

Diastereoselective Synthesis of All Eight l-Hexoses from l-Ascorbic Acid

[reaction: see text] A novel versatile method for the synthesis of all eight diastereomerically pure L-hexoses was developed. L-Ascorbic acid was converted to two diastereomers A. These alpha-hydroxy esters were transformed into four gamma-alkoxy-alpha,beta-unsaturated esters C via the intermediates B and subsequent Wittig olefination reactions. Each one of compounds C was subjected to dihydroxylation to provide a set of two diols D. Anti/syn-differentiation in diol formation was manipulated by using (DHQD)2PHAL and (DHQ)2PHAL as chiral ligands. Further two-step reaction sequence affords all eight diastereopure L-hexoses.

Synthesis of fused imidazo azepine derivatives by sequential van Leusen/enyne metathesis reactions

A facile synthesis of fused imidazo azepine derivatives by a van Leusen/intramolecular enyne metathesis synthetic sequence is reported. The two-step reaction sequence generates compounds of significant molecular complexity from simple starting materials in an expedient fashion with good overall yields.

Synthesis of fused triazolo-imidazole derivatives by sequential van Leusen/alkyne–azide cycloaddition reactions

A facile synthesis of fused triazolo imidazole derivatives by a van Leusen/alkyne–azide cycloaddition synthetic sequence is reported. The two-step reaction sequence generates compounds of significant molecular complexity from simple starting materials in an expedient fashion with good overall yields.

The variation of coordination modes of aromatic imines in iron carbonyl complexes: Is there a correlation between bond lengths in organometallic model compounds and the reactivity of the ligands in catalytic C–C bond formation reactions?

The reaction of aromatic imines with Fe 2(CO) 9 proceeds via a two-step reaction sequence. A C–H activation reaction in ortho-position with respect to the exocyclic imine function is followed by an intramolecular hydrogen transfer reaction towards the former imine carbon atom. The resulting dinuclear iron carbonyl complexes show an aza-ferra-cyclopentadiene ligand which is apically coordinated by the second iron tricarbonyl moiety. Comparing the bond lengths of 43 different compounds, which were synthesized and structurally characterized in our group shows that the iron iron bond length correlates with one of the iron carbon bond lengths. The longer the iron carbon bond between the apically coordinated iron atom and the carbon atom next to the former imine carbon atom is, the shorter is the iron iron bond. The same ligands may be used as the substrates in ruthenium catalyzed C–C bond formation reactions. Whereas most of the imines react via the formal insertion of CO and/or ethylene into the C–H bond in ortho-position to the imine function, the ligands that show the longest iron carbon bond lengths in the model compounds under the same reaction conditions produce different types of isoindolones.

Microwave‐Assisted One‐Pot Synthesis of 2,3‐Disubstituted 3H‐Quinazolin‐4‐ones.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Microwave-assisted one-pot synthesis of 2,3-disubstituted 3 H-quinazolin-4-ones

A practical synthesis of 2,3-disubstituted 3 H-quinazolin-4-ones 1 with broad chemistry scope is described. The key step is the microwave promoted one-pot, two-step reaction sequence combining anthranilic acids, carboxylic acids, and amines providing efficient access to this important class of heterocycles.