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1.
Metabolite-sensing mRNAs, or "riboswitches," specifically interact with small ligands and direct expression of the genes involved in their metabolism. Riboswitches contain sensing "aptamer" modules, capable of ligand-induced structural changes, and downstream regions, harboring expression-controlling elements. We report the crystal structures of the add A-riboswitch and xpt G-riboswitch aptamer modules that distinguish between bound adenine and guanine with exquisite specificity and modulate expression of two different sets of genes. The riboswitches form tuning fork-like architectures, in which the prongs are held in parallel through hairpin loop interactions, and the internal bubble zippers up to form the purine binding pocket. The bound purines are held by hydrogen bonding interactions involving conserved nucleotides along their entire periphery. Recognition specificity is associated with Watson-Crick pairing of the encapsulated adenine and guanine ligands with uridine and cytosine, respectively.  相似文献   
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The trans–iso isomerization of the dimeric borohydride cluster anion [B20H18]2– in the course of the silver(I) complexation reaction with triphenylphosphine has been described for the first time. The isomeric complexes [Ag2(Ph3P)6[trans-B20H18]] and [Ag2(Ph3P)6[iso-B20H18]] have been isolated and characterized by physicochemical methods. The structures of the compounds have been determined by X-ray crystallography.  相似文献   
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Transition states of elementary reactions of H2 molecule elimination from [B n H n + 1]? anions (n = 6–9, 11) in which nucleophilic/electrophilic vacancies form at boron atoms have been localized by the density functional theory method (in the B3LYP/6-311++G** approximation). For a series of [B n H n + 1]? anions (n = 6–12), the activation barriers to H2 elimination have been compared to consider the possibility of substitution for exopolyhedral hydrogen atoms by the mechanism with the first rate-limiting stage of formation of [B n H n ? 1]? (n = 6–12) intermediates with a vacant “bare” vertex of the boron cluster. For the [B n H n ]2?, [B n H n + 1]?, and [B n H n ? 1]? anions (n = 6–12), the electronic chemical potential μ and Pearson hardness η have been evaluated since these characteristics make it possible to assess the propensity of different reagents to react with each other in terms of the empirical HSAB principle (soft with soft and hard with hard). The application of this principle is exemplified by the interaction of the [B10H9]? and [B12H11]? anions with acetonitrile CH3CN, furan C4H4O, and 18-crown-6.  相似文献   
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A new binuclear cadmium(II) complex with neutral ligands, 1,2-diaminobenzene (DMB) and dimethylformamide (DMF), [Cd2(Ph(NH2)2)5(DMFA)4](B10H10)2, was synthesized and studied by IR spectroscopy and X-ray diffraction. The crystals are monoclinic, a = 26.198(3) ?, b = 12.742(3) ?, c = 21.658(3) ?, β = 119.985(10)°, Z = 8, space group C2/c. The distorted octahedral environment of Cd is formed by four nitrogen atoms of three DAB molecules and two oxygen atoms of DMF molecules. Three independent DAB molecules perform different functions: one chelates the Cd atom, another is linked to cadmium as a monodentate ligand, and the third one bridges two Cd atoms, thus forming the dimer. The amino groups of the DAB molecules are involved in the N-H⋯O and N-H⋯N hydrogen bonds and in N-H⋯B and N-H⋯H-B specific interactions with the cluster boron anion. Original Russian Text ? E.A. Malinina, V.V. Drozdova, L.V. Goeva, I.N. Polyakova, N.T. Kuznetsov, 2007, published in Zhurnal Neorganicheskoi Khimii, 2007, Vol. 52, No. 6, pp. 922–926.  相似文献   
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Conclusions The formation of a neutral monocomplex of Ce(IV) with diethylenetriaminepentaacetic acid was found by a spectrophotometric method. The instability constant of the complex at =1.0 and pH 1.4, the value of which was equal to (0.8±0.2)·10–34, was calculated.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2186–2189, October, 1970.  相似文献   
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The efficiency of quenching of the B 2Σ 1 2/+ state of HgBr* and HgI* exciplex molecules by vapors of mercury dihalides (HgBr2 and HgI2) in the gas-discharge plasma of a HgBr/HgI radiation source is determined. The measurement technique used is substantiated. The quenching rate constants of mercury monobromide and monoiodide exciplex molecules by mercury dihalides are 3.4 × 10?10 and 1.1 × 10?10 cm3/s, respectively.  相似文献   
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We present the results of a study of the emission characteristics of a longitudinal glow discharge based on a mixture of helium and chlorine. Study of the emission characteristics of the discharge showed that it emits most efficiently in the bands of the chlorine molecule with maxima at 200 nm and 258 nm in an He-Cl2 mixture (P = 0.2–0.3 kPa). We present the optimization of the average emission power of a UV emitter as a function of the glow discharge parameters, the pressure, and the composition of the working gas mixture. Based on solution of the Boltzmann equation for the electron energy distribution function in a discharge based on the optimal mixture (according to experimental data), we calculated the power losses per unit pressure going toward elementary processes, the electron transport characteristics, the ionization and attachment coefficients as a function of the parameter E/P. The calculated parameters of the plasma for the UV emitter allowed us to provide a basis for the qualitative pattern of physicochemical processes in the studied plasma. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 1, pp. 121–124, January–February, 2008.  相似文献   
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