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31.
Voegtli WC Sommerhalter M Saleh L Baldwin J Bollinger JM Rosenzweig AC 《Journal of the American Chemical Society》2003,125(51):15822-15830
The R2 subunit of Escherichia coli ribonucleotide reductase contains a dinuclear iron center that generates a catalytically essential stable tyrosyl radical by one electron oxidation of a nearby tyrosine residue. After acquisition of Fe(II) ions by the apo protein, the resulting diiron(II) center reacts with O(2) to initiate formation of the radical. Knowledge of the structure of the reactant diiron(II) form of R2 is a prerequisite for a detailed understanding of the O(2) activation mechanism. Whereas kinetic and spectroscopic studies of the reaction have generally been conducted at pH 7.6 with reactant produced by the addition of Fe(II) ions to the apo protein, the available crystal structures of diferrous R2 have been obtained by chemical or photoreduction of the oxidized diiron(III) protein at pH 5-6. To address this discrepancy, we have generated the diiron(II) states of wildtype R2 (R2-wt), R2-D84E, and R2-D84E/W48F by infusion of Fe(II) ions into crystals of the apo proteins at neutral pH. The structures of diferrous R2-wt and R2-D48E determined from these crystals reveal diiron(II) centers with active site geometries that differ significantly from those observed in either chemically or photoreduced crystals. Structures of R2-wt and R2-D48E/W48F determined at both neutral and low pH are very similar, suggesting that the differences are not due solely to pH effects. The structures of these "ferrous soaked" forms are more consistent with circular dichroism (CD) and magnetic circular dichroism (MCD) spectroscopic data and provide alternate starting points for consideration of possible O(2) activation mechanisms. 相似文献
32.
Clean gold and platinum surfaces have been shown to be hydrophilic in borate and sulphate media at all potentials between hydrogen and oxygen evolution. Finite contact angles were observed at the nitrogen/metal/solution interface immediately after polishing with diamond paste, but electrochemical evidence demonstrated that such surfaces were contaminated. After cleaning the surface chemically or electrochemically, zero contact angles were observed. 相似文献
33.
Some new quaternary compounds of the type BaLnMQ3 (Ln = rare earth or Sc; M = Cu, Ag; Q = S, Se) have been synthesized by the reaction of the constituent binary chalcogenides and elements at 1000°C. The crystal structures of two of these compounds have been determined by single-crystal X-ray diffraction techniques and are isostructural. Crystal data: BaErCuS3—space group D172h —Cmcm, M= 464.32, Z = 4 , a = 3.987(1), b = 13.377(3), c = 10.101(2) Å (T = 115 K), V = 538.7(4) Å3, Rw (F2) = 0.095 for 848 observations and 24 variables, R(F) = 0.037 for 840 observations having F20 > 2σ (F20); BaYAgSe—space group D172h —Cmcm, M = 571.0, Z = 4, a = 4.239(1), b = 14.030(2), c = 10.636(2) Å (T = 115 K), V = 632.6(2) Å3, Rw (F2) = 0.057 for 645 observations and 24 variables, R(F) = 0.023 for 595 observations having F20 > 2σ(F20). These two compounds adopt the layered KZrCuS3 structure type. The layers, which are separated by Ba2+ ions, consist of edge-sharing octahedral chains and corner-sharing tetrahedral chains. The other compounds synthesized crystallize either with this same structure or with that of β-BaLaCuSe3, a slightly distorted variation, which is isostructural with Eu2CuS3. The diffuse reflective UV-visible spectra of several of these compounds have been measured. From magnetic susceptibility measurements, both BaNdCuS3 and BaGdCuS3 show Curie-Weiss behavior, whereas BaCeCuS3 and BaCeCuSe3 show in addition temperature-independent paramagnetism. 相似文献
34.
Porphyrin monomers, 5-(monopyridyl)-10,15,20-(triphenyl)porphyrin ( 1 ), 5,10-(dipyridyl)-15,20-(diphenyl)-porphyrin ( 2 ), and 5,15-(dipyridyl)-10,20-{diphenyl)porphyrin ( 3 ), were linked by hydrocarbon chains to form a series of dimers, trimers and polymers. The 5-(monopyridyl)-10,15,20-(triphenyl)porphyrin monomers were linked by 2, 4, 6, 8 and 10 carbon chains through the alkylation of the pyridine nitrogens using the appropriate diiodoalkane to form positively charged linked dimers 4–8 . A trimer 12 was synthesized from two 5-(monopyridyl)-10,15,20-(triphenyl)porphyrin and one 5,10-(dipyridyl)-15,20-(diphenyl)porphyrin linked by a six carbon chain. Hydrocarbon linked (5,10-(dipyridyl)-15,20-(diphenyl)porphyrin)n ( 13 ) and (5,15-(dipyridyl)-10,20-(diphenyl)porphyrin)n ( 14 ) were also prepared. 相似文献
35.
Richard A. Gardner 《Journal of solid state chemistry》1974,9(4):336-344
The kinetics of the reduction of SiO2 in hydrogen have been examined using a thermalgravimetric method. The mechanism of the reaction SiO2 + H2 = SiO + H2O can be described on the basis of a reaction interface moving at a constant velocity toward the center of the reacting sample. The velocity of the moving interface depends on the reaction temperature and water vapor content of the ambient gas. The reaction was studied in the temperature range 1115–1630°C and was found to be isokinetic in this range. The activation energy for the reaction is 85 kcal/mole in pure, dry hydrogen and 135 kcal/mole in hydrogen with a dew point of 24°C. 相似文献
36.
Koren AB Curtis MD Francis AH Kampf JW 《Journal of the American Chemical Society》2003,125(17):5040-5050
Syntheses are reported of new 4,4'-dialkyl-2,2'-bithiazole oligomers that have alkenoxy side chains that are capable of easy conversion to oligomers with functionalized side chains, e.g., terminally substituted hydroxy chains. The crystal structures of two representative oligomers (4,4',4' ',4' "-tetra-(2-propenoxymethyl)-2,2',5',5' ',2' ',2' "-quaterthiazole (3P2) and 4,4',4' ',4' "-tetra-(3-hydroxypropyloxymethyl)-2,2',5',5' ',2' ',2' "-quaterthiazole (3H2)) were determined; 3P2 crystallizes in a pi-stacked motif with two molecules per unit cell, whereas 3H2 forms pi-stacks that are linked with hydrogen bonds to form infinite two-dimensional sheets with one molecule per unit cell. A comparison of the UV-vis spectra of the compounds in solution and in the solid state provides unequivocal evidence for the presence of a Davydov splitting, W(D) approximately 0.2 eV, in solid 3P2. The spectra are interpreted in the framework of molecular exciton theory to extract a value of the intermolecular transfer integral, J approximately 0.2 eV, for a total exciton bandwidth of ca. 0.8 eV. Monte Carlo calculations were used to determine the density of states of the exciton band and the absorption and emission line shapes of the 0 <-- 0 transition. It is suggested that the "three-humped" absorption profile typical of partially crystalline, regioregular polymers is the "optical signature" of pi-stacking. 相似文献
37.
Gar B. Hoflund Steven D. Gardner David R. Schryer Billy T. Upchurch Erik J. Kielin 《Reaction Kinetics and Catalysis Letters》1996,58(1):19-26
The influence of promoters on Pt/SnOx/SiO2 and Au/MnOx low-temperature CO oxidation catalysts has been investigated under stoichiometric reaction conditions with no CO2 added to the feed gas. The performance of Pt/SnOx/SiO2 catalysts is improved significantly by the addition of 1 wt.% Fe but reduced by the addition of 5 wt.%Fe, 1 wt.% Sb, 5 wt.% Sb, 1 wt.% As, 5 wt.%As and 1.8 wt.% P. The performance of Au/MnOx is improved significantly by the addition of 1 at.% Ce but reduced by the addition of 1 at.% Co. For the catalysts and conditions examined, the Au/MnOx catalysts are superior to the Pt/SnOx/SiO2 catalysts with respect to both activity and decay characteristics. 相似文献
38.
D'Souza F Chitta R Gadde S Zandler ME McCarty AL Sandanayaka AS Araki Y Ito O 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(15):4416-4428
Two types of structurally well-defined, self-assembled zinc porphyrin-fullerene conjugates were formed by "two-point" binding strategies to probe the effect of axial ligation or pi-pi-type interactions on the photochemical charge stabilization in the supramolecular dyads. To achieve this, meso-tetraphenylporphyrin was functionalized to possess one or four [18]crown-6 moieties at different locations on the porphyrin macrocycle while fullerene was functionalized to possess an alkyl ammonium cation, and a pyridine or phenyl entities. As a result of the crown ether-ammonium cation complexation, and zinc-pyridine coordination or pi-pi-type interactions, stable zinc porphyrin-fullerene conjugates with defined distance and orientation were formed. Evidence for the zinc-pyridine complexation or pi-pi-type interactions was obtained from the spectral and computational studies. Steady-state and time-resolved emission studies revealed efficient quenching of the zinc-porphyrin singlet excited state in these dyads, and the measured rates of charge separation, k(CS) were found to be slightly better in the case of the dyads held by axial coordination and crown ether-cation complexation. Nanosecond transient absorption studies provided evidence for the electron transfer reactions, and these studies also revealed charge stabilization in these dyads. The lifetimes of the radical ion pairs were found to depend upon the type of porphyrins utilized to form the dyads, that is, porphyrin possessing the crown ether moiety at the ortho position of one of the phenyl rings yielded prolonged charge stabilized states. Addition of pyridine to the supramolecular dyads eliminated the zinc-pyridine coordination or pi-pi-type interactions of the "two-point" bound systems due to the formation of a new zinc-pyridine axial bond thus giving a unique opportunity to probe the effect of axial coordination or pi-pi interactions on k(CS) and k(CR). Under these conditions, the measured electron transfer rates revealed faster k(CS) and slower k(CR) as compared to those obtained in the absence of added pyridine. The evaluated lifetimes of the radical ion-pairs were found to be hundreds of nanoseconds and were longer in the presence of pyridine. 相似文献
39.
Kesanli B Fettinger J Gardner DR Eichhorn B 《Journal of the American Chemical Society》2002,124(17):4779-4786
[Sn(9)Pt(2)(PPh(3))](2)(-) (2) was prepared from Pt(PPh(3))(4), K(4)Sn(9), and 2,2,2-cryptand in en/toluene solvent mixtures. The [K(2,2,2-cryptand)](+) salt is very air and moisture sensitive and has been characterized by ESI-MS, variable-temperature (119)Sn, (31)P, and (195)Pt NMR and single-crystal X-ray diffraction studies. The structure of 2 comprises an elongated tricapped Sn(9) trigonal prism with a capping PtPPh(3), an interstitial Pt atom, a hypercloso electron count (10 vertex, 20 electron) and C(3)(v)() point symmetry. Hydrogenation trapping experiments and deuterium labeling studies showed that the formation of 2 involves a double C-H activation of solvent molecules (en or DMSO) with the elimination of H(2) gas. The ESI-MS analysis of 2 showed the K[Sn(9)Pt(2)(PPh(3))](1)(-) parent ion, an oxidized [Sn(9)Pt(2)(PPh(3))](1)(-) ion, and the protonated binary cluster anion [HSn(9)Pt(2)](1)(-). 2 is highly fluxional in solution giving rise to a single time-averaged (119)Sn NMR signal for all nine Sn atoms but the Pt atoms remain distinct. The exchange is intramolecular and is consistent with a rigid, linear Pt-Pt-PPh(3) rod embedded in a liquidlike Sn(9) matrix. [Sn(9)Ni(2)(CO)](3)(-) (3) was prepared from Ni(CO)(2)(PPh(3))(2), K(4)Sn(9), and 2,2,2-cryptand in en/toluene solvent mixtures. The [K(2,2,2-cryptand)](+) salt is very air and moisture sensitive, is paramagnetic, and has been characterized by ESI-MS, EPR, and single-crystal X-ray diffraction. Complex 3 is a 10-vertex 21-electron polyhedron, a slightly distorted closo-Sn(9)Ni cluster with an additional interstitial Ni atom and overall C(4)(v)() point symmetry. The EPR spectrum showed a five-line pattern due to 4.8-G hyperfine interactions involving all nine tin atoms. The ESI-MS analysis showed weak signals for the potassium complex [K(2)Sn(9)Ni(2)(CO)](1-) and the ligand-free binary ions [K(2)Sn(9)Ni(2)](1)(-), [KSn(9)Ni(2)](1)(-), and [HSn(9)Ni(2)](1)(-). 相似文献
40.
Gottfried AC Wang J Wilson EE Beck LW Banaszak Holl MM Kampf JW 《Inorganic chemistry》2004,43(24):7665-7670
The combination of 2 equiv of bis[bis(trimethylsilyl)amide]germylene (5) with 2 equiv of 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) in tetrahydrofuran (THF) results in the ring-opening of 1 equiv of THF to form 2,2,8,8-tetrakis(1,1,1,3,3,3-hexamethyl-disilazan-2-yl)-5,16-diphenyl-7,9,14-trioxa-1,3,5,16,18,19-hexaaza-2,8-digerma-tricyclo[13.2.1.13,6]nonadeca-6(19),15(18)-diene-4,17-dione (6). This fast and nearly quantitative reaction builds a 15-membered ring from five different molecules. The new ring, structurally assigned by X-ray crystallography, contains a flexible methylene chain that moves rapidly on the NMR time scale. 相似文献