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Showing papers by "Bernt Krebs published in 1998"


Journal ArticleDOI
TL;DR: Based on biochemical, spectroscopic and the presented structural data, a catalytical mechanism is proposed in which one of the oxygen atoms of the diphenolic substrate binds to CuB ofThe oxygenated enzyme.
Abstract: Catechol oxidases are ubiquitous plant enzymes containing a dinuclear copper center. In the wound-response mechanism of the plant they catalyze the oxidation of a broad range of ortho-diphenols to the corresponding o-quinones coupled with the reduction of oxygen to water. The crystal structures of the enzyme from sweet potato in the resting dicupric Cu(II)-Cu(II) state, the reduced dicuprous Cu(I)-Cu(I) form, and in complex with the inhibitor phenylthiourea were analyzed. The catalytic copper center is accommodated in a central four-helix-bundle located in a hydrophobic pocket close to the surface. Both metal binding sites are composed of three histidine ligands. His 109, ligated to the CuA site, is covalently linked to Cys 92 by an unusual thioether bond. Based on biochemical, spectroscopic and the presented structural data, a catalytical mechanism is proposed in which one of the oxygen atoms of the diphenolic substrate binds to CuB of the oxygenated enzyme.

749 citations


Journal ArticleDOI
TL;DR: In this paper, a series of manganese(II)-substituted polyoxometalates were synthesized and characterized by X-ray structure analyses, and the use of these oxidatively and solvolytically stable heteropolyanions as homogeneous catalysts for the epoxidation of dienes was investigated.
Abstract: A series of novel manganese(II)-substituted polyoxometalates, [(MnII(H2O)3)2(WO2)2(BiW9O33)2]10- (1), [(MnII(H2O))3(SbW9O33)2]12- (2), and [(MnII(H2O)3)2(MnII(H2O)2)2(TeW9O33)2]8- (3), were synthesized and characterized by X-ray structure analyses. The use of these oxidatively and solvolytically stable heteropolyanions as homogeneous catalysts for the epoxidation of dienes was investigated by gas chromatography/mass spectrometry, IR spectroscopy, UV−visible studies, and cyclic voltammetric measurements. The catalytic performance is exemplified by the model substrate (R)-(+)-limonene, at ambient temperatures in a biphasic system, with excellent regioselectivities, >99%, and very high turnovers even with only a small molar excess of hydrogen peroxide.

230 citations


Journal ArticleDOI
TL;DR: In this article, two catechol oxidases have been isolated from sweet potatoes (Ipomoea batatas) and purified to homogeneity, and the two isozymes have been characterized by EXAFS, EPR-, UV/Vis-spectroscopy, isoelectric focusing, and MALDI-MS and have been shown to contain a dinuclear copper center.

151 citations


Journal ArticleDOI
TL;DR: In this paper, the unique properties of a new family of peroxocopper(II) complexes are discussed and their synthesis, structure, and spectroscopic and magnetic properties are described.
Abstract: The unique properties of a newfamily of peroxocopper(II) complexes are discussed. They are stable at room temperature and contain the µ4-peroxo structural unit (right). Their synthesis, structure, and spectroscopic and magnetic properties are described.

57 citations


Journal ArticleDOI
TL;DR: It was found that compound 6a having a 2‐methoxyphenyl group at position 5 and a benzylidene group at positions 2 was the most potent compound in this series.
Abstract: In this study, thirty six new 2-benzylidene-7-methyl-3-oxo-5- phenyl-2,3-dihydro-5H-thiazolo[3,2-alpha]pyrimidine-6-carboxylic acid methyl esters were synthesized and characterized by spectral, crystallographic, and elemental analysis. The antiinflammatory activity of the compounds was tested by the carrageenan hind paw edema test. It was found that compound 6a having a 2-meth-oxyphenyl group at position 5 and a benzylidene group at position 2 was the most potent compound in this series. All the compounds that were tested for ulcer activity gave positive results.

44 citations


Journal ArticleDOI
TL;DR: The tridentate NS2 ligands 2-HSC6H4C(Me)NNHC(S)SCH2Ph (H2L1), 2-HCN4C (Me)NNHCPh3 (H 2L2L2) and 1-Ph-3-Me-5-HS-C3N2-CHNNC(SH)NHC Ph3 (SH) were determined by single-crystal X-ray analysis Complexes 1 and 2 each consist of a central cis-MoO2 unit with
Abstract: Reaction of the new tridentate NS2 ligands 2-HSC6H4C(Me)NNHC(S)SCH2Ph (H2L1), 2-HSC6H4C(Me) NNHC(S)NHCPh3 (H2L2), 2-HSC6H4CHNNHC(S)NHCPh3 (H2L3) and 1-Ph-3-Me-5-HS-C3N2-CHNNC(SH)NHCPh3 (H2L4) with dioxomolybdenum(VI) precursors yielded mononuclear molybdenum(VI) complexes [MoO2(L1)(pic)] 1 (pic = 4-methylpyridine), [MoO2L2(MeOH)]·225 MeOH 2 and the dinuclear molybdenum(V) complexes [Mo2O3L32]·3CH2Cl2 3 and [Mo2O3L42] 4 The molecular structures of 1–3 were determined by single-crystal X-ray analysis Complexes 1 and 2 each consist of a central cis-MoO2 unit with a mer co-ordinating tridentate dianionic ligand and one neutral donor molecule completing the octahedral environment of molybdenum(VI) Complex 3 exhibits an anti-Mo2O3 group co-ordinated by two mer chelating ligands The symmetry-related molybdenum(V) centres are in a square pyramidal environment Infrared, NMR and MS studies evidenced a comparable structure for 4 The reduction of the dioxomolybdenum(VI) compounds 1 and 2, achieved by addition of phosphines like PPh3, also leads to dinuclear µ-oxo-bridged complexes The reverse oxidations of the oxomolybdenum(V) complexes are restricted by the nature of the ligand and the oxidizing agents

28 citations


Journal ArticleDOI
TL;DR: In this paper, Thioborate als Ionenleiter von interesse ebenso wie ein durch Hochtemperatursynthese zugangliches, persubstituiertes B12-Ikosaeder, das als Besonderheit einen planaren B3Se2-Ring unter Beteiligung seiner chelatisierenden Liganden aufweist.
Abstract: Eine immense Bandbreite an Strukturmotiven wird durch die herausfordernde Chemie der Bor-Schwefel- und Bor-Selen-Verbindungen abgedeckt. Charakteristische Strukturbesonderheiten der binaren Borsulfide konnen zum Verstandnis und zur Beschreibung der dreidimensionalen Atomanordnung in makromolekularen, anionischen Gerusten wie des im Bild dargestellten Li6+2x[B10S18]Sx (x≈2; ○: S, •: B, •: Li) herangezogen werden. Daruber hinaus sind Thioborate als Ionenleiter von Interesse ebenso wie ein durch Hochtemperatursynthese zugangliches, persubstituiertes B12-Ikosaeder, das als Besonderheit einen planaren B3Se2-Ring unter Beteiligung seiner chelatisierenden Liganden aufweist.

19 citations



Journal ArticleDOI
01 Jan 1998-Analyst
TL;DR: In this article, the formation of a coloured adduct in the reaction of a dinuclear iron(III) complex with hydrogen peroxide is used for the quantification of the peroxide.
Abstract: A selective method for the determination of hydrogen peroxide in the presence of other oxidants is presented. The formation of a coloured adduct in the reaction of a dinuclear iron(III) complex with hydrogen peroxide is used for the quantification of the peroxide. The reaction product is detected at wavelengths between 570 and 600 nm. The major advantage of the method is the immediate reaction. Other peroxides exhibit little interfence, typically contributing to an error of 0.1% for the same concentration. The method was optimized for the determination of hydrogen peroxide in household products using microplate spectrophotometry. The limit of detection is 3 µmol l–1 (100 ppb) under these conditions. Calibration functions are linear over the range 9–300 µmol l–1 (300 ppb–10 ppm). The method was compared with established methods for peroxide analysis, including the spectrophotometric titanyl method and an HPLC method based on the oxidation of triphenylphosphine. The method described is suitable for the analysis of real samples.

11 citations


Journal ArticleDOI
C Schoeberl1, R Boehner1, Bernt Krebs1, C Mueller1, A Barnekow1 
TL;DR: A considerable decrease in total cellular phosphotyrosine content in treated infected cells in vivo is measured, possibly due to a reduced rate of v-src translation caused by the inhibitory effect of POM1 on the RSV encoded reverse transcriptase activity (RT) activity.
Abstract: We examined the recently synthesized and characterized polyoxometalate compound (NH4)10[Co2Sb2W20 O70(H2O)6] (POM1). The inhibitory potency of POM1 was studied in tissue culture experiments with uninfected and Rous sarcoma virus (RSV)-infected chicken embryo fibroblasts (CEF) in vivo. We measured a considerable decrease in total cellular phosphotyrosine content in treated infected cells in vivo. POM1 treatment of SR-RSV-A infected CEF in vivo resulted in decreased pp60v-src activity, possibly due to a reduced rate of v-src translation caused by the inhibitory effect of POM1 on the RSV encoded reverse transcriptase activity (RT) activity. In further studies we were able to demonstrate the inhibitory effect of this complex on the RT activity of RSV in vitro and in vivo.

10 citations


Journal ArticleDOI
TL;DR: In this article, a review is given on the synthesis of a series of novel heteropolytungstales and -molybdates with polymeric network structures containing bismuth, chromium, copper, ruthenium, rhodium, cerium and europium as heteroatoms.
Abstract: A review is given on the synthesis of a series of novel heteropolytungstales and -molybdates with polymeric network structures containing bismuth, chromium, copper, ruthenium, rhodium, cerium and europium as heteroatoms. Their crystal structures were determined by single crystal X-ray methods. The compounds reported are: Na 6 [Rh 4 (CH 3 COO) 8 W 12 O 38 (OH) 2 ]· 25H 2 O ( 1 ), K 6 Na 2 [Ru(H 2 O) 2 (W 12 O 40 (OH) 2 ] · 10H 2 O ( 2 ), K 20 Na 6 [(Cu(H 2 O) 2 W 12 O 40 (OH) 2 ) 2 (W 12 O 40 (OH)) 2 ]· 31H 2 O ( 3 ), Ce(H 2 O) 7 [CrMo 6 O 18 (OH) 6 ] · 4H 2 O ( 4 ), and Na 3 H 2 [Eu 3 (H 2 O) 18 (Bi 2 W 22 O 76 )]· 29H 2 O ( 5 ). As a common feature they all contain oligomeric isopolymetalate or heteropolymetalate groups of known structure and size which are linked by various transition metal or rare earth metal centers to form a number of novel extended polymeric heteropolymetalate networks. A polymeric layer structure is formed in 1 by two-dimensional linking of dirhodium tetraacetate units and [W 12 O 38 (OH) 2 ] 6− isopolyamons. In 2 paratungstate-B units are linked by octahedral RuO 4 (H 2 O) 2 polyhedra to form polymeric anionic chains. Similar chains are present in 3, however with Cu being in a square planar CuO 2 (H 2 O) 2 coordination, and additional molecular non-coordinated paratungstate anions are present in the crystal. In 4 nine-coordinate CeO 2 (H 2 O) 7 polyhedra share corners with two neighbouring anions of composition [CrMo 6 O 18 (OH) 6 ] 3− to form polymeric chain molecules of alternating cerium polyhedra and hexamolybdochromate(III) units. In 5 structurally remarkable [Bi 2 W 22 O 76 ] 14− groups share edges with eight-coordinate EuO 2 (H 2 O) 6 polyhedra, again to form three-dimensional polymeric heleropoly networks.

Journal ArticleDOI
TL;DR: In this paper, the preparation and X-ray structure analysis of the compounds [PPh 4 ] 2 [Cl 2 (S 2 )Ta(μ-O)(μ-S 2 ),Ta(S 2 ]Cl 2 ] ( 1 ) and[PPh4 ] 2 -[Cl 2 S 2 )NbNb(μ O)(μ S 2 ),Nb Nb(M 2 )Cl 2 )

Journal ArticleDOI
TL;DR: The results indicate that polyoxoanions are cytotoxic for malignant glioma cells and that the most promising compound investigated here was POA5, which was superior to cisplatin.
Abstract: We studied the cytotoxicity of five polyoxoanions on two human malignant glioma cell lines (T98G and 86HG39), a rat glioma cell line (C6) and a human fibroblast cell line (NIH-3T3) using MTT tests to measure the drug concentration killing 50% of the cells (LC50). Cisplatin was used as a reference agent. Cisplatin had the highest efficacy in three of the four cell lines. Only in T98G cells, one of the components (POA5) had a lower LC50 value (1.3 x 10(-6) mol/l) than cisplatin (2.5 x 10(-6)). POA5 was also the most cytotoxic polyoxoanion when the LC50 values of all four cell lines were averaged (6.6 x10(-6)). Average LC50 values of the other compounds were 10.9, 12.6, 19.0 and 19.2 x 10(-6) mol/l in POA1, POA2, POA3 and POA4, respectively. When the benign fibroblasts were used to calculate a therapeutic index as LC50 in fibroblasts divided by LC50 in glioma cells, POA5 was superior to cisplatin. These results indicate that polyoxoanions are cytotoxic for malignant glioma cells and that the most promising compound investigated here was POA5.

Journal ArticleDOI
TL;DR: The reported novel compounds [H3O·(dibenzo-18crown-6)][Te2Br9], [H5O2]·Te2Cl9]·2C4H8O2 and [Se3Br8]2] highlight the large structural and chemical variety of halogen chalcogen chemistry as mentioned in this paper.
Abstract: The reported novel compounds [H3O·(dibenzo-18-crown-6)][Te2Br9], [H5O2][Te2Cl9]·2C4H8O2 and [H5O2·(bis-dibromo-dibenzo-24-crown-8)]2[Se3Br8] highlight the large structural and chemical variety of halogen chalcogen chemistry. Both [Te2X9]− anions (X = Cl, Br) consist of one square pyramidal TeX5 fragment connected to a TeX6 octahedron by a common edge. The linear [Se3Br8]2− anion is built by three square planar SeBr4-fragments. The three counterions offer a convenient possibility for controlled synthesis of supramolecular oxonium systems.

Journal ArticleDOI
TL;DR: A comprehensive overview on the structural diversity of thio- and selenoborates is given with special focus to the molecular features of boron-selenium chemical in binary and ternary compounds in this paper.
Abstract: A comprehensive overview on the structural diversity of thio- and selenoborates is given with special focus to the molecular features of boron-selenium chemical in binary and ternary compounds.