Record 1 of 23
Author(s): Matsui, J; Park, H; Otsuka, K; Oyama, M
Title: Kinetics and mechanisms of the reactions of 9-substituted anthracene cation radicals with water or methanol in acetonitrile
Source: JOURNAL OF ELECTROANALYTICAL CHEMISTRY, 558: 49-57 OCT 30 2003
Abstract: The reactions of 9-substituted anthracene cation radicals (XA (+) (.) s) with water and methanol in acetonitrile were analyzed using an electron transfer stopped-flow (ETSF) method. By adopting the tris(2,4-dibromophenyl)amine cation radical (TDBPA (+ .)) as a reaction initiator, five XA (+ .) s, i.e. phenyl, bromo, acetyl, benzyl, and methyl derivatives, could be generated quantitatively by mixing XA with TDBPA (+ .) and the chemical reaction processes of XA (+ .) s with water or methanol (ROH) could then be followed by observing dynamic transformation processes of the absorption spectra. Consequently, the reaction orders of XA (+ .) and ROH as well as the reaction rates could be determined. The order of the reactivity was anthracene (+ .) > 9-methylanthracene (+ .) > 9-benzylanthracene (+ .) > 9-acetylanthracene (+ .), 9-bromoanthracene (+ .) > 9-phenylanthracene (+ .), and diverse changes in the reaction orders of both XA (+ .) and ROH were observed indicating some mechanistic changes. The order of the reactivity was difficult to explain only on the basis of the character of XAs or XA (+ .)s. However, as a hypothesis, it can be assumed that when the electron transfer between XA (+ .) and XA(ROH) (+ .) is favorable to formation of XA(ROH)(2+), the reaction order of [XA (+ .)] is second, and the reaction rates become faster. (C) 2003 Elsevier B.V. All rights reserved.

Record 2 of 23
Author(s): Headspith, J; Salvini, G; Thomas, SL; Derbyshire, G; Dent, A; Rayment, T; Evans, J; Farrow, R; Anderson, C; Cliche, J; Dobson, BR
Title: XSTRIP - a silicon micro strip-based X-ray detector for ultra-fast X-ray spectroscopy studies
Source: NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT, 512 (1-2): 239-244 OCT 11 2003
Abstract: For a number of years, an exciting and important area of synchrotron radiation science has been X-ray absorption spectroscopy fine structure studies of dynamically changing samples on the sub-second time-scales. By utilizing this technique, precise measurement of detailed structural changes can be investigated during a chemical or phase change reaction without the need for repeated experiments or expensive stopped flow techniques. Until recently, instrumentation to facilitate these studies has been based on commercially available detectors developed predominantly for other applications. Whilst these systems have yielded quality science, they have been subject to a number of fundamental limitations, particularly their speed, linearity and dynamic range. We have developed a new detector, XSTRIP, to overcome some of these. This new instrument marries dedicated silicon microstrip technology with specialist low noise, custom developed, fast readout integrated circuits, to yield an instrument that will unlock whole new areas of science to researchers. This paper will discuss some of the drawbacks of historical systems, give details of the XSTRIP system and also present the operating parameters of the system. In addition, some of the initial scientific experimental results will also be presented. (C) 2003 Published by Elsevier B.V.

Record 3 of 23
Author(s): Rahman, MBA; Evans, J; Dent, AJ
Title: Time-resolved energy dispersive EXAFS measurement of oligomerisation reaction catalysed by nickel homogeneous catalyst on addition of triphenylphosphine and AlEt2(OEt) as Co-catalyst
Source: ASIAN JOURNAL OF CHEMISTRY, 15 (2): 1097-1102 APR-JUN 2003
Abstract: Energy dispersive EXAFS (EDE) data have been analysed for the nickel (3-diketonate catalysts, Ni(dpM)(2), using curved wave theory with ab initio phase shift's. The derived parameters for metal compounds were in good agreement, within experimental error of 0.02-0.05 Angstrom for the bond distances. Time-resolved EDE studies on Ni(dpm)(2) : PPh3 : AlEt2(OEt): 1-hexene of I : 2 : 4 : 20 were acquired within 0.24 s for each spectrum. The growth of a pre-edge feature and change at the top of the edge can be clearly seen to show that the reaction reached completion in less than 6 s. A stopped flow system was used for in situ time-resolved EDE studies on the nickel systems at room temperature. EDE measurements using a new rectangular Si(III) monochromator and Hamamatsu S4874 photodiode array at Station 9.3, Daresbury Laboratory have yielded consecutive spectral data taken at 1-1000 accumulations with 2-30 ms integration time to follow the changes during initiation of the catalytic reaction.

Record 4 of 23
Author(s): Rahman, MBBA; Bolton, PR; Evans, J; Dent, AJ; Harvey, I; Diaz-Moreno, S
Title: Application of stopped flow techniques and energy dispersive EXAFS for investigation of the reactions of transition metal complexes in solution: Activation of nickel beta-diketonates to form homogeneous catalysts, electron transfer reactions involving iron(III) and oxidative addition to iridium(I)
Source: FARADAY DISCUSSIONS, 122: 211-222 2003
Abstract: Stopped-flow techniques of rapid mixing have been combined with energy dispersive X-ray absorption spectroscopy to monitor the reaction of Ni(dpm)(2) {dpm = (BuC)-C-t(O)CHC(O)Bu-t} by aluminium alkyls (AlEt2X, X = OEt and Et) to form the active species for the catalytic di- and tri-merisation of hex-1-ene. Acquisition times down to ca. 30 ms were achieved on Station 9.3 of the SRS using a photodiode array detector. The EXAFS features of the resulting solution complexes are of the form [Ni(O-O)(R) (alkene)]. In the presence of PPh3, [Ni(O-O)(R)(PPh3)] appears to be the predominant type of species. The reduction of aqueous Fe(III) by hydroquinone was investigated on ID24 at the ESRF by Fe K-edge energy dispersive EXAFS with a CCD camera as detector; spectra were obtained in 1 ms or longer. No intermediate inner sphere complex was detected prior to the formation of aqueous Fe(II). Finally the oxidative addition of CH3SO3CF3 to [IrI2(CO)(2)](-) was monitored on Station 9.3 with a silicon microstrip detector. A single acquisition of 400 mus was feasible, with spectra recorded in multiples of 1.2 ms. In that time, the first stage of the reaction had been completed, with a slower stage thereafter. The results are consistent with the two-stage ionic oxidative addition mechanism.

Record 5 of 23
Author(s): Rao, LF; Zhang, ZC; Friese, JI; Ritherdon, B; Clark, SB; Hess, NJ; Rai, D
Title: Oligomerization of chromium(III) and its impact on the oxidation of chromium(III) by hydrogen peroxide in alkaline solutions
Source: JOURNAL OF THE CHEMICAL SOCIETY-DALTON TRANSACTIONS, (2): 267-274 2002
Abstract: Monomeric, dimeric and trimeric chromium(III) species in solution were separated by ion exchange and characterized with UV/Vis absorption and Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS). The kinetics of the oxidation of the separated species by hydrogen peroxide in alkaline solutions were studied by conventional and stopped-flow UV/Vis absorption spectroscopy. Results indicate that the intensity of Cr-Cr scattering in the EXAFS spectra (d(Cr-Cr) similar to2.99 Angstrom), a measure of the degree of oligomerization, increases as the solution alkalinity is increased. As the oligomerization proceeds, the rate of oxidation by hydrogen peroxide in alkaline solutions decreases in the order: monomer > dimer > trimer > aged/unseparated alkaline chromium(III) solution where higher oligomers dominate. The dominant redox pathway has an inverse order with respect to C-NaOH. The data suggest that the rate-determining step involves the weakening of the bridging bonds in the oligomer and a concomitant release of one hydroxyl group from the chromium(III) moiety upon the attack by hydrogen peroxide.

Record 6 of 23
Author(s): de Seny, D; Heinz, U; Wommer, S; Kiefer, M; Meyer-Klaucke, W; Galleni, M; Frere, JM; Bauer, R; Adolph, HW
Title: Metal ion binding and coordination geometry for wild type and mutants of metallo-beta-lactamase from Bacillus cereus 569/H/9 (BcII) - A combined thermodynamic, kinetic, and spectroscopic approach
Source: JOURNAL OF BIOLOGICAL CHEMISTRY, 276 (48): 45065-45078 NOV 30 2001
Abstract: One high affinity (nm) and one low affinity (IM) macroscopic dissociation constant for the binding of metal ions were found for the wild-type metallo-beta -lactamase from Bacillus cereus as well as six single-site mutants in which all ligands in the two metal binding sites were altered. Surprisingly, the mutations did not cause a specific alteration of the affinity of metal ions for the sole modified binding site as determined by extended x-ray absorption fine structure (EXAFS) and perturbed angular correlation of gamma -rays spectroscopy, respectively. Also UV-visible absorption spectra for the mono-cobalt enzymes clearly contain contributions from both metal sites. The observations of the very similar microscopic dissociation constants of both binding sites in contrast to the significantly differing macroscopic dissociation constants inevitably led to the conclusion that binding to the two metal sites exhibits negative cooperativity. The slow association rates for forming the binuclear enzyme determined by stopped-flow fluorescence measurements suggested that fast metal exchange between the two sites for the mononuclear enzyme hinders the binding of a second metal ion. EXAFS spectroscopy of the mono- and di-zinc wild type enzymes and two di-zinc mutants provide a definition of the metal ion environments, which is compared with the available x-ray crystallographic data.

Record 7 of 23
Author(s): Inamo, M; Kamiya, N; Inada, Y; Nomura, M; Funahashi, S
Title: Structural characterization and formation kinetics of sitting-atop (SAT) complexes of some porphyrins with copper(II) ion in aqueous acetonitrile relevant to porphyrin metalation mechanism. Structures of aquacopper(II) and Cu(II)-SAT complexes as determined by XAFS spectroscopy
Source: INORGANIC CHEMISTRY, 40 (22): 5636-5644 OCT 22 2001
Abstract: The formation of the sitting-atop (SAT) complexes of 5,10,15,20-tetraphenylporphyrin (H(2)tpp), 5,10,15,20-tetrakis(4-chlorophenyl)porphyrin (H(2)t(4-Clp)p), 5,10,15,20-tetramesitylporphyrin (H(2)tmp), and 2,3,7,8,12,13,17,18-octaethylporphyrin (H(2)oep) with the Cu(H) ion was spectrophotometrically confirmed in aqueous acetonitrile (AN), and the formation rates were determined as a function of the water concentration (Cw). The decrease in the conditional first-order rate constants with the increasing Cw was reproduced by taking into consideration the contribution of [Cu(H2O)(an)(5)](2+) in addition to [Cu(an)(6)](2+) to form the Cu(II)-SAT complexes. The second-order rate constants for the reaction of [Cu(an)(6)](2+) and [Cu(H2O)(an)(5)](2+) at 298 K were respectively determined as follows: (4.1 +/- 0.2) x 10(5) and (3.6 +/- 0.2) x 10(4) M-1 s(-1) for H(2)tpp, (1.15 +/- 0.06) x 10(5) M-1 s(-1) and negligible for H(2)t(4-Clp)p, and (4.8 +/- 0.3) x 10(3) and (1.3 +/- 0.3) x 10(2) M-1 s(-1) for H(2)tmp. Since the reaction of H(2)oep was too fast to observe the reaction trace due to the dead time of 2 ms for the present stopped-flow technique, the rate constant was estimated to be greater than 1.5 x 10(6) M-1 s(-1). According to the structure of the Cu(II)-SAT complexes determined by the fluorescent XAFS measurements, two pyrrolenine nitrogens of the meso-substituted porphyrins (H(2)tpp and H(2)tmp) bind to the Cu(Il) ion with a Cu-N(pyr) distance of ca. 2.04 Angstrom, while those of the beta -pyrrole-substituted porphyrin (H(2)oep) coordinate with the corresponding bond distance of 1.97 Angstrom. The shorter distance of H(2)oep is ascribed to the flexibility of the porphyrin ring, and the much greater rate for the formation of the Cu(H)-SAT complex of H(2)oep than those for the meso-substituted porphyrins is interpreted as due to a small energetic loss at the porphyrin deformation step during the formation of the Cu(H)SAT complex. The overall formation constants, beta (n), of [Cu(H2O)(n)(an)(6-n)](2+) for the water addition in aqueous AN were spectrophotometrically determined at 298 K as follows: log(beta (1)/M-1) = 1.19 +/- 0.18, log(beta (2)/M-2) = 1.86 +/- 0.35, and log(beta (3)/M-3) = 2.12 +/- 0.57. The structure parameters around the Cu(II) ion in [Cu(H2O)(n)(an)(6-n)](2+) were determined using XAFS spectroscopy.

Record 8 of 23
Author(s): Funahashi, S; Inada, Y; Inamo, M
Title: Dynamic study of metal-ion incorporation into porphyrins based on the dynamic characterization of metal ions and on sitting-atop complex formation
Source: ANALYTICAL SCIENCES, 17 (8): 917-927 AUG 2001
Abstract: We succeeded in the detection of the sitting-atop (SAT) copper(II) complex of TPP (5,10,15,20-tetraphenylporphyrin) in acetonitrile (AN) as a solvent with a very low Bronsted basicity, where two pyrrolenine nitrogens in the Cu(II)-SAT complex coordinate to the metal ion and two protons still remain on the pyrrole nitrogens. The structure parameters around the copper(II) ion in the Cu(II)-SAT complex, as determined by a fluorescent EXAFS method, suggest an axially elongated and equatorially distorted six-coordinate geometry. We measured the rates of the formation reaction of the SAT complexes for a series of transition metal(II) ions in AN using the stopped-flow technique. We propose the mechanism where there is a rapid deformation equilibrium of the porphyrin ring prior to the rate-determining step of the bond rupture of a coordinated solvent molecule on the metal(II) ion. Furthermore, we measured the rates of the deprotonation reaction of the Cu(II)-SAT complex by some Bronsted bases and indicated that the rate -determining step is the attack of the base on the proton of the pyrrole nitrogen in the SAT complex. Finally, a unified mechanism relevant to the porphyrin metalation mechanism has been proposed.

Record 9 of 23
Author(s): Matocha, CJ; Sparks, DL; Amonette, JE; Kukkadapu, RK
Title: Kinetics and mechanism of birnessite reduction by catechol
Source: SOIL SCIENCE SOCIETY OF AMERICA JOURNAL, 65 (1): 58-66 JAN-FEB 2001
Abstract: The complex interactions of oxidizable organic ligands with soil Mn(III,IV) (hydr)oxide minerals have received little study by in situ spectroscopic techniques. We used a combination of an in situ electron paramagnetic resonance stopped-flow (EPR-SF) spectroscopic technique and stirred-batch studies to measure the reductive dissolution kinetics of birnessite (delta -MnO2), a common Mn mineral in soils, by catechol (1,2-dihydroxybenzene). The reaction was rapid, independent of pH, and essentially complete within seconds under conditions of excess-catechol at pH 4 to 6. The overall empirical second-order rate equation describing the reductive dissolution rate was d[Mn(II)]/ dt = k[CAT](1.0)[SA](1.0) where k = 4 (+/-0.5) (10(-3) L m(-2) s(-1) and [CAT] and [SA] are the initial concentrations in molarity and meters square per liter. In the process, catechol was oxidized to the two-electron o-quinone product. The energy of activation (E.) for the reaction was 59 (+/-7) kJ mol(-1) and the activation entropy (SI) was -78 +/- 225 mol(-1) K-1, suggesting that the reaction was surface-chemical controlled and occurs by an associative mechanism. Rates of catechol disappearance from solution with simultaneous Mn(II) and o-quinone production were comparable. These data strongly suggest that precursor surface-complex formation is rate-limiting and that electron transfer is rapid, The rapid reductive dissolution of birnessite by catechol has significant implications for C and Mn cycling in soils and the availability of Mn to plants.

Record 10 of 23
Author(s): Inada, Y
Title: Local structure analysis of short-lived reaction intermediates by means of a time-resolved stopped-flow extended X-ray absorption fine-structure technique
Source: BUNSEKI KAGAKU, 49 (8): 563-573 AUG 2000
Abstract: Three types of time-resolved stopped-flow extended X-ray absorption fine-structure (EXAFS) instruments have been developed by combining an EXAFS spectrometer equipped with the original time-resolved detection system and a stopped-flow unit to analyze the local structure around the metal center in short-lived reaction intermediates. The whole X-ray energy region used to obtain an EXAFS spectrum could be measured simultaneously for the instrument in the dispersive mode, and a. time-resolution on the order of ms has been acheived in the case of die instrument using synchrotron radiation as an X-ray source. By using these time-resolved stopped-flow EXAFS instruments, we have analyzed the local structure around the metal center in some short-lived reaction intermediates, such as a peroxochromium intermediate formed in a reduction reaction of chromate ion by hydrogen peroxide in an acidic aqueous solution; a Cu(II)/Hg(II) heterodinuclear porphyrin intermediate formed in the Cu(II) ion incorporation process into 5,10,15,20-tetrakis (4-sulfonatophenyl) porphyrin in the presence of the Hg(II) ion in water, and a chloride-bridged precursor intermediate formed in the inner-sphere reduction of CoCl(NH3)(5)(2+) by Fe2+ in N,N-dimethylformamide. According to the obtained local structure of the reaction intermediate, the reaction mechanisms and the structural characteristics of the reaction intermediates have been microscopically clarified.

Record 11 of 23
Author(s): Ozutsumi, K; Ohtaki, H
Title: Structure analysis of reaction intermediates in metal substitution reactions in solution by the stopped-flow EXAFS method
Source: BULLETIN OF THE CHEMICAL SOCIETY OF JAPAN, 72 (9): 1947-1956 SEP 1999
Abstract: Improvements of XAFS (X-Ray Absorption Fine Structure) measurements in the time domain are summarized and several examples of structure determination of reaction intermediates by the EXAFS (Extended X-Ray Absorption Fine Structure) method are given. The local structure around the copper(II) or cobalt(II) ion is described in the reaction intermediate in the metal substitution reaction of the homodinuclear mercury(II) porphyrinato (5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin; H(6)tpps) complex in an acetate buffer determined by the stopped-flow EXAFS (SF-EXAFS) method, which was developed by us. Comparison of the bond lengths in the intermediates with those in the reactants and the products of the metal substitution reaction, the structure of which has been determined by the same method, revealed the cause of the instability of the reaction intermediates.

Record 12 of 23
Author(s): Cheung, KC; Strange, R; Harvey, I; Dobson, B; Derbyshire, G; Kay, J; Binsted, N; Linford, R; Hasnain, S
Title: Conversion of the oldest XAFS station at the first dedicated SR source to a state-of-the-art XAFS facility
Source: JOURNAL OF SYNCHROTRON RADIATION, 6: 161-163 Part 3 MAY 1 1999
Abstract: Station 7.1 was the first XAFS station to operate at the Synchrotron Radiation Source (SRS) starting in 1981. A major upgrade of 7.1 has been taking place aimed towards a multidimensional XAFS and single crystal facility for 3d elements (Ca to Zn). The optical upgrade included a collimating mirror and QuEXAFS monochromator, also a highly compact and rapid count-rate multi-element solid state detector has been designed, built and recently commissioned. A stopped-flow system is commissioned and a diffractometer stage is planned. The necessary analytical tools for 3D XAFS refinement using crystallographic data as the starting 3D model have been developed for solution XAFS.

Record 13 of 23
Author(s): Dent, A; Evans, J; Newton, M; Corker, J; Russell, A; Abdul Rahman, MB; Fiddy, S; Mathew, R; Farrow, R; Salvini, G; Atkinson, P
Title: High-quality energy-dispersive XAFS on the 1 s timescale applied to electrochemical and catalyst systems
Source: JOURNAL OF SYNCHROTRON RADIATION, 6: 381-383 Part 3 MAY 1 1999
Abstract: A long program involving the development of a fast linear and stable detector, and the recent upgrade of the bent monochromator to a 4-point one at station 9.3 Daresbury Laboratory is showing exciting results such that it has now become possible to obtain analysable XAFS data on 50mM solutions in time scales of a second or less. Three examples of such data are shown: a homogeneous Ni catalyst reaction using a stopped flow system, a heterogeneous catalyst reduction showing time dependent data collected in a microreactor, and an electrochemical system showing the oxidation of small Pt particles as the potential is changed. The improvements have come from two major developments - the monochromator and the detector.

Record 14 of 23
Author(s): Ohtaki, H
Title: Development of modern solution chemistry: a search of new fields
Source: COORDINATION CHEMISTRY REVIEWS, 186: 735-759 MAY 1999
Abstract: In order to investigate new fields and how one can approach a new field of chemistry, studies carried out by Nobel laureates are surveyed from the historical point of view and are classified into five categories. A brief discussion shows how they created new fields of chemistry and how one can open a gate to a new world. A short history of the development of solution chemistry is reviewed in connection with a view for the development of theories and methodologies used for investigations in solution chemistry. New fields and new methodologies have often been discovered by extension and combination of existing theories and methodologies with anew vision recognising existing chemistry from another side of the traditional face of chemistry. A:recently developed method to determine the structure of short-lived reaction intermediates in Solution, which has not been determinable by traditional analytical methods, is shown as an example. This is a method combining a stopped-flow technique and the EXAFS method. Some results obtained for metal substitution reactions of porphyrin complexes are given, and emphasis is placed on an effective combination of different methodologies to create a new method of investigation. (C) 1999 Elsevier Science S.A. All rights reserved.

Record 15 of 23
Author(s): Nishimoto, J; Tabata, M; Eguchi, T; Takauchi, J
Title: Kinetics and mechanism of the acid-base reaction of the diacid salts of porphyrin with pyridine in chloroform
Source: JOURNAL OF THE CHEMICAL SOCIETY-PERKIN TRANSACTIONS 2, (7): 1539-1543 JUL 1998
Abstract: The acid-base reaction of the diacid salts of 5,10,15,20-tetraphenylporphyrin [(H(4)tpp)(2+)(X-)(2); X- = Cl- and Br-] with pyridine is so slow in chloroform that the reaction has been followed using a conventional stopped-flow apparatus. The reaction rate is first-order with respect to the concentrations of porphyrin and pyridine, The reaction rate is enhanced by water and suppressed by 1,4-dioxane. The rate equation is given by the following equation:
-d[(H(4)tpp)(2+)(X-)(2)]/dt = k(1)'[(H(4)tpp)(2+)(X-)(2)][H2O][py] + k(2)'[(H(4)tpp)(2+)(X-)(2)][H2O](2)[py] = k(1)[(H(4)tpp)(2+)(X-)(2)(H2O)] [py] + k(2)[(H(4)tpp)(2+) (X-)(2)(H2O)(2)][py]
The values of k(1)' and k(2)' are (4.5 +/- 0.2) x 10(4) dm(6) mol(-2)s(-1) and (4.3 +/- 0.3) x 10(5) dm(9) mol(-3)s(-1) for Br-, and (1.95 +/- 0.07) x 10(4) dm(6) mol(-2)s(-1) and(1.11 +/- 0.14) x 10(5) dm(9) mol(-3)s(-1) for Cl- at 25 degrees C, respectively. The reaction mechanism is discussed from the viewpoint of hydration of the diacid porphyrin, pyridine and 1,4-dioxane.

Record 16 of 23
Author(s): Ozutsumi, K; Ohnishi, S; Ohtaki, H; Tabata, M
Title: Structure of the short-lived intermediate formed during the metal substitution reaction of the Mercury(II) porphyrin complex with cobalt(II) ion in aqueous solution determined by the stopped-flow EXAFS method
Source: ZEITSCHRIFT FUR NATURFORSCHUNG SECTION B-A JOURNAL OF CHEMICAL SCIENCES, 53 (4): 469-475 APR 1998
Abstract: The local structure around the cobalt(II) ion in the reaction intermediate formed during the metal substitution reaction of the homodinuclear mercury(II) porphyrin (5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin; H(2)tpps(4-)) complex with a cobalt(II) ion in an acetate buffer has been determined by the stopped-flow EXAFS method. The structure of the reactant and the product of the above reaction has also been determined by the same method. The coordination geometry around the cobalt(II) ion in the heterodinuclear intermediate, [Hg(tpps)Co-II](2-) is six-coordinate octahedral with four additional water and/or acetate oxygen atoms. The Co-II- N and Co-II-O bond lengths in the intermediate are 212(2) and 221(1) pm, respectively. The product, [Co-II(tpps)](4-), has a six-coordinate octahedral structure, the Co-II-N and Co-II-O bond lengths being 203(1) and 215(1) pm, respectively. The Co-II-N bond length in the intermediate is ca. 9 pm longer than that in the product. The Co-II-O bond length in the intermediate is also ca. 9 pm longer than that of 212(1) pm in the reactant, the cobalt(II) acetato complex, and ca. 6 pm longer than that in the product. The longer Co-II-O bond in the intermediate as compared to those in the reactant and in the product appears to be responsible for the instability of the intermediate. The oxidized product, [Co-III(tpps)](3-), has a six-coordinate structure with two additional Co-III-O bonds. The Co-III-N and Co-III-O bond lengths are 189(1) and 197(2) pm, respectively, and are much shorter than those in [Co-II(tpps)](4-).

Record 17 of 23
Author(s): Klabunde, T; Krebs, B
Title: The dimetal center in purple acid phosphatases
Source: METAL SITES IN PROTEINS AND MODELS, 89: 177-198 1997
Book series title: STRUCTURE AND BONDING
Abstract: Purple acid phosphatases (PAPs) contain a dinuclear metal center in their active site and hydrolyze phosphoric acid esters at low pH. Characteristic of this group of acid phosphatases is their resistance to inhibition by tartrate and their purple color, due to the presence of a tyrosine residue ligated to a ferric iron. The mammalian enzymes all contain a mixed-valent di-iron unit in their catalytic active form, first identified in the bovine spleen and porcine uterus enzymes, while a heterodinuclear Fe(III)Zn(II) unit has been characterized for the most studied plant enzyme from kidney bean. The enzymes from porcine uterus and bovine spleen can be converted into active FeZn forms and the plant enzyme can be transformed into an active FeFe form. In recent years the dimetal center of PAPs has been studied using numerous spectroscopic methods such as Mossbauer spectroscopy, EPR, NMR, EXAFS, magnetic, electrochemical and resonance Raman studies characterizing most of the metal coordinating residues, the metal-metal separation and providing evidence of the similarity between enzymes from different sources. Analysis of the products of hydrolysis of a substrate containing a chiral phosphorus by P-31 NMR, stopped-flow measurements and kinetic studies all support a reaction path involving nucleophilic attack of a Fe(III)-bound hydroxide ligand on the phosphate ester. The recently solved crystal structure of the plant enzyme provides the structural basis for the understanding of the two-metal ion mechanism of this class of enzymes.

Record 18 of 23
Author(s): Inada, Y; Funahashi, S
Title: Laboratory stopped-flow XAFS apparatus. Structure determination of the short-lived peroxochromium intermediate formed during reduction process of chromate(VI) ion by hydrogen peroxide
Source: ZEITSCHRIFT FUR NATURFORSCHUNG SECTION B-A JOURNAL OF CHEMICAL SCIENCES, 52 (6): 711-718 JUN 1997
Abstract: In order to determine the structure parameters of the reaction intermediate formed during the reduction process of chromate(VI) ion by hydrogen peroxide, a laboratory stopped-flow X-ray absorption fine structure (XAFS) apparatus, which was constructed by a rotating anode X-ray generator, an energy scanning spectrometer, a stopped-flow assembly, and X-ray detectors, was newly developed. Using the apparatus, the Cr K-edge XAFS spectrum of the reaction intermediate was measured. One oxo group, two peroxo groups, and one water molecule are coordinated to the Cr(VI) center in the intermediate, CrO(O-2)(2)(H2O), with Cr-O bond lengths of 157, 168, and 193 pm, respectively. The geometry around the Cr(VI) center is probably 6-coordinate pentagonal pyramidal.

Record 19 of 23
Author(s): YOSHIDA, N; NAGAMURA, T
Title: STOPPED-FLOW TIME-RESOLVED EXTENDED X-RAY-ABSORPTION FINE-STRUCTURE TECHNIQUE FOR STUDYING REACTION DYNAMICS IN SOLUTION
Source: REVIEW OF SCIENTIFIC INSTRUMENTS, 66 (1): 52-54 Part 1 JAN 1995

Record 20 of 23
Author(s): ROWLETT, RS; CHANCE, MR; WIRT, MD; SIDELINGER, DE; ROYAL, JR; WOODROFFE, M; WANG, YFA; SAHA, RP; LAM, MG
Title: KINETIC AND STRUCTURAL CHARACTERIZATION OF SPINACH CARBONIC-ANHYDRASE
Source: BIOCHEMISTRY, 33 (47): 13967-13976 NOV 29 1994
Abstract: We have carried out kinetics studies of spinach carbonic anhydrase (CA) using stopped-flow spectrophotometry at steady state and C-13-NMR exchange at chemical equilibrium. We found that the rate of CO2 reversible arrow HCO3- exchange catalyzed by spinach CA at pH 7.0 to be 3-5 times faster than the maximal k(cat) for either CO2 hydration or HCO3- dehydration at steady state, suggesting a rate-determining H+ transfer step in the catalytic mechanism. Correspondingly, we measured a pH-independent solvent deuterium isotope effect on k(cat) of approximately 2.0, and found that the rate of catalysis was significantly decreased at external buffer concentrations below 5 mM. Our results are consistent with a zinc-hydroxide mechanism of action with for spinach CA, similar to that of animal carbonic anhydrases. We have also collected X-ray absorption spectra of spinach CA. Analysis of the extended fine structure (EXAFS) suggests that the coordination sphere of Zn in spinach CA must have one or more sulfur ligands, in contrast to animal CAs which have only nitrogen and oxygen ligands. The models which best fit the data have average Zn-N(O) distances of 1.99-2.06 Angstrom, average Zn-S distances of 2.31-2.32 Angstrom, and a total coordination number of 4-6. We conclude that animal and spinach CAs are convergently evolved enzymes which are structurally quite different, but functionally equivalent.

Record 21 of 23
Author(s): OHTAKI, H; INADA, Y; FUNAHASHI, S; TABATA, M; OZUTSUMI, K; NAKAJIMA, K
Title: DIRECT STRUCTURAL DETERMINATION OF THE SHORT-LIVED INTERMEDIATE FORMED IN THE DYNAMIC INCORPORATION OF A COPPER(II) ION INTO A MERCURY(II) PORPHYRIN BY THE STOPPED-FLOW EXAFS METHOD
Source: JOURNAL OF THE CHEMICAL SOCIETY-CHEMICAL COMMUNICATIONS, (8): 1023-1025 APR 21 1994
Abstract: Structure parameters around the Cull ion in a reaction intermediate formed in the course of Cu-II ion incorporation into an Hg-II porphyrin were directly determined using a new stopped-flow EXAFS apparatus, and elongation of the Cu-N bond in the unstable heterodinuclear porphyrin intermediate was observed


Record 22 of 23
Author(s): OHTAKI, H
Title: A RAPID DIRECT ANALYSIS OF THE STRUCTURE OF REACTION INTERMEDIATES BY THE EXAFS METHOD COMBINED WITH A STOPPED-FLOW TECHNIQUE
Source: PURE AND APPLIED CHEMISTRY, 65 (12): 2589-2592 DEC 1993
Abstract: A partial structure of the copper(II) ion of a reaction intermediate at the formation of a copper(II) porphyrin complex in the metal substitution reaction of the mercury(II) porphyrin complex with the copper(II) ion in an aqueous acetate buffer solution has been determined by using a newly developed rapid EXAFS apparatus, which is constructed by combining a stopped-flow and a laboratory-scale EXAFS equipments. The porphyrin used is 5,10,15,20-tetrakis(4-sulfonatophenyl)porphine. Two Cu-N bonds in the reaction intermediate have been found to be 204 pm, which are longer than the Cu-N bonds (200 pm) in the final product of the copper(II) porphyrin complex, and it has been estimated that the copper(II) ion in the intermediate is about 40 pm out from the center of the porphine ring, and thus the mercury(II) and copper(II) ions are almost in contact in the heterodinuclear complex. Since we did not clearly see the Hg...Cu nonbonding interaction in the EXAFS spectrum, the position of the atoms may be significantly fluctuating due to the repulsion between the atoms.

Record 23 of 23
Author(s): INADA, Y; FUNAHASHI, S
Title: IN-LABORATORY STOPPED-FLOW EXTENDED X-RAY-ABSORPTION FINE-STRUCTURE APPARATUS IN THE DISPERSIVE MODE FOR DETERMINATION OF THE STRUCTURE OF SHORT-LIVED INTERMEDIATES
Source: REVIEW OF SCIENTIFIC INSTRUMENTS, 65 (1): 18-24 JAN 1994
Abstract: We have constructed an in-laboratory stopped-flow extended x-ray absorption fine structure (EXAFS) apparatus by combining laboratory EXAFS equipment in the dispersive mode and a stopped-flow unit with two 150-mum-thick boron nitride windows. The rotating anode x-ray generator with a Mo target was used as an x-ray source. The self-scanning photodiode array was adopted as a position sensitive-linear x-ray detector. In order to obtain a suitable x-ray flux intensity, the accumulation of weak intensities during a short gate time was carried out by repeating the measurements. We have confirmed the performance of our apparatus by repeatedly measuring a number of static EXAFS spectra of 0.20 mol dm-3 aqueous solution of copper(II) sulfate. By use of our newly developed apparatus, we have succeeded in the direct determination of the structure of the reaction intermediate, the heterodinuclear copper(II)/mercury(II) complex, formed in the copper(II) ion incorporation into the homodinuclear mercury(II) complex of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphine.