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  • 1
    ISSN: 1432-1327
    Keywords: Key words Pentacyanoferrate(III) complexes ; Oxymyoglobin ; Electron transfer ; Kinetics ; Electrostatic potential
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Abstract  The mechanism of the reduction of pentacyanoferrate(III) complexes by oxymyoglobin has been studied by conventional and high-pressure kinetic methods, and also by structural modelling. The results of this and an earlier study show that an outer-sphere mechanism is operating for electron transfer between oxymyoglobin and FeIII(CN)5L n –, independent of the lability of the ligand L. The electron transfer process is preceded by precursor formation at a specific site on the protein close to the protein heme pocket.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1434-1948
    Keywords: Iron(III) ; Peroxo Complexes ; Kinetics ; Hydrogen Peroxide ; Catalase ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The new diiron complex [Fe2(tbpo){O2As(CH3)2}(CH3O)(CH3OH)](ClO4)3 · 5 CH3OH · 2 H2O (1) containing a (μ-alkoxo)(μ-dimethylarsinato)diiron(III) core was synthesized using the heptadentate ligand N,N,N′,N′-Tetrakis(2-benzimidazolylmethyl)-1,3-diamino-2-propanol (Htbpo). The complex was characterized structurally by X-ray crystallography. 1reproduces the coordination mode and the stoichiometry of the proposed purple acid phosphatase-arsenate inhibitor complex. More importantly, 1 is a good functional model for the activation of small molecules, since the solvent molecule in the coordination sphere of each iron ion can be substituted very easily by a small substrate molecule. This is confirmed by the comparatively high pH-dependent catalase-like activity of 1. In order to study the influence of the cacodylate bridge on the formation of the metastable adduct with hydrogen peroxide, the analogous hydroxo-bridged complex [Fe2(tbpo)(OH)(NO3)2](NO3)2 · CH3OH · 2 H2O (2) was employed. The reactions of 1and 2 with H2O2 were studied as a function of [H2O2], pH, temperature, and pressure, and the kinetic results including the activation parameters are reported. In the case of compound 2 the reaction proceeds in one step, and the observed first order rate constant, kobs, shows a linear dependence on the hydrogen peroxide concentration with a zero intercept. For complex 1 the kinetic traces could be fitted to two exponential functions. One of the observed pseudo-first-order rate constants, kobs1, exhibits a linear dependence on the hydrogen peroxide concentration with a zero intercept, whereas the other rate constant, kobs2, was independent of the hydrogen peroxide concentration. A mechanistic interpretation is presented.Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2005/1999/99068_s.pdf or from the author.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1998 (1998), S. 389-396 
    ISSN: 1434-1948
    Keywords: Substitution mechanism ; Platinum ; Pressure dependence ; Kinetics ; Dinuclear complex ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Both reaction steps observed for the substitution of water by thiourea in the complexes [Pt(en)(OH2)2]2+ and [Pt(phen)(OH2)2]2+ (en = ethylenediamine, phen = 1,10-phenanthroline) were investigated under pseudo-first-order conditions using the stopped-flow technique. The substitution of the second water molecule in each complex was also studied under high pressure. The observed pseudo-first-order rate constants kobs (s-1) obeyed the equation k1,2obs= k1,2[tu] (tu = thiourea), where “1” and “2” refer to the first and the second substitution reactions, respectively. Kinetic parameters associated with the substitution process are: k1en (25.0°C, pH = 3.0, I = 0.1 M) = 25.6 M-1 s-1, ΔH# = 51 kJ mol-1, ΔS# = -48 J K-1 mol-1; k2en (same conditions) = 12.1 M-1 s-1, ΔH# = 30 kJ mol-1, ΔS# = -124 J K-1 mol-1, ΔV# = -7 cm3 mol-1; k1phen (25.0°C, pH = 1.0, I = 0.1 M) = 2900 M-1 s-1, ΔH# = 41 kJ mol-1, ΔS# = -41 J K-1 mol-1; k2phen (same conditions) = 1170 M-1 s-1, ΔH# = 37 kJ mol-1, ΔS#= -61 J K-1 mol-1, ΔV# = -5 cm3 mol-1. The temperature and pressure dependence of all the processes studied suggest an associative substitution mechanism. The hydroxo-bridged dinuclear complex [{Pt(phen)(μ-OH)}2]2+ is formed from [Pt(phen)(OH2)2]2+ in aqueous solution unless the solution is very dilute and highly acidic. The X-ray structure of [{Pt(phen)(μ-OH)}2](F3CSO3)2 · 2 H2O was determined. It belongs to the triclinic space group P1­ and has one formula unit in the unit cell. The unit cell dimensions are a = 7.126(5), b = 9.665(5), c = 12.774(7) Å; α = 71.85(5), β = 85.52(5), γ = 73.12(5) deg; V = 799.9(8) Å3. The structure was solved with the Patterson method and refined to R = 0.061. A square planar coordination of the platinum centers is observed, with no deviations from planarity but distortions due to the small bite angle of phen and the four-membered ring. No significant lengthening of the Pt-O bond [mean value: 2.03(1) Å] is observed in comparison with [{Pt(NH3)2(μ-OH)}2]2+.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1999 (1999), S. 447-454 
    ISSN: 1434-1948
    Keywords: Kinetics ; Oxidations ; Iron ; Oxygen ; High pressure ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The complicated oxidation kinetics of the reaction of [FeII(cdta)] [cdta = 1,2-(N,N′- cyclohexanediamine)tetraacetate] with molecular oxygen was investigated as a function of [FeII], [O2], pH, temperature and pressure. In the presence of an excess of [FeII(cdta)] three steps could be observed, for which the following rate constants were found at 25°C; k1 = 1080 ± 16 M-1 s-1, k2 = 103 ± 4 M-1 s-1 and k3 = 59 ± 5 M-1 s-1. These reaction steps can be accounted for in terms of the following mechanism: (1) [FeII(cdta)H2O]2- reacts with O2 by a substitution process to form [FeII(cdta)O2]2-; (2) electron-transfer to form an FeIII-superoxo species; (3) subsequent bridge formation followed by electron-transfer to give [(cdta)FeIII-O22--FeIII(cdta)]4-; and (4) a fast decomposition of the peroxide intermediate yielding the monomeric [FeIII(cdta)] and H2O2. Rate and activation parameters for these steps are reported and discussed in terms of the postulated mechanism and in reference to available literature data.Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2005/1999/98232_s.pdf or from the author.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 1434-1948
    Keywords: Cobalt(III) complexes ; Tetrapodal pentadentate ligand ; Hydrolysis ; Kinetics ; Mechanism ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Multi-wavelength stopped-flow spectrophotometry was used to study the kinetics of base hydrolysis of the octahedral cobalt(III) complex CoLCl2+ (2), in which the tetrapodal pentadentate ligand L has an NN4 donor set and forms a square-pyramidal coordination cap [L = 2,6-bis(1′,3′-diamino-2′-methylprop-2′-yl)pyridine, 1]. The kinetic investigation, carried out at different temperatures, pressures and ionic strengths I, led to second-order kinetics, rate = kOH [2][OH-], with kOH = 0.139 ± 0.001 M-1s-1 (I = 0.1 M) and kOH = 0.0570 ± 0.0004 M-1s-1 (I = 1.0 M) at 298 K. The temperature and pressure dependence of kOH resulted in ΔH
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 130 (1997), S. 1791-1799 
    ISSN: 0009-2940
    Keywords: Platinum substitution ; Metal-carbon bonds ; trans-Labilization ; Kinetics ; Activation parameters ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The substitution reactions of cis-[PtMe,(dmso),] with pyridine (py) to produce cis-[PtMe2py2] in toluene proceeds in two steps. In the absence of added dimethylsulfoxide (dmso), these steps can not be separated due to the rate constants being very similar. In the presence of added dmso, the rate of the first step, the formation of the monopyridine complex is retarded, which is indicative of a dissociative mechanism. A parallel associative reaction path with pyridine could also be observed. This pathway is independent of the concentration of added dmso. Above a 40-fold excess of dmso, the dissociative pathway is suppressed and only the associative reaction occurs. A plot of Kobs vs the pyridine concentration for this pathway is linear at low [py], but shows a saturation at high [py]. This suggests that the reaction occurs via the formation of a precursor-complex, for which the formation constant was found to be 0.32 · 0.03 M-1. The volume of activation at a high pyridine concentration is -11.4 · 0.8 cm3 mol-1, which indicates that the ligand interchange process is of the associative type. The second step, the formation of the bispyridine complex, can clearly be separated from the first reaction step. This step occurs via a dissociative mechanism, as demonstrated by the decrease in kobs with increasing pyridine concentration. The dissociation of dmso was characterized by a rate constant of (8.1 · 0.9).10-45-1 at 25°C, ·H = 116 ·9 kJ mol-1 and AS. = 86 · 29 JK-1 mol-1. At higher pyridine concentrations evidence for a parallel associative reaction was found, for which the rate constant is (1.3 · 0.2).10-3 M-1S-1 25°C. The results are discussed in reference to available literature data.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
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  • 7
    ISSN: 1434-1948
    Keywords: Cytochrome c ; Chromium(v) ; Saturation kinetics ; Separation of K and kET ; Volume profile ; Kinetics ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The irreversible outer-sphere electron-transfer reaction between trans-bis(2-ethyl-2-hydroxybutanoato(2-))oxochromate(v) and cytochrome cII was investigated as a function of pH, concentration, temperature and pressure. The plot of the observed pseudo-first order rate constant as a function of the CrV concentration shows a clear trend towards saturation at higher CrV concentrations, from which the precursor formation constant and the electron-transfer rate constant could be separated (K = 37 ± 5 M-1 and kET = 1510 ± 180 s-1 at pH 4.8 and 279 K). In the low CrV concentration range the second-order electron-transfer rate contants were measured as a function of temperature (ΔH# = 20.9 ± 0.6 kJ mol-1; ΔS# = -79.9 ± 2.1 J K-1 mol-1; ΔG# (279 K) = 43.2 kJ mol-1). High-pressure experiments were performed at two different pH values. The kinetic (stopped-flow) and thermodynamic (electrochemical) measurements as a function of pressure enabled the construction of a volume profile for the system at 279 K. The activation volumes for the redox process are -9.2 ± 0.2 (pH 5.0) and -11.1 ± 0.8 cm3 mol-1 (pH 4.7), and the overall reaction volumes were estimated to be -7 ± 2 (pH 5.0) and -10 ± 2 cm3 mol-1 (pH 4.7) . The transition state of the redox reaction lies to a large extent on the product side and can be described as “late”. The results are discussed in comparison to earlier measurements using cobalt and ruthenium complexes as reaction partners for cytochrome c.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
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