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1.
Proton NMR studies of N,N-diethylformamide (def) exchange on [M(Me 6tren)def] 2+ where M = Co and Cu yield: kex (298.2K) = 26.3 ± 2.2, 980 ± 70 s −1; Δ H≠ = 58.3 ± 1.7, 36.3 ± 0.9 kJ mol −1; Δ S≠= −22.2 ± 4.6, −65.9 ± 2.5 J K −1 mol −1; and Δ V≠ = −1.3 ± 0.2, 5.3 ± 0.3 cm 3 mol −1 respectively. These data which are consistent with a and d activation modes operating when M = Co and Cu respectively are compared with data for related systems. 相似文献
2.
1H NMR line broadening is found to be an effective complimentary method to chemical trapping for determining the rates and activation parameters for organo-metal bond homolysis events that produce freely diffusing radicals. Application of this method is illustrated by measurement of bond homolysis activation parameters for a series of organo-cobalt porphyrin complexes ((TPP)Co-C(CH 3) 2CN (Δ H≠ = 19.5±0.9 kcal mol −1, Δ S≠ = 12±3 cal°K −1 mol −1), (TMP)Co-C(CH 3) 2CN (Δ H≠ = 20±1 kcal mol −1,Δ S≠ = 13±2 cal°K −1 mol −1), (TAP)Co-C(CH 3) 2CO 2CH 3 (Δ H≠ = 18.2±0.5 kcal mol −1, Δ S≠ = 12±2 cal °K −1 mol −1), (TAP)Co-CH(CH 3)C 6H 5 (Δ H≠ = 22.5±0.5, Δ S≠ = 17±2 cal °K −1 mol −1)). The line broadening method is particularly useful in determining activation parameters for dissociation of weakly bonded organometallics where the rate of homolysis can exceed the range measurable by conventional chemical trapping methods. 相似文献
3.
Oxygenation of [Cu II(fla)(idpa)]ClO 4 (fla=flavonolate; IDPA=3,3′-iminobis( N, N-dimethylpropylamine)) in dimethylformamide gives [Cu II(idpa)( O-bs)]ClO 4 ( O-bs= O-benzoylsalicylate) and CO. The oxygenolysis of [Cu II(fla)(idpa)]ClO 4 in DMF was followed by electronic spectroscopy and the rate law −d[{Cu II(fla)(idpa)}ClO 4]/d t= kobs[{Cu II(fla)(idpa)}ClO 4][O 2] was obtained. The rate constant, activation enthalpy and entropy at 373 K are kobs=6.13±0.16×10 −3 M −1 s −1, Δ H‡=64±5 kJ mol −1, Δ S‡=−120±13 J mol −1 K −1, respectively. The reaction fits a Hammett linear free energy relationship and a higher electron density on copper gives faster oxygenation rates. The complex [Cu II(fla)(idpa)]ClO 4 has also been found to be a selective catalyst for the oxygenation of flavonol to the corresponding O-benzoylsalicylic acid and CO. The kinetics of the oxygenolysis in DMF was followed by electronic spectroscopy and the following rate law was obtained: −d[flaH]/d t= kobs[{Cu II(fla)(idpa)}ClO 4][O 2]. The rate constant, activation enthalpy and entropy at 403 K are kobs=4.22±0.15×10 −2 M −1 s −1, Δ H‡=71±6 kJ mol −1, Δ S‡=−97±15 J mol −1 K −1, respectively. 相似文献
4.
The rate of Hg 2+-assisted chloride release from several mer-[CrCl(diamine)(triamine)] 2+ complexes has been measured as a function of pressure, Hg 2+ concentration and temperature. The calculated activation volumes are independent of [Hg 2+] and temperature and kinetic parametes 10 4 kHg (25 °c) (M −1 s −1), Δ H‡ (kJ mol −1), Δ S‡ (J K −1 mol −1), Δ V‡ (cc mol −1) are: (en)(dpt): 6.44. 75.5, −52, −5.0; (ibn)(dpt): 5.81, 89.5, −6, −0.03; (Me 2tn)(dpt): 22.2, 84.9, −11, −0.5; (tn)(dpt): 29.1, 87, −1, +0.3; (en)(2,3-tri): 1.94, 87.0, −24, −5.7; (en)(Medpt): 0.417, 94.6, −11, −0.8; (tn)(Medpt): 9.14, 98.3, +26, +1.8. 相似文献
5.
Carbonylation of the anionic iridium(III) methyl complex, [MeIr(CO) 2I 3] − (1) is an important step in the new iridium-based process for acetic acid manufacture. A model study of the migratory insertion reactions of 1 with P-donor ligands is reported. Complex 1 reacts with phosphites to give neutral acetyl complexes, [Ir(COMe)(CO)I 2L 2] (L = P(OPh) 3 (2), P(OMe) 3 (3)). Complex 2 has been isolated and fully characterised from the reaction of Ph 4As[MeIr(CO) 2I 3] with AgBF 4 and P(OPh) 3; comparison of spectroscopic properties suggests an analogous formulation for 3. IR and 31P NMR spectroscopy indicate initial formation of unstable isomers of 2 which isomerise to the thermodynamic product with trans phosphite ligands. Kinetic measurements for the reactions of 1 with phosphites in CH 2Cl 2 show first order dependence on [1], only when the reactions are carried out in the presence of excess iodide. The rates exhibit a saturation dependence on [L] and are inhibited by iodide. The reactions are accelerated by addition of alcohols (e.g. 18× enhancement for L = P (OMe) 3 in 1:3 MeOH-CH 2Cl 2). A reaction mechanism is proposed which involves substitution of an iodide ligand by phosphite, prior to migratory CO insertion. The observed rate constants fit well to a rate law derived from this mechanism. Analysis of the kinetic data shows that k1, the rate constant for iodide dissociation, is independent of L, but is increased by a factor of 18 on adding 25% MeOH to CH 2Cl 2. Activation parameters for the k1 step are Δ H≠ = 71 (±3) kJ mol −, Δ S≠ = −81 (±9) J mol −1 K −1 in CH 2Cl 2 and Δ H≠ = 60(±4) kJ mol −1, Δ S≠ = −93(± 12) J mol −1 K −1 in 1:3 MeOH-CH 2Cl 2. Solvent assistance of the iodide dissociation step gives the observed rate enhancement in protic solvents. The mechanism is similar to that proposed for the carbonylation of 1. 相似文献
6.
The kinetics of substitution reactions of [η-CpFe(CO) 3]PF 6 with PPh 3 in the presence of R-PyOs have been studied. For all the R-PyOs (R = 4-OMe, 4-Me, 3,4-(CH) 4, 4-Ph, 3-Me, 2,3-(CH) 4, 2,6-Me 2, 2-Me), the reactions yeild the same product [η 5-CpFe(CO) 2PPh 3]PF 6, according to a second-order rate law that is first order in concentrations of [η 5-CpFe(CO) 3]PF 6 and of R-PyO but zero order in PPh 3 concentration. These results, along with the dependence of the reaction rate on the nature of R-PyO, are consistent with an associative mechanism. Activation parameters further support the bimmolecular nature of the reactions: Δ H≠ = 13.4 ± 0.4 kcal mol −1, Δ S≠ = −19.1 ± 1.3 cal k −1 mol −1 for 4-PhPyO; Δ H≠ = 12.3 ± 0.3 kcal mol −1, Δ S≠ = 24.7 ±1.0 cal K −1 mol −1 for 2-MePyO. For the various substituted pyridine N-oxides studied in this paper, the rates of reaction increase with the increasing electron-donating abilities of the substituents on the pyridine ring or N-oxide basicities, but decrease with increasing 17O chemical shifts of the N-oxides. Electronic and steric factors contributing to the reactivity of pyridine N-oxides have been quantitatively assessed. 相似文献
7.
The rate of electron transfer between reduced cytochrome ƒ and plastocyanin (both purified from parsley) has been measured as k = 3.6 · 10 7 M −1 · s −1, at 298 °K and pH 7.0, with activation parameters Δ H‡ = 44 kJ · mole −1 and Δ S‡ = +46 J · mole −1 · °K −1. Replacement of cytochrome ƒ with red algal cytochrome c-553, Pseudomonas cytochrome c-551 and mammalian cytochrome c gave rates at least 30 times slower: k = 5 · 10 5, 7.5 · 10 5 and 1.0 · 10 6 M −1 · s −1, respectively. Similar measurements made with azurin instead of plastocyanin gave k = 6 · 106 and approx. 2 · 107 M−1 · s−1 for reaction of reduced azurin with cytochrome ƒ and algal cytochrome respectively. Rate constants of 115 and 80 M−1 · s−1 were found for reduction of plastocyanin by ascorbate and hydroquinone at 298 °K and pH 7.0. The rate constants for the oxidation of plastocyanin, cytochrome ƒ, Pseudomonas cytochrome c-551 and red algal cytochrome c-553 by ferricyanide were found to be between 3 · 104 and 8 · 104 M−1 · s−1. The results are discussed in relation to photosynthetic electron transport. 相似文献
8.
Density functional theory (DFT) computations at the B3LYP/Lanl2DZ level were used to elucidate the oxygen atom transfer (OAT) and coupled electron proton transfer (CEPT) reaction steps involved in the biomimetic catalytic cycle performed by polymer-supported Mo VIO 2( NN′) 2 complexes [ NN′ = phenyl-(pyrrolato-2-ylmethylene)-amine] with water as oxygen source, trimethyl-phosphane as oxygen acceptor and one-electron oxidising agents. The DFT method employed has been validated against experimental data [X-ray crystal structures of a NN′ ligand and a Mo VIO 2( NN′) 2 complex as well as kinetic data]. The rate-limiting step in the forward-OAT from [Mo VIO 2] to PMe 3 is the attack of PMe 3 at an oxo ligand with Δ G≠ (298 K) = 64.6 kJ mol −1. Dissociation of the product OPMe 3 is facile with Δ G≠ (298 K) = 26.3 kJ mol −1 giving a mono-oxo [Mo IVO] complex which fills its coordination sphere with a further PMe 3 substrate with Δ G≠ (298 K) = 39.2 kJ mol −1. One-electron oxidation to a Mo(V) phosphane complex precedes the coordination of water/hydroxide. Additionally, the comproportionation of [Mo VIO 2] and [Mo IVO] to dinuclear oxo-bridged [OMo V–O–Mo VO] species has been calculated as the thermodynamic sink in this system and the back-OAT from dmso to mono-oxo [Mo IVO] to give [Mo VIO 2] has been shown to involve an equilibrium between stereoisomeric [Mo VIO 2] complexes with an activation barrier of Δ G≠ (298 K) = 113.1 kJ mol −1. 相似文献
9.
σ-Methyl-(η 5-indenyl) chromium tricarbonyl (III) rearranges quantitatively into η 6-1-endo-methylindene) chromium tricarbonyl (IV) in C 6D 6 solution at 30–60°C. Methyl group attachment to the positions 2 or 3 of indenyl ligand in (III) has no influence on the activation parameters of this ricochet inter-ring haptotropic rearrangement (Δ G#=23.6 kcal mol −1; Δ H#=18.9±0.2 kcal mol −1; Δ S#=−18.6±0.2 cal K −1 mol−1). (IV) undergoes further irreversible isomerization at 60–120° into (ν 6-3-methylindene) chromium tricarbonyl (V) with a higher activation barrier (Δ G#=28.5±0.1 kcal mol −1) via two consecutive [1,5]-sigmatropic hydrogen shifts. The mechanisms of both rearrangements have been studied in detail using density functional theory (DFT) calculations with extended basis sets. Calculations show that the rearrangement (III) → (IV) proceeds in two steps. Methyl group migration from chromium into position 1 of the indenyl ligand is the rate-determining step leading to the formation of the 16-electron intermediate (VII). The calculated activation barrier ( Ea=19.6 kcal mol −1) is in good agreement with the experimental one. Further rearrangement (VII) → (V) proceeds via a trimethylenemethane-type transition state (XVIII) with an activation barrier 11.8 kcal mol −1. The coordination of the chromium tricarbonyl group at the six-membered ring has only minor influence on the kinetic parameters of the hydrogen [1,5]-sigmatropic shift in indene. 相似文献
10.
The purpose of this study was to measure heats involved in the hydrolysis process for the industrial production of 6-aminopenicillanic acid (6-APA) using dynamic calorimetry techniques. The experimental design was planned using Hess' law. The information derived from the calorimeter was correlated mathematically to determine the heat released during enzymatic hydrolysis. This is important for temperature control systems and reactor design. The results obtained with the calorimetric measurements at 308 K and pH 7.5 are the penicillin hydrolysis, ΔH hydrol, at 35.9 ± 5.7 kJ mol −1 and phenyl acetic acid neutralization, ΔH neut, at −47.1 ± 3.8 kJ mol −1. 相似文献
11.
The reactions of complex (C 5Me 5)Ir(Cl) (CO) (Me) (1a) with cyclohexylisocyanide and phosphines (L=CyNC, PHPh 2, PMePh 2, PMe 2Ph) give the products of alkyl migratory insertion (C 5Me 5Ir(Cl) (COMe) (L), in toluence or tetrahydrofuran at 323 K or higher temperature. The phenyl analogue (C 5Me 5)Ir(Cl)(CO)(Ph) or the iodide complexes (C 5Me 5)Ir(I) (CO) (R) (R=Me, Ph_are not reactive under the same conditions. The reaction of (C 5Me 5)Ir(Cl)(CO)(Me) with PMePh 2 and PMe 2Ph in acetonitrile yields the chloride substitution product [(C 5Me 5)Ir(CO)(L)(Me)] +Cl −. Kinetic measurements for the reactions of (C 5Me 5)Ir(Cl)(CO)(Me) in toluene are first order in the iridium complex and exhibit a saturation dependence on the incoming donors L. Analysis of the data suggests a two-step process involving (i) rapid formation of a molecular complex [(C 5Me 5)Ir(Cl)(CO)(Me), (L)], in which the structure of 1a is unperturbed within the limits of spectroscopic analysis, and (ii) rate determining methyl migration. The reaction parameters are K for the pre-equilibrium step ( K = 1.5 (CyNC), 7.3 (PHPh 2), 7.1 (PMePh 2) dm 3 mol −1 at 323 K) and k2 for the slow carbon---carbon bond formation ( k2 (10 5) = 6.9 (CyNC), 1.2 (PHPh 2), 1.0 (PMePh 2) s −1 at 323 K). The activation parameters for the methyl migration step in the reaction with PMePh 2 obtained between 308 and 338 K, are Δ H≠ = 106±16 kJ mol −1 and Δ S≠ = − 14±5 J K −1 mol −1. The reaction of 1a with PMePh 2 proceeds at similar rates in tetrahydrofuran ( K = 3.7 dm 3 mol −1, k2 (10 5) = 1.2 s −1, 323 K). The crystal structure of (C 5Me 5)Ir(Cl)(COMe) (PMe 2Ph) has been determined by X-ray diffraction. C 20H 29ClOPIr: Mr = 544.1, monoclinic, P2 1/ n, A = 8.084 (2), B = 9.030(2), C = 28.715 (3) Å, β = 91.41 (3)°, Z = 4, Dc = 1.71 g cm −3, V = 2095.5 Å 3, room temperatyre, Mo K, γ = 0.71069, μ = 65.55 cm −1, F(000) = 1044, R = 0.037 for 2453 independent observed reflections. The complex shows a deformed tetrahedral coordination assuming the η 5-C 5Me 5 molecular fragment as a single coordination site. The iridium-chlorine bond is staggered with respect to two adjacent C(ring)-methyl bonds, while the Ir---P and the Ir---COMe bonds are eclipsed with respect to C(ring)-methyl bonds. 相似文献
12.
Crocin in aqueous solution is oxidized by ferrylmyoglobin, MbFe(IV)=O, in a second order reaction with k = 183 1 · mol -1 · s -1, AH ‡298 = 55.0 kJ · mol -1, and ΔLS ‡298 = -17 J · mol -1 K -1 (pH = 6.8, ionic strength 0.16 (NaCl), 25°C), as studied by stopped-flow spectroscopy. The reaction has 1:1 stoichiometry to yield metmyoglobin, MbFe(III), and has AG o = -11 kJ · mol -1, as calculated from the literature value E 0 = +0.85 V (pH = 7.4) vs. NHE for MbFe(IV)=O/MbFe(III) and from the half-peak potential +0.74 V ( vs. NHE in aqueous 0.16 NaCl, pH = 7.4) determined by cyclic voltammetry for the one-electron oxidation product of crocin, for which a cation radical structure is proposed and which has a half-peak potential of +0.89 V for its formation from the two-electron oxidation product of crocin. The fer-rylmyoglobin protein-radical, MbFe(IV)=O, reacts with crocin with 2:l stoichiometq to yield MbFe(IV)= 0, as determined by ESR spectroscopy, in a reaction faster than the second order protein-radical generating reaction between H 2O 2 and MbFe(III), for which latter reaction k = 137 L · mol -1 · s -1, ΔH ‡298 = 51.5 kJ · mol -1, and ΔH ‡298 = -31 J · mol -1 · K -1 (pH = 6.8, ionic strength = 0.16 (NaCI), 25°C) was determined. Based on the difference between the stoichiometry for the reaction between crocin and each of the two hypervalent forms of myoglobin, it is concluded in agreement with the determined half peak reduction potentials, that the crocin cation radical is less reducing compared to crocin, as the cation radical can reduce the protein radical but not the iron(IV) centre in hypervalent myoglobin. 相似文献
13.
The reversible equilibrium conversion under H 2 of [RuCl(dppb) (μ-Cl)] 2 (1) to generate (η 2-H 2) (dppb) (μ-Cl) 3RuCl(dppb) in CH 2Cl 2 (dppb = Ph2P( CH2) 4PPh2) has been studied at 0–25 °C by UV-Vis and 31P{ 1H} NMR spectroscopy, and by stoppe kinetics; the equilibrium constant and corresponding thermodynamic parameters, and the forward and reverse rate constants at 25 °C have been determined. A measured Δ H° value of 0 kJ mol −1 allows for an estimation of an exothermicity of 60 kJ mol −1 for binding an η 2-H 2 at an Ru(II) centre; a Δ S° value of 60 J mol −1 K −1 indicates that in solution 1 contain s coordinated CH 2Cl 2. The kinetic and thermodynamic data are compared to those obtained from a previously studied hydrogenation of styrene catalyzed by 1. Preliminary findings on related systems containing Ph 2P(CH 2) 3PPh 2 and (C 6H 11) 2P(C 6H 11) 2 are also noted. 相似文献
14.
High-pressure liquid-chromatography and microcalorimetry have been used to determine equilibrium constants and enthalpies of reaction for the disproportionation reaction of adenosine 5′-diphosphate (ADP) to adenosine 5′-triphosphate (ATP) andadenosine 5′-monophosphate (AMP). Adenylate kinase was used to catalyze this reaction. The measurements were carried out over the temperature range 286 to 311 K, at ionic strengths varying from 0.06 to 0.33 mol kg −1, over the pH range 6.04 to 8.87, and over the pMg range 2.22 to 7.16, where pMg = -log a(Mg 2+). The equilibrium model developed by Goldberg and Tewari (see the previous paper in this issue) was used for the analysis of the measurements. Thus, for the reference reaction: 2 ADp 3− (ao) AMp 2− (ao)+ ATp − (ao), K° = 0.225 ± 0.010, Δ G° = 3.70 +- 0.11 kJ mol −1, Δ H° = −1.5 ± 1. 5 kJ mol −1, °S ° = −17 ± 5 J mol −1 K−1, and ACP p°≈ = −46 J mo1l −1 K−1 at 298.15 K and 0.1 MPa. These results and the thermodynamic parameters for the auxiliary equilibria in solution have been used to model the thermodynamics of the disproportionation reaction over a wide range of temperature, pH, ionic strength, and magnesium ion morality. Under approximately physiological conditions (311.15 K, pH 6.94, [Mg 2+] = 1.35 × 10 −3 mol kg −1, and I = 0.23 mol kg −1) the apparent equilibrium constant ( KA′ = m(ΣAMP) m(ΣATP)/[ m(ΣADP)] 2) for the overall disproportionation reaction is equal to 0.93 ± 0.02. Thermodynamic data on the disproportionation reaction and literature values for this apparent equilibrium constant in human red blood cells are used to calculate a morality of 1.94 × 10 −4 mol kg −1 for free magnesium ion in human red blood cells. The results are also discussed in relation to thermochemical cycles and compared with data on the hydrolysis of the guanosine phosphates. 相似文献
15.
Fourteen flavonoid aglycones, and the flavonoid glyco-side rutin, with redox potentials ranging from 0.20 (myricetin) to 0.83 V (chrysin) vs. NHE, as determined by cyclic voltammetry at 23°C in aqueous 50 mM phosphate, ionic strength 0.16 (NaCI) with pH = 7.4 and compared with redox potentials determined for four cinnamic acid derivatives, were all found to reduce ferrylmyoglobin, MbFe(IV)=O, to metmyoglo-bin, MbFe(III). Reaction stoichiometry depends strongly on the number of hydroxyl groups in the flavonoid B-ring. All compounds with 3',4'-dihydroxy substitution reduce 2 equivalents of MbFe(IV)=O, whereas naringenin, hesperitin and kaempferol, with one hydroxyl group in the B-ring, reduce with a one-to-one stoichiometry. As studied spectrophotometrically under pseudo-first-order conditions with flavonoids in excess, rutin and apigenin react with MbFe(IV)=O with very similar and moderately high activation enthalpies of ΔH‡ 298 = 69 ± 1kJ mol -1 and ΔH‡ 298 = 65 ± 3kJ mol- 1, respectively, and with positive activation entropies of ΔH‡ 298 = 23 ± 4Jmol- 1 K -1 and ΔS‡ 298 = 13 ± 9Jmol -1K- 1, respectively, in agreement with outer-sphere electron transfer as rate determining. For the fifteen plant polyphenols only qualitative relations exist between redox potential and rate constants rather than a linear free energy relationship ( r2 = 0.503), and especially the flavone apigenin was found more efficient as reducing agent. For the flava-nones, a linear relation ( r2 = 0.971) indicate that, in the absence of a 2,3 double bond, removal of the 4-carbonyl group or addition of a 3-hydroxy group only has minor effect on reactivity. The flavonols are the most efficient reducing agents, effectively reducing MbFe(IV)=O to MbFe(III) and establishing a steady state distribution between the flavonol and MbFe(III) and oxymyoglobin, MbFe(II)O 2. Oxidised flavonols reduces MbFe(III) to MbFe(II)0 2 very efficiently and much faster than the parent flavonol. 相似文献
16.
The enthalpies of reaction of HMo(CO) 3C 5R 5 (R = H, CH 3) with diphenyldisulfide producing PhSMo(CO) 3C 5R 5 and PhSH have been measured in toluene and THF solution (R = H, Δ H= −8.5 ± 0.5 kcal mol −1 (tol), −10.8 ± 0.7 kcal mol −1 (THF); R = CH 3, Δ H = −11.3±0.3 kcal mol −1 (tol), −13.2±0.7 kcal mol −1 (THF)). These data are used to estimate the Mo---SPh bond strength to be on the order of 38–41 kcal mol −1 for these complexes. The increased exothermicity of oxidative addition of disulfide in THF versus toluene is attributed to hydrogen bonding between thiophenol produced in the reaction and THF. This was confirmed by measurement of the heat of solution of thiophenol in toluene and THF. Differential scanning calorimetry as well as high temperature calorimetry have been performed on the dimerization and subsequent decarbonylation reactions of PhSMo(CO) 3Cp yielding [PhSMo(CO) 2Cp] 2 and [PhSMo(CO)Cp] 2. The enthalpies of reaction of PhSMo(CO) 3Cp and [PhSMo(CO) 2Cp] 2 with PPh 3, PPh 2Me and P(OMe) 3 have also been measured. The disproportionation reaction: 2[PhSMo(CO) 2Cp] 2 → 2PhSMo(CO) 3Cp + [PhSMP(CO)Cp] 2 is reported and its enthalpy has also been measured. These data allow determination of the enthalpy of formation of the metal-sulfur clusters [PhSMo(CO) nC 5H 5] 2, N = 1,2. 相似文献
17.
1. 1. The superoxide anion radical (O2−) reacts with ferricytochrome c to form ferrocytochrome c. No intermediate complexes are observable. No reaction could be detected between O2− and ferrocytochrome c. 2. 2. At 20 °C the rate constant for the reaction at pH 4.7 to 6.7 is 1.4 · 106 M−1 · s−1 and as the pH increases above 6.7 the rate constant steadily decreases. The dependence on pH is the same for tuna heart and horse heart cytochrome c. No reaction could be demonstrated between O2− and the form of cytochrome c which exists above pH ≈ 9.2. The dependence of the rate constant on pH can be explained if cytochrome c has pKs of 7.45 and 9.2, and O2− reacts with the form present below pH 7.45 with k = 1.4 · 106 M−1 · s−1, the form above pH 7.45 with k = 3.0 · 105 M−1 · s−1, and the form present above pH 9.2 with k = 0. 3. 3. The reaction has an activation energy of 20 kJ mol−1 and an enthalpy of activation at 25 °C of 18 kJ mol−1 both above and below pH 7.45. It is suggested that O2− may reduce cytochrome c through a track composed of aromatic amino acids, and that little protein rearrangement is required for the formation of the activated complex. 4. 4. No reduction of ferricytochrome c by HO2 radicals could be demonstrated at pH 1.2–6.2 but at pH 5.3, HO2 radicals oxidize ferrocytochrome c with a rate constant of about 5 · 105–5 · 106 M−1 · s−1
. 相似文献
18.
The dynamics of the metal atom motion in sym octamethyl ferrocene (OMF) has been elucidated over the temperature range 85≤ T≤350 K by 57Fe Mössbauer effect spectroscopy, and shows a marked increase in the mean-square-amplitude of vibration at 348 K, some 80° below the melting point of the neat solid. Differential scanning calorimetry shows an endothermic peak at about the same temperature, and Δ H is 1.50 kJ mol −1 and Δ S is 4.31 J mol −1 K −1. Corresponding data for OMF +PF 6− can be fitted by a relaxation algorithm and confirm the intra-molecular nature of the transition. The spin-lattice relaxation over the above temperature range is fast compared to the characteristic Mössbauer time scale and can be accounted for by a Raman process in the high temperature limit. The transition at 348 K is associated with the onset of ring rotation/libration in the neat solid. 相似文献
19.
The role of the hydroxyl group of tyrosine 6 in the binding of Schistosoma japonicum glutathione S-transferase has been investigated by isothermal titration calorimetry (ITC). A site-specific replacement of this residue with phenylalanine produces the Y6F mutant, which shows negative cooperativity for the binding of reduced glutathione (GSH). Calorimetric measurements indicated that the binding of GSH to Y6F dimer is enthalpically driven over the temperature range investigated. A concomitant net uptake of protons upon binding of GSH to Y6F mutant was detected carrying out calorimetric experiments in various buffer systems with different heats of ionization. The entropy change is favorable at temperatures below 26 °C for the first site, being entropically favorable at all temperatures studied for the second site. The enthalpy change of binding is strongly temperature-dependent, arising from a large negative Δ C° p1=−3.45±0.62 kJ K −1 mol −1 for the first site, whereas a small Δ C° p2=−0.33±0.05 kJ K −1 mol −1 for the second site was obtained. This large heat capacity change is indicative of conformational changes during the binding of substrate. 相似文献
20.
Rates of stepwise anation of cis-Cr(ox) 2(H 2O 2) − with SCN −/N 3−, Cr(acac) 2(H 2O) 2+ with SCN − and Cr(atda)(H 2O) 2 with SCN − have been investigated in weakly acidic aqueous solutions. Rate constants, kI and kII for the two steps in each system, are composite as kx = kx0+ kxX[X −] ( x = I, II; X − = SCN −, N 3−). These rate constants have been evaluated also as the corresponding Δ H≠ and Δ S≠ values. The results obtained and the plausible I d mechanism seem to suggest Cr---OOC bond dissociation (hence a strongly negative Δ S≠) generating the transition state in each system with outer-sphere association forming the precursor complex in the X − dependent paths. 相似文献
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