The exchange reaction of acac(acetylacetonate) in UO2(acac)2dmf (dmf=N,N-dimethylformamide) in o-dichlorobenzene has been studied by the NMR line-broadening method. The exchange rate depends on the concentration of the enol isomer of acetylacetone in its low region, and approaches the limiting value in its high region. It is proposed that the exchange reaction proceeds through the mechanism in which the dissociation of one end of the chelated acac is the rate-determining step. The kinetic parameters for this step are as follows: k (25 °C)=5.03 × 10−3 s−1, ΔH3=91.6 ± 3.8 kJ mol−1, and ΔS3 =17.2 ± 10.5 JK−1 mol−1. The exchange rate becomes slower by the addition of free DMF. This may be due to the competition of DMF with the enol isomer of acetylacetone in attacking a four-coordinated intermediate in the equatorial plane. 相似文献
The axial adduct formation of the iron(II) complex of 2,3,9,10-tetraphenyl-l,4,8,11-tetraaza-1,3,8,10-cyclotetradecatetraene (L) with imidazole in dimethyl sulfoxide has been investigated spectrophotometrically at various temperatures and pressures. In the presence of a large excess of imidazole the reaction with the two phases has been observed. The first faster reaction is the formation of the monoimidazole complex of FeL2+, and the second slower reaction corresponds to the formation of the bisimidazole complex. Activation parameters are as follows: for the first step with k1 (25.0°C) = (6.8 ±0.2)×105 mol−1 kg s−1, ΔH31 = 47.5 ± 4.9 kJ mol−1, ΔS31 = 26±16 J K−1 mol−1, and ΔV31 (30.0°C) = 27.2±1.5 cm3 mol−1; for the second step with k2 (25.0°C) = 26.8±0.8 mol−1 kg s−1, ΔH32 = 91.6± 0.8 kJ mol−1, ΔS32 = 90±3 J K−1 mol−1, and ΔV32 (35.0°C) = 21.8±0.9 cm3 mol−1. The large positive activation volumes strongly indicate a dissociative character of the activation process. 相似文献
Calorimetric studies of the reduction of free oxygen in solution by sodium dithionite are in agreement with a stoichiometry of 2 moles Na2S2O4 per mole of oxygen. The reaction is biphasic with ΔHt - 118±7 kcal mol?1 (?494 ± 29 kJ mol?1). The initial phase of the reaction proceeds with an enthalpy change of ca ?20 kcal (?84 kJ) and occurs when 0.5 moles of dithionite have been added per mole dioxygen present. This could be interpreted as the enthalpy change for the addition of a single electron to form the superoxide anion. Further reduction of the oxygen to water by one or more additional steps is accompanied by an enthalpy change of ca ?100 kcal (?418. 5 kJ). Neither of these reductive phases is consistent with the formation of hydrogen peroxide as an intermediate. The reduction of hydrogen peroxide by dithionite in 0.1 M phosphate buffer, pH 7.15, is a much slower process and with an enthalpy change of ca ? 74 kcal mol?1 (?314 kJ mol?1). Dissociation of oxyhemoglobin induced by the reduction of free oxygen tension with dithionite also shows a stoichiometry of 2 moles dithionite per mole oxygen present and an enthalpy change of ca. ?101 ±9 kcal mol?1 (?423± 38 kJ mol?1). The difference in the observed enthalpies (reduction of dioxygen vs. oxyhemoglobin) has been attributed to the dissociation of oxyhemoglobin, which is 17 kcal mol?1 (71 kJ mol?1). 相似文献
In CD3CN solutions the kinetic parameters characterising rotation about the CNMe2 and CNH2 bonds in [UO2(1,1-DMU)5]2+ (1,1-DMU = 1,1- dimethylurea) were determined as: k(265 K) = 39.1 ± 0.4 and 2960 ± 60 s?1, ΔH3 = 49.1 ± 0.76 and 61.1 ±0.5 kJ mol?1, ΔS2 = ?28.3 ± 2.7 and 53.1 ± 2.2 J K?1 mol?1 respectively from 1H NMR studies. Resonances arising from the three isomeric 1,3-DMU (= 1,3-dimethylurea) ligands were observed for [UO2(1,3-DMU)5]2+ in CD3CN solution and the kinetic parameters characterising their isomerisations were also determined. The three isomers of 1,3-DMU have not previously been detected in solution and it appears that coordination of 1,3-DMU to UO22+ increases the barrier to rotation about the carbon nitrogen bond, as is also shown to be the case for 1,1-DMU. 相似文献
The temperature dependences of the P870+Q?A → P870QA and P870+Q?B → P870QB recombination reactions were measured in reaction centers from Rhodopseudomonas sphaeroides. The data indicate that the P870+Q?B state decays by thermal repopulation of the P870+Q?A state, followed by recombination. ΔG° for the P870+Q?A → P870+Q?B reaction is ?6.89 kJ · mol?1, while ΔH° = ?14.45 kJ · mol?1 and ?TΔS° = + 7.53 kJ · mol?1. The activation ethalpy, , for the P870+Q?A Δ P870+Q?B reaction is +56.9 kJ · mol?1, while the activation entropy is near zero. The results permit an estimate of the shape of the potential energy curve for the P870+Q?A → P870+Q?B electron transfer reaction. 相似文献
The kinetics of malonate replacement in bis- (malonato)oxovanadate(IV), [VO(mal)2H2O]2−(hereafter water molecule will be omitted), by oxalate has been studied by the stopped-flow method. The reaction was found to consist of two consecutive steps (k1 and k2: first-order rate constants) passing through a mixed ligand complex, [VO(mal)(ox)]2−. The rates for each step depended linearly on the concentrations of free oxalate species, Hox− and ox2−. The second-order rate constants for the replacement by ox2− were much larger in the k1 step than in the k2 step and the activation parameters were determined as follows: ΔH≠= 43.5 ± 5.6 kJ mol−1, ΔS±-53 ± 19 J K−1 mol−1 and ΔH≠= 43.6 ± 0.5 kJ mol−1, δS≠ = -62 ± 2 J K−l mol−1 for the k1 and k2 steps, respectively. The volume of activation was determined to be -0.65 ± 0.75 cm3 mol−1 at 20.2 °C by the high-pressure stopped-flow method for the apparent rate constants. 相似文献
The thermodynamic parameters, ΔH′, ΔG′, and ΔS′, and the stoichiometry for the binding of the substrate 2′-deoxyuridine-5′-phosphate (dUMP) and the inhibitor 5-fluoro-2′-deoxyuridine-5′-phosphate (FdUMP) to Lactobacillus casei thymidylate synthetase (TSase) have been investigated using both direct calorimetric methods and gel filtration methods. The data obtained show that two ligand binding sites are available but that the binding of the second mole of dUMP is extremely weak. Binding of the first mole of dUMP can best be illustrated by dUMP + TSase + H+?(dUMP-TSase-H+). [1] The enthalpy, ΔH1′, for reaction [1] was measured directly on a flow modification of a Beckman Model 190B microcalorimeter. Experiments in two different buffers (I = 0.10 m) show that ΔH1′ = ?28 kJ mol?1 and that 0.87 mol of protons enters into the reaction. Analysis of thermal titrations for reaction [1] indicates a free energy change of ΔG1′ = ?30 kJ mol?1 (K1 = 1.7 × 105 m?1). From these parameters, ΔS1′ was calculated to be +5 J mol?1 degree?1, showing that the reaction is almost totally driven by enthalpy changes. Gel filtration experiments show that at very high substrate concentrations, binding to a second site can be observed. Gel filtration experiments performed at low ionic strength (I = 0.05 m) reveal a stronger binding, with ΔG1′ = ?35 kJ mol?1 (K1 = 1.2 × 106 m?1), suggesting that the forces driving the interaction are, in part, electrostatic. Addition of 2-mercaptoethanol (0.10 m) had the effect of slightly increasing the dUMP binding constant. Binding of FdUMP to TSase is best illustrated by 2FdUMP + TSase + nHH+?FdUMP2 ? TSase ? (H+)nH. [2] The enthalpy for this reaction, ΔH2, was also measured calorimetrically and found to be ?30 kJ mol?1 with nH = 1.24 at pH 7.4 Assuming two FdUMP binding sites per dimer as established by Galivan et al. [Biochemistry15, 356–362 (1976)] our calorimetric results indicate different binding energies for each site. Based on the binding data, a thermodynamic model is presented which serves to rationalize much of the confusing physical and chemical data characterizing thymidylate synthetase. 相似文献
The extraction equilibrium of the hydronium-uranium(VI)-dicyclohexano-24-crown-8 complex was carried out in the crown ether1,2-dichloroethaneHCl aqueous solution system at different temperatures. The extraction complex has the overall composition (L)2·(H3O+·χH2O)2·UO2Cl42− (L = dicyclohexano-24-crown-8). The values of the extraction equilibrium constants (Kex) increase steadily with a decrease in temperature: 13.5 (298 K), 7.96 (301 K), 4.20 (303 K) and 2.07 (305 K). A plot of log Kex against 1/T shows a straight line. The value of the enthalpy change, ΔH°, was calculated from the slope and equals −212 kJ mol−1. The value of the entropy change, ΔS°, was calculated from ΔH° and Kex and equals −690 J K−1 mol−1, whereas ΔG° = −6.45 kJ mol−1. Comparing these thermodynamic parameters with those of the dicyclohexano-18-crown-6 isomer A [1] (ΔS° = −314 J K−1 mol−1, ΔH° = −101 kJ mol−1 and ΔG° = −8.37 kJ mol−1), it can be seen that ΔH° and ΔS° are more negative for the former than for the latter, and both are enthalpy-stabilized complexes. The molecular structure of the complex has the feature that there are two H5O2+ ions in it, in contrast to the H3O+ ions in the dicyclohexano-18-crown-6 isomer A complex [1]. Each of the H5O2+ ions is held in the crown ether cavity by four hydrogen bonds. The H5O2+ ion has a central bond. The uranium atom forms UO2Cl42− as a counterion away from the crown ether. The formation of this complex is in good agreement with more negative entropy change and less negative free energy change, as mentioned above. 相似文献
We have measured the thermodynamic parameters of the slow-fast tail-fiber reorientation transition on T2L bateriophage. Proportions of the virus in each form were determined from peak-height measurements in sedimention-velocity runs and from average diffusion coefficients obtained by quasielastic laser light scattering. Computer simulation of sedimentation confirmed that there were no undetected intermediates in the transition, which was analyzed as a two-state process. Van't Hoff-type plots of the apparent equilibrium constant and of the pH midpoint of the transition as function of reciprocal temperature led to the following estimates of the thermodynamic parameters for the transition at pH 6.0 and 20°C: ΔH° = ?139 ± 18Kcal mol?1, ΔS° = ?247 ± 46 cal K?1 mol?1, and ΔG° = ?66 ± 22 kcal mol?1. Per mole of protons taken up in the transition, the analogous quantities were ?15.9 ± 1.7 kcal mol?1, ?26.3 ± 2.2 cal K?1 mol?1, and ?8.22 ± 1.8 kcal mol?1. The net number of protons taken up was about 8.5 ± 1.5. The large values of the thermodynamic functions are consistent with a highly cooperative reaction and with multiple interactions between the fibres and the remainder of the phage. The negative entropy of the transition is probably due to immobilization of the fibres. 相似文献
Kinetic studies of the reduction of ferrioxamine B (Fe(Hdesf)+) by Cr(H2O)62+, V(H2O)62+, and dithionite have been performed. For Cr(H2O)62+ and V(H2O)62+, the rate is ?d[Fe(Hdesf)+]/dt = k[Fe(Hdesf)+][M2+]. For Cr(H2O)62+, k = 1.19 × 104 M?1 sec?1 at 25°C and μ = 0.4 M, and k is independent of pH from 2.6 to 3.5. For V(H2O)62+, k = 6.30 × 102 M?1 sec?1 at 25°C, μ = 1.0 M, and pH = 2.2. The rate is nearly independent of pH from 2.2 to 4.0. For Cr(H2O)62+ and V(H2O)62+, the activation parameters are ΔH≠ = 8.2 kcal mol?1, ΔS≠ ?12 eu and ΔH≠ = 1.7 kcal mol?1, ΔS≠ = ?40 eu (at pH 2.2) respectively. Reduction by Cr(H2O)62+ is inner-sphere, while reduction by V(H2O)62+ is outer-sphere. Reduction by dithionite follows the rate law ?d[Fe(Hdesf)+]/dt =kK[Fe(Hdesf)+][S2O42?] where K is the equilibrium constant for dissociation of S2O42? into SO2? radicals. The value of k at 25°C and μ = 0.5 is 2.7 × 103 M?1 sec?1 at pH 5.8, 3.5 × 103 M?1 sec?1 at pH 6.8, and 4.6 × 103 M?1 sec?1 at pH 7.8, and ΔH≠ = 6.8 kcal mol?1 and ΔS≠ = ?19 eu at pH 7.8. 相似文献
The enthalpy of the bioluminescent reaction has been studied by direct calorimetric methods. Bacterial luciferase, isolated from Beneckea harveyi (formerly strain MAV) has been used to catalyze the oxidation of reduced flavin mononucleotide (FMNH2) and a long chain aliphatic aldehyde (dodecanal, RCHO) by molecular oxygen to give the indicated products and blue-green light. The enthalpy measured for this process was found to be ΔHL = ?338.9 k.J (mol FMN)?1 (?81.0 kcal) at 25.00 °C and ?402.9 kJ (mol FMN)?1 (?96.3 kcal) at 7.00 °C. Calculations based on redox electrode potentials indicate a corresponding value of the free energy change, ΔGL = ?464.8 kJ (mol FMN)?1 (?111.1 kcal), at 25 °C. Measurements were performed in 0.15 m phosphate buffer, pH 7.0 and the values were arrived at by correcting the observed heats for the heat associated with the autoxidation process: FMNH2 + O2 ? FMN + H2O2; ΔHD = ?158.5 kJ (mol FMN)?1 (?37.8). These data and a detailed thermodynamic analysis have demonstrated the need for two parameters, referred to as the intrinsic free energy, ΔG1, and intrinsic enthalpy, ΔH1, which are functionally defined by the relations ΔGI = ΔGL ? uhvΔHI = ΔHL ? uhv, where u is the quantum yield of the reaction expressed in einsteins mole?1.These parameters reflect the thermochemistry of the bioluminescent reaction corrected for emitted photons. Thus, they are useful for comparing the thermochemistry of a chemiluminescent process. Their values for the bacterial luciferase system at 25 °C and pH 7.0 are ?391.6 and ?266.9 kJ (mol FMN)?1 (?93.6 and ?63.8 kcal), respectively, assuming a value of 0.3 for the quantum yield. The calorimetric data also suggest the existence of a long-lived species which persists after photon emission. 相似文献
A methodological study has been made with a syringe titration unit attached to an LKB batch microcalorimeter. The presicion and accuracy of the instrument assembly have been evaluated by neutralization reactions and by dilution of sucrose solutions. As an example, heat quantities on the order of 10 mJ accompanying the addition of 10 μl titrant solution could be determined with an accuracy of better than 1%. A stepwise titration procedure was used to characterize the binding of indole-3-propionic acid to α-chymotrypsin. The following thermodynamic data were obtained (25°C, acetate buffer, pH 5.80): ΔG0 = ?18.46±0.17 kJ·mol?1, ΔH0 = ?15.26±0.20 kJ·mol?1, ΔS0 = 10.85±1.21 JK?·mol?1. 相似文献
The interaction between K2Cr2O7 and urease was investigated using fluorescence, UV-vis absorption, and circular dichroism (CD) spectroscopy. The experimental results showed that the fluorescence quenching of urease by K2Cr2O7 was a result of the formation of K2Cr2O7–urease complex. The apparent binding constant KA between K2Cr2O7 and urease at 295, 302, and 309 K were obtained to be 2.14?×?104, 1.96?×?104, and 1.92?×?104 L mol?1, respectively. The thermodynamic parameters, ΔH° and ΔS° were estimated to be ?5.90 kJ mol?1, 43.67 J mol?1 K?1 according to the Van’t Hoff equation. The electrostatic interaction played a major role in stabilizing the complex. The distance r between donor (urease) and acceptor (K2Cr2O7) was 5.08 nm. The effect of K2Cr2O7 on the conformation of urease was analyzed using UV-vis absorption, CD, synchronous fluorescence spectroscopy, and three-dimensional fluorescence spectra, the environment around Trp and Tyr residues were altered. 相似文献
We report for the first time kinetic and thermodynamic properties of soluble acid invertase (SAI) of sugarcane (Saccharum officinarum L.) salt sensitive local cultivar CP 77-400 (CP-77). The SAI was purified to apparent homogeneity on FPLC system. The crude enzyme was about 13 fold purified and recovery of SAI was 35%. The invertase was monomeric in nature and its native molecular mass on gel filtration and subunit mass on SDS-PAGE was 28 kDa. SAI was highly acidic having an optimum pH lower than 2. The acidic limb was missing. Proton transfer (donation and receiving) during catalysis was controlled by the basic limb having a pKa of 2.4. Carboxyl groups were involved in proton transfer during catalysis. The kinetic constants for sucrose hydrolysis by SAI were determined to be: km = 55 mg ml?1, kcat = 21 s?1, kcat/km = 0.38, while the thermodynamic parameters were: ΔH* = 52.6 kJ mol?1, ΔG* = 71.2 kJ mol?1, ΔS* = ?57 J mol?1 K?1, ΔG*E–S = 10.8 kJ mol?1 and ΔG*E–T = 2.6 kJ mol?1. The kinetics and thermodynamics of irreversible thermal denaturation at various temperatures 53–63 °C were also determined. The half -life of SAI at 53 and 63 °C was 112 and 10 min, respectively. At 55 °C, surprisingly the half -life increased to twice that at 53 °C. ΔG*, ΔH* and ΔS* of irreversible thermal stability of SAI at 55 °C were 107.7 kJ mol?1, 276.04 kJ mol?1 and 513 J mol?1K?1, respectively. 相似文献
An attempt to estimate the importance of general acid-base catalysis in enzymic catalysis has been made, using the hydrolysis of the ester group of N,O-diacetylserinamide as a model for the deacylation of acyl-chymotrypsins. General base catalysis of this reaction by imidazole is estimated to reduce the activation energy by at least 31 kJ mol?1. The rate of reaction, however, is not greatly enhanced because of an unfavourable change in the entropy of activation from ?132 to ?197 JK?1 mol?1. At about 300 K, a typical temperature for enzyme-catalysed reactions, the reduction in activation energy would cause a rate enhancement of about 3 × 105-fold if the unfavourable entropy change did not occur. For specific acyl-chymotrypsins the entropy of activation for deacylation is about ?89 J K?1 mol?1, allowing the full effect of general base catalysis by imidazole to be realized. It is, therefore, postulated that in the active site of an enzyme, a properly oriented imidazole side chain may catalyse the rate of a reaction 105-fold by general base catalysis. 相似文献
The enthalpy change for phosphorylation of ADP3? by PEP3? catalysed by pyruvate kinase has been determined at 25°C using flow microcalorimetry. Measurements were made at pH 8 in three buffer systems TRIS, TEA and HEPES and also at pH 8.5 in TRIS buffer. The values of ΔH obtained, ?8.75 kJ mol?1 in TRIS, ?7.39 kJ mol? in TEA and ?6.19 kJ mol?1 in HEPES surprisingly display a dependence on the buffer system used. The enthalpy change was combined with free energy data to calculate the entropy change for the catalysed reaction. 相似文献
The kinetics and mechanism of the oxidation of L- ascorbic acid by trisoxalatocobaltate(III) were studied as a function of pH, ascorbate concentration, ionic strength and temperature in a weakly basic aqueous solution. The pH dependence of the process can be ascribed to the oxidation of the doubly deprotonated ascorbate ion for which k = 20 M−1 s−1 at 25 °C, ΔH# = 34 ± 2 kJ mol−1 and ΔS# = −108 ± 7 J K−1 mol−1. The results are discussed in reference to literature data for this reaction in weakly acidic medium and for the oxidation by a series of other oxidants. 相似文献
The complexes CodptX3 and [Codpt(H2O)X2]ClO4 (X = Cl, Br; dpt = dipropylenetriamine = NH(CH2CH2CH2NH2)2) have been prepared and characterized. Rate constants (s−1) for aqueous solution at 25 °C and μ = 0.5 M (NaClO4), for the acid-independent sequential ractions. have been measured spectrophotometrically. For X = Cl: k1 ⋍ 2 × 10−2, k2 = 1.7 × 10−4 and k3 = 4.8 × 10−6, and for X = Br: k1 ⋍ 2 × 10−2, k2 = 5.25 × 10−4 and k3 = 2.5 × 10−5 The primary equation was found to be acid independent, while the secondary and tertiary aquations were acid-inhibited reactions. For the second step, the rate of the reaction was given by the rate equation where Ct is the complex concentration in the aqua-and hydroxodihalo species, k′2 is the rate constant for the acid-dependent pathway and Ka is the equilibrium constant between the hydroxo and aqua complex ions. The activation parameters were evaluated, for X = Cl: ΔH≠2 = 106.3 ± 0.4 kJ mol−1 and ΔS≠2 = 40.2 ± 1.7 J K−1 mol−, and for X = Br: ΔH≠2 = 91.6 ± 0.4 kJ mol−1 and ΔS≠2 = 0.4 ± 1.7 J K−1 mol−1. The results are discussed and detailed comparisons of the reactivities of these complexes with other haloaminecobalt(III) species are presented. 相似文献
Kinetic data for the oxidations of d-fructose and l-sorbose by chromium(VI) and vanadium(V) in perchloric acid medium are reported. The addition of perchloric acid and sodium perchlorate increases the pseudo-first-order rate constants. Change of the reaction medium from water to deuterium oxide appreciably affects the rates of chromium(VI) oxidations, but does not affect those of vanadium(V) oxidations. The activation parameters are ΔH3 = 46.6 ±3.4 (fructose) and 50.6 ±6.3 (sorbose) kJ.mol?1, and ΔS3 = ?105 ±11 (fructose) and ?100 ±20 (sorbose) J.deg?1.mol?1 for chromium(VI) oxidations, and, for the other reactions, ΔH3 = 53.2 ±4.2 (fructose) and 52.3 ±6.3 (sorbose) kJ.mol?1, and ΔS3 = ?139.0 ±14 (fructose) and ?137 ±20 (sorbose) J.deg?1.mol?1. The kinetics of the oxidations of ketohexoses by chromium(VI) indicate no intermediate-complex formation, whereas those for vanadium(V) indicate the formation of a 1:1 intermediate complex between ketohexoses and vanadium(V). 相似文献