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 enthalpies of the hexokinase-catalyzed phosphorylation or glucose, mannose, and fructose by ATP to the respective hexose 6-phosphates have been measured calorimetrically in TRIS/TRIS HCl buffer at 25.0, 28.5, and 32.0°C. The effects on the measured enthalpy of the glucose/hexokinase reaction due to variation of pH (over the range 6.7 to 9.0) and ionic strength (over the range 0.02 to 0.25) have been examined. Correction for enthalpy of buffer protonation leads to δHo and δCpo values for the processes: eq-D-hexose + ATP4− = eq-D-hexose 6-phosphate2− + ADP3−+ H+. Results are δHo = −23.8 ± 0.7 kJ · mol−1 and δCpo = −156 ± 280 J·mol−1·K−1 for glucose. δHo = −21.9 ± 0.7 kJ·mol−1 and δCpo = 10 ± 140 J·mol−1·K−1 for mannose, and δHo = −15.0 ± 0.9 kJ·mol−1 and δCpo = −41 ± 160 J·mol−1·K−1 for fructose. Combination of these measured enthalpies with Gibbs energy data for hydrolysis of ATP4− and that for the hexose 6-phosphates lead to δSo values for the above hexokinase-catalyzed reactions. 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
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
Nuclear magnetic resonance line-widths data have been used to determine the rate of solvent exchange from the first coordination sphere of ferro-and ferriprotoporphyrin(IX) dimethylester (Fe-PPD) in pyridine/chloroform. The average values of kinetic parameters for pyridine (PY) exchange indicate an SN2 mechanism tor Fe(III)-PPD(ΔH&;#; = 36 kJ · mol−1 ; ΔS&;#; = −53 J·mol−1K−1; TM(298 K) = 0.07 msec) and an SNI mechanism for Fe(II)-PPD (ΔH&;#; = 67 kJ·mol−1; ΔS&;#; = 42 J · mol−1K−1; TM(298 K) = 0.06 msec). Parallel to the accelerated ligand exchange rate at rising temperatures a redistribution of the electrons causing a transition of the metal porphyrin from the low-spin state to the high-spin state is observed. Enthalpy and entropy of the thermodynamic equilibrium between low- and high-spin Fe-PPD have been determined from experimental values of the average magnetic moment. A mean lifetime of low-spin Fe(III)-PPD was estimated from line. widths changes (TL→H(298 K)≈ 20 msec) and the corresponding activation parameters have been obtained (ΔH&;#;L→H(298 K) = 26 kJ · mol−1; ΔS&;#;L→H(298K) = −125 J · mol−1K−1). 相似文献
One isomer of [CrCl(N-Me-tn)(dien)]ZnCl4 has been isolated from the reaction of CrCl3·6H2O, dehydrated in DMF, with the polyamines N-methyl- 1,3-diaminopropane and diethylenetriamine. This complex is isomorphous with δλ-(R,S)usft-[CoCl(N-Me-tn)(dien)] ZnCl4 and thus has the unsym-fac- configuration with the N-Me group trans to the sec-NH group of the coordinated triamine. The Cr(III) complex has been resolved with NH4BCS and the chiroptical parameters of (-)488-[CrCl(N-Me-tn)(dien)]- ZnCl4, derived from the less soluble diastereoisomeride by metathesis, are similar to those obtained for the less soluble (-)534-λ-(S)-a,cb,edf-Co(III) analogue, of known absolute configuration. Kinetic parameters for the rates of thermal aquation (μ= 1.0 M, HClO4) and Hg2+-assisted chloride release (μ= 1.0 M) for usft-[CrCl(N-Me-tn)(dien)]ZnCl4 are kH= 3.7 × 10−6 s−1, Ea=93 ± 8 kJ mol−1, ΔS2984t#= −45 ± 16 J K−1 mol−1 and kHg=2.01 × 10−3 M−1 s−1, Ea=64.2 ± 3.3, ΔS298#=−89.5 ± 7, respectively at 298 K. 相似文献
The effects of pressure on the kinetics of redox reactions in and around the chloroplast cytochrome bf complex were studied using a reconstituted system consisting of Photosystem I (PS I) particles, cytochrome bf complex and plastocyanin (PC), all derived from pea chloroplasts. There were no significant permanent effects of pressure in the range 0.1–191 MPa on the reaction kinetics, or on the shape of the absorption spectra of components studied. Discernable effects on rate-coefficients of increasing pressure were observed on the reduction of P700+ by PCI, on the reduction of PCII by ascorbate, and on the oxidation of decyl plastoquinol by the bf complex. The volumes of activation ΔV# were determined from the dependence of the rate-coefficient on pressure using: $$(\partial lnk/\partial P)_T = - \Delta V^\# /RT.$$ The volume of activation is the difference in partial molar volume between the activated state and the reactants for the redox reaction. Such data was sought to help define in detail those redox reactions and the corresponding activated states. For the reduction of P700+ by PCI and the oxidation of decyl plastoquinol by the bf complex, the rate coefficient decreased with increase in pressure, whilst for the reduction of PCII by ascorbate it increased. The corresponding volumes of activation were 9.6±0.6×10-6 m3 mol-1, 18±2×10-6 m3 mol-1 and -14±1×10-6 m3 mol-1, respectively. Much of the pressure-dependence of PCII reduction by ascorbate was ascribed to an increase in ascorbate ionisation with increase in pressure. There was little effect of pressure on the kinetics of oxidation of ferrocytochrome f by PCII, or on the equilibrium constant of the redox pair ferrocytochrome f/ferricytochrome f: PCII/PCI. Possible physical bases for these activation volumes are discussed, and they are compared with literature values. 相似文献
The rate of reaction of [Ce(EDTA)(OH)nn−] with H2O2 in 0.10 M KNO3 solution was investigated at various temperatures. The presence of a peroxy intermediate is inferred from spectrophotometric measurements. The general rate equation, is valid for pH 7-9 with n= 1 and 2 complexes involved. The rate constants kl and k2 were determined at 25 °C to be 0.054 and 0.171 M−1 s−1 respectively. The corresponding activation enthalpies, as calculated from Arrhenius plots, were δH1#= 51.3 ± 14.8 and δH2#= 41.8 ± 5.3 kJ m−1 and the activation entropies were δS1#=-97 ± 47 and ΔS2#=−119±17 J K−1 m−1. 相似文献
Binding of [3H]aflatoxin B1 to rat plasma was investigated in vivo and vn vitro. Column chromatographic and polyacrylamide gel electrophoretic analysis clearly demonstrated that aflatoxin B1 bound primarily plasma albumin. Very little binding activity was shown by other plasma proteins. Spectrofluorimetric studies were undertaken to gain some insight into the nature of the aflatoxin-albumin interaction. Quenching of the lone tryptophan fluorescence intensity upon aflatoxin binding was due, at least in part, to a ligand-induced conformational change in the albumin molecule. Aflatoxin B1 binds an apolar site with an association constant of 30 mM−1 at pH 7.4 and 20°C. Neither charcoal treatment of rat albumin nor the presence of 0.15 M NaCl had many significant effect on the interaction. The association constant was pH-dependent, increasing about 1.7-fold as the pH increased from 6.1 to 8.4. This pH dependence is ascribed to a pH-induced conformational change in the albumin molecule. Thermodynamic studies indicated that the aflatoxin-albumin interaction was exothermic (ΔH = −29.3 kJ·mol−), with a ΔS value of −13.8 J·mol−1·K−1. 相似文献
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
We aimed to synthesize hydroxyethyl starch (HES) 200/0.5-loaded bovine serum albumin nanoparticles (HBNs) and investigate the compatibility and binding mechanism in simulated physiological environments. Here, to elucidate the morphology, biocompatibility, and formation mechanism of HBNs, techniques such as scanning electron microscopy, haemolysis test, fluorescence, and circular dichroism spectroscopy were applied. The thermodynamic parameters at body temperature (ΔS° = −26.7 J·mol−1·K−1, ΔH° = −3.20 × 104 J·mol−1, and ΔG = −2.35 × 104 J·mol−1) showed a 1:1 binding stoichiometry, which was formed by hydrogen bonds and van der Waals interactions. In addition, the conformational analysis showed that the microenvironment of fluorophores was altered with the adaptational protein secondary structural changes. Energy transfer occurred from the fluorophores to HES with a high possibility. All these results provided accurate and complete primary data for demonstrating the interaction mechanisms of HES with BSA, which helps to understand its pharmaceutical effects in blood. 相似文献
We measured by batch microcalorimetry the standard enthalpy change ΔH° of the binding of Mn2+ to apo-bovine α-lactalbumin; ΔH° = −90 ± 4kJ·mol−1. The binding constants, KMn2+, calculated from the calorimetric and circular dichroism titration curves, are (4.6±1) · 105M−1, respectively. Batch calorimetry confirms the competitive binding of Ca2+, Mn2+ and Na+ to the same site. The relatively small enthalpy change for Mn2+ binding compared to Ca2+ binding favours a model of a rigid and almost ideal Ca2+-complexating site, different from the well-known EF-hand structures. Cation binding to the high-affinity site most probably triggers the movement of an α-helix which is directly connected to the complexating loop. 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
The reaction of CrCl3 · 6H2O (dehydrated in DMSO) with 1,5,9-triazanonane (3,3-tri) gives mer- CrCl3(3,3-tri), the configuration being established by isomorphism with the corresponding Co(III) complex. This non-electrolyte is hydrolyzed in aqueous acidic solution and mer-[CrCl2(3,3-tri)- (OH2)]ClO4 can be isolated by anation with HCl in the presence of HClO4. Reaction of mer-CrCl3- (3,3-tri) in DMF with diamines produces complexes of the type [CrCl(diamine)(3,3-tri)] Cl2 [diamine= 1,2-diaminoethane (en), 1.2-diaminopropane (pn), 1,3-diaminopropane (tn), 2,2-dimethyl-1,3-diaminopropane (Me2tn) and cyclohexanediamine (chxn, cis plus trans mixture; two isomers A and B)] and these have been characterized as the ZnCl42− salts. The configuration of the triamine ligand in these complexes has been established as mer-(H↓)- by a single crystal X-ray analysis of [CrCl(en)(3,3-tri)]- ZnCl4, monoclinic, P21, a=7.932, b= 14.711, c= 8.312 Å, β=104.6° and Z=2, refined to a conventional R factor of 0.034. The kinetics of the Hg2+- assisted chloride release from [CrCl(diamine)(3,3- tri)]ZnCl4 salts were measured spectrophotometrically (μ=1.0 M HClO4 or HNO3) over 15 K temperature ranges to give, in order, 104kHg (298.2 K) (M−1 s−1), Ea(kJ mol−1), ΔS# (J K−1 mol−1): en- (HClO4): 5.95, 78.1, -53; pn(HClO4); 5.24, 81.2; -44; tn(HClO4): 26.7, 85.6, -15; Me2tn(HClO4): 21.8, 78.6, -40; A-chxn(HNO3): 7.60, 81.0,-41; B-chxn(HNO3): 18.3, 56.8, -115. A ‘non-replaced ligand effect’ on the rate is observed for the first time in this series of homologous Cr(III) complexes. The kinetics of the thermal aquation (kH, 0.1 M HClO4) were measured titrimetrically for CrCl(diamine) (3,3-tri)2+ to give the following kinetic parameters: diamine=en: 107kH (298.2)=5.34 s−1, Ea=99.2 kJ mol−1, ΔS#=-40 J K−1 mol-1; diamine =tn: 107kH (298.2)=5.04 s−1, Ea= 82.8, ΔS#= -96. 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
The qualitative separation performance of a C18, C8 and C4 reversed-phase column was investigated for the separation of histidine and its metabolites histamine, 1-methyihistamine and trans- and cis-urocanic acid. Trans- and cis-urocanic acid were baseline separated from their precursor histidine on all three columns using isocratic elution with a mobile phase composed of 0.01 M aqueous TEAP pH 3.0 and acetonitrile at a ratio of 98:2 (v/v). However, histidine was not separated from histamine and 1-methyihistamine. Selecting the C8 column and introducing 0.005 M of the ion pairing reagent 1-octanesulfonic acid sodium salt into the aqueous solution and acetonitrile at a ratio of 90:10 (v/v), significantly improved the separation. The separation was also followed by a change in the retention times and the order of elution. The sequence of elution was histidine, cis-urocanic acid, trans-urocanic acid, histamine and 1-methylhistamine with retention times of 5.58±0.07, 7.03±0.15, 7.92±0.18, 18.77±0.24 and 20.79±0.21 min (mean±SD; n=5). The separation on the C8 column in the presence of the ion-pairing reagent was further improved with gradient elution that resulted in a reduction in the retention times and elution volumes of histamine and 1-methylhistamine. The detection limits of histidine and trans-urocanic acid at a wavelength of 210 nm and an injection volume of 0.05 ml were 5×10−8 mol l−1 (n=3). The kinetic of the in-vitro conversion of trans- into the cis-isomer after UV irradiation was depending on the time of exposure and the energy of the light source. UVB light induced a significantly faster conversion than UVA light. TUCA and cUCA samples kept at −25°C were stable for up to 50 weeks. Samples, eluted from human skin showed various concentrations of histidine and trans- and cis-urocanic acid with an average of 1.69±0.33×10−5 mol l−1, 1.17±0.43×10−5 mol l−1 and 1.67±0.33×10−5 mol l−1, respectively (n=8). 相似文献
High hydrostatic pressure inhibits growth in most organisms; this may be explained by a deactivation of enzymes involved in essential metabolic pathways. In order to check this hypothesis the enzymic activity of rabbit muscle lactic dehydrogenase and yeast glyceraldehyde-3-phosphate dehydrogenase was investigated in the presence of the coenzyme and excess of substrate at pressures up to 2 kbar.Kinetic analysis of an initial phase of pressure induced activation and of a second phase of reversible deactivation shows that the two enzymes respond to high pressures in different ways leading to a volume of activation of ΔV3(LDH) = 0 ± 1 cm3 mol−1 and ΔV3(GAPDH) = 60 ± 4 cm3 mol−1, respectively. Comparing the lower limits of pressure deaclivation, LDH is found to be more stable towards pressure than GAPDH. At p ≈ 2 kbar total deactivation of both enzymes is observed. A concentration dependent lag of GAPDH reactivation proves dissociation to participate in the process of deactivation, while the effects for LDH are explicable on the basis of reversible denaturation alone. 相似文献
In coordinating solvents, the complex 1, 4, 8, 11- tetramethyl-1, 4, 8, 11-tetraazacyclotetradecane nickel(II) bisperchlorate exists as an equilibrium mixture involving four coordinate R,S,R,S-[Ni(tmc)]2+ and five coordinate R,S,R,S-[Ni(tmc)(solvent)]2+ species. Spectrophotometric measurements of this equilibrium in a number of solvents have been conducted over a range of temperatures and pressures. The stability order for the five coordinate complex in the solvents investigated is CH3CN>DMF>DMSO>C6H5CN> H2O>ClCH2CN at 25 °C. Differences in stability are considered in terms of the measured thermodynamic parameters ΔH° and ΔS°. Both steric and electronic factors were found to influence solvent coordination with the macrocyclic complex.For the equilibrium in CH3CN, C6H5CN, DMF and H2O, reaction volumes, ΔV°, of −3.2±0.5, −4.2±0.5, −0.2±0.5 and −0.5±0.5 cm3 mol−1 respectively have been determined. Each is significantly smaller than the corresponding solvent molar volume. The ΔV° for the equilibrium in CH3CN is comparable with the previously determined activation volume for exchange of this solvent on R, S, R, S- [Ni(tmc)(CH3CN)]2+. The equilibrium and measured volume parameters are discussed in relation to the mechanism for solvent exchange. 相似文献
We have used two techniques to characterize the gelation of deoxyhemoglobin S, a high sensitivity heat-flow calorimeter to measure the heat of gelation and a simple light-transmission method to measure the optical birefringence resulting from the alignment of deoxyhemoglobin S fibers in the gel. A theory for the interpretation of the birefringence measurements is presented. We combine the results of the calorimetric and optical measurements with those of sedimentation experiments to obtain enthalpy changes for gelation. The enthalpy change obtained from scanning and isothermal calorimetric measurements (0.25 m-potassium phosphate, 0.05 m-sodium dithionite, pH 6.9) varies from 4000 to 2200 cal mol−1 hemoglobin between 16 and 25 °C. There is a large apparent heat capacity change of −130 to −190 cal deg.−1 mol−1. The apparent enthalpy change estimated from solubility measurements and birefringence melting experiments is 2200 ± 500 cal mol−1 in qualitative agreement with the calorimetric results. Analysis of the time dependence of the calorimetric and optical progress curves at 20 °C leads to a rough estimate of 1800 to 4000 and −800 to 1500 cal mol−1 hemoglobin for the enthalpies of polymerization and alignment of fibers, respectively. The small magnitude of the observed enthalpy change is in accord with the view that no large conformational change takes place in the deoxyhemoglobin S molecule upon gelation. 相似文献