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Stopped-flow Fourier transform infrared spectroscopy of nitromethane oxidation by the diiron(IV) intermediate of methane monooxygenase 总被引:1,自引:0,他引:1
Muthusamy M Ambundo EA George SJ Lippard SJ Thorneley RN 《Journal of the American Chemical Society》2003,125(37):11150-11151
The hydroxylase component (MMOH) of soluble methane monooxygenase from Methylococcus capsulatus (Bath) was reduced to the diiron(II) form and then allowed to react with dioxygen to generate the diiron(IV) intermediate Q in the first phase of a double-mixing stopped-flow experiment. CD3NO2 was then introduced in the second phase of the experiment, which was carried out in D2O at 25 degrees C. The kinetics of the reaction of the substrate with Q were monitored by stopped-flow Fourier transform infrared spectroscopy, observing the disappearance of the asymmetric NO2 bending vibration at 1548 cm-1. The data were fit to a single-exponential function, which yielded a kobs of 0.45 +/- 0.07 s-1. This result is in quantitative agreement with a kobs of 0.39 +/- 0.01 s-1 obtained by observing the disappearance of Q by double-mixing stopped-flow optical spectroscopy at its absorption maximum of 420 nm. These results provide for the first time direct monitoring of the hydroxylation of a methane-derived substrate in the MMOH reaction pathway and demonstrate that Q decay occurs concomitantly with substrate consumption. 相似文献
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Saturated aliphatic lactones and dihydrocoumarins have been characterized by the formation of their crystalline 1:1-adducts (hydroxyalkanohydrazides) with hydrazine hydrate. A new procedure, which involves the use of azeotropic distillation to remove the water present, greatly facilitates the reaction by reducing the time required. Three macrocyclic lactones, that did not react in either of two short procedures, readily formed the same type of 1:1-adducts after several hours of refluxing of the components in alcoholic solution. The unsaturated macrocyclic lactone gave two products of cleavage; one was the expected ω-hydroxyalkenohydrazide, but the other was its saturated analog, in which the double bond had been reduced by the excess hydrazine employed. β-Lactones are a special case, requiring variations in conditions; the yield of the simplest adduct was more than doubled by a new procedure. In one instance products in which the lactone ring had been opened in both possible ways were obtained for the first time. A “stream-lined extracter” was useful with the low melting adducts. Coumarins themselves gave either 1:2 adducts or pyrazolidones. The latter could also be prepared from the adducts. A study of 4-hydroxybutanohydrazide in a mass spectrometer showed that both γ-butyro-lactone and 5-methyl-3-pyrazolidone are thermal products. Pure 5-methyl-3-pyrazolidone has now been isolated from the syrupy reaction product of ethyl crotonate and hydrazine, It has been converted to benzylidene derivatives; spectral data of the latter are consistent with their polarized structures. 相似文献
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The electron-transfer kinetics for each of three copper(II/I) tripodal ligand complexes reacting with multiple reducing and oxidizing counter reagents have been examined in aqueous solution at 25 degrees C, mu = 0.10 M. For all of the ligands studied, an amine nitrogen serves as the bridgehead atom. Two of the ligands (PMMEA and PEMEA) contain two thioether sulfurs and one pyridyl nitrogen as donor atoms on the appended legs while the third ligand (BPEMEA) has two pyridyl nitrogens and one thioether sulfur. Very limited kinetic studies were also conducted on two additional closely related tripodal ligand complexes. The results are compared to our previous kinetic study on a Cu(II/I) system involving a tripodal ligand (TMMEA) with thioether sulfur donor atoms on all three legs. In all systems, the Cu(II/I) electron self-exchange rate constants (k(11)) are surprisingly small, ranging approximately 0.03-50 M(-)(1) s(-)(1). The results are consistent with earlier studies reported by Yandell involving the reduction of Cu(II) complexes with four similar tripodal ligand systems, and it is concluded that the dominant reaction pathway involves a metastable Cu(II)L intermediate species (designated as pathway B). Since crystal structures suggest that the ligand reorganization accompanying electron transfer is relatively small compared to our earlier studies on macrocyclic ligand complexes of Cu(II/I), it is unclear why the k(11) values for the tripodal ligand systems are of such small magnitude. 相似文献
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The Kinetics of the binding of librium to bovine serum albumin were investigated at an ionic strength of 0.15 M, in the pH-range between 5.5 and 10.5, using the temperature-jump method. Two relaxation times were observed. The first, rapid one is assigned to the equilibrium between reactants and complex, whereas the second, slower relaxation time measures a conformational change of this complex. Both relaxation times are independent of pH up to about 8.0 when they increase sharply to a new plateau at a pH of about 9.3. A possible parallelism between this behavior and the N ? B transition of albumin is discussed. The overall equilibrium constant K, as well as the equilibrium constant between the two conformers, K2, were estimated from our kinetic results. Whereas the change of K with changing pH was within the experimental error, the equilibrium between the two conformers of the complex is somewhat shifted by an increase of the pH. At all values of pH, however, they are present in comparable concentrations. The value of K was also measured spectrophotometrically. The results of the two methods showed reasonable agreement. The relaxation times were affected very little by a change in temperature between 12 and 25°C from which it is concluded that the isomerization has a high negative value of ΔS?. 相似文献
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Marli Roehsig Daniela Mendes Louzada de Paula Sidnei Moura Edna Maria de Albuquerque Diniz Mauricio Yonamine 《Journal of separation science》2010,33(14):2115-2122
A number of fatty acid ethyl esters (FAEEs) have recently been detected in meconium samples. Several of these FAEEs have been evaluated as possible biomarkers for in utero ethanol exposure. In the present study, a method was optimized and validated for the simultaneous determination of eight FAEEs (ethyl laurate, ethyl myristate, ethyl palmitate, ethyl palmitoleate, ethyl stearate, ethyl oleate, ethyl linoleate and ethyl arachidonate) in meconium samples. FAEEs were extracted by headspace solid‐phase microextraction. Analyte detection and quantification were carried out using GC‐MS operated in chemical ionization mode. The corresponding D5‐ethyl esters were synthesized and used as internal standards. The LOQ and LOD for each analyte were <150 and <100 ng/g, respectively. The method showed good linearity (r2>0.98) in the concentration range studied (LOQ – 2000 ng/g). The intra‐ and interday imprecision, given by the RSD of the method, was lower than 15% for all FAEEs studied. The validated method was applied to 63 authentic specimens. FAEEs could be detected in alcohol‐exposed newborns (>600 ng/g cumulative concentration). Interestingly, FAEEs could also be detected in some non‐exposed newborns, although the concentrations were much lower than those measured in exposed cases. 相似文献
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Edna Furman-Haran Dov Grobgeld Hadassa Degani 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1997,128(2):161-171
High resolution, dynamic GdDTPA-enhanced images of MCF7 human breast tumors in immunodeficient mice were analyzed at pixel resolution. The analysis, based on a physiological model, was performed by applying a nonlinear least-square algorithm using a color coded scale. The final output mapped at pixel resolution capillary permeability times surface area and fraction of extracellular volume, for each tumor slice. In addition, the output included assessment of the fit to the model by determining the proportion of variability (R2) for each pixel. The spatial variation in theR2values served to identify regions where the predominant mechanism of enhancement was leakage from the intravascular volume to the extracellular volume (R2close to 1). In regions with lowR2other mechanisms of enhancement appear to be dominating presumably diffusion within the extracellular space. As expected, in necrotic regions lacking microcapillaries and identified by analyzingT2-weighted images of the same tumors, the model failed to fit the dynamic contrast enhanced data. The heterogeneous distribution of the determined pathophysiological features demonstrates the importance of recording and analyzing breast tumor images at high spatial resolution. 相似文献
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It is shown that thep-summing norm of any operator withn-dimensional domain can be well-aproximated using only “few” vectors in the definition of thep-summing norm. Except for constants independent ofn and logn factors, “few” meansn if 1<p<2 andn
p/2 if 2<p<∞.
Supported in part by NSF #DMS90-03550 and the U.S.-Israel Binational Science Foundation.
Supported in part by the U.S.-Israel Binational Science Foundation. 相似文献
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