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1.
It is well recognized that estradiol (E 2) is one of the most important hormones supporting the growth and evolution of breast cancer. Consequently, to block this hormone before it enters the cancer cell or in the cell itself, has been one of the main targets in recent years. In the present study we explored the effect of the progestin, nomegestrol acetate, on the estrone sulfatase and 17β-hydroxy-steroid dehydrogenase (17β-HSD) activities of MCF-7 and T-47D human breast cancer cells. Using physiological doses of estrone sulfate (E 1S: 5 × 10 −9 M), nomegestrol acetate blocked very significantly the conversion of E 1S to E 2. In the MCF-7 cells, using concentrations of 5 × 10 −6 M and 5 × 10 −5 M of nomegestrol acetate, the decrease of E 1S to E 2 was, respectively, −43% and −77%. The values were, respectively, −60% and −71% for the T-47D cells. Using E 1S at 2 × 10 −6 M and nomegestrol acetate at 10 −5 M, a direct inhibitory effect on the enzyme of −36% and −18% was obtained with the cell homogenate of the MCF-7 and T-47D cells, respectively. In another series of studies, it was observed that after 24 h incubation of a physiological concentration of estrone (E 1: 5 × 10 −9 M) this estrogen is converted in a great proportion to E 2. Nomegestrol acetate inhibits this transformation by −35% and −85% at 5 × 10 −7 M and 5 × 10 −5 M, respectively in T-47D cells; whereas in the MCF-7 cells the inhibitory effect is only significant, −48%, at 5 × 10 −5 M concentration of nomegestrol acetate. It is concluded that nomegestrol acetate in the hormone-dependent MCF-7 and T-47D breast cancer cells significantly inhibits the estrone sulfatase and 17β-HSD activities which converts E 1S to the biologically active estrogen estradiol. This inhibition provoked by this progestin on the enzymes involved in the biosynthesis of E 2 can open new clinical possibilities in breast cancer therapy. 相似文献
2.
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
3.
1. The alteration of the Ca 2+ requirements of the ATPase activity of fibrils from rabbits and crabs at varying ionic strength, pH and concentration of MgATP ( i.e. MgATP 2− + MgHATP −) was investigated. 2. Under physiological conditions, it was found that the ATPase activity of rabbit and crab fibrils after an initial increase decreased steeply when the Ca2+ concentration is raised above 1×10−4 M. This is a primary effect of the over-optimal Ca2+ concentration and not a secondary one caused by the influence of accompanying ions. 3. The Ca2+ requirements for ATP splitting by rabbit fibrils remain constant at an ionic strength from 0.1 to 0.2 and for a MgATP concentration in the range from 0.5 to 10 mM. At I = 0.05 it is about 5 times smaller than at 0.1. When the pH is decreased from 8 to 7, the Ca2+ requirements are increased some 10 times but only 3 times when the pH is varied between 7 and 6. 4. In crab fibrils, there is no alteration of the Ca2+ requirements when the ionic strength is varied between 0.05 and 0.2, but a reduction of the pH from 8.0 to 6.0 raises the Ca2+ requirements for half activation and for threshold by a factor of 10. Changing the MgATP concentration increases the Ca2+ requirements only in the range from 1 to 5 mM, while the concentration required in 0.5 mM is identical with that at 1 mM, and 10 mM corresponds to 5 mM. 5. It can be deduced from the experimental results that at a pH above 6.0 maximal activation is always obtained if the Ca2+ concentration is 5×10−5 M. By contrast, relaxation is only achieved when the Ca2+ concentration is below 1×10−7 M for pH 7.0 and I > 0.1 or below 1×10−8 for pH > 7.0 or I < 0.1. 6. To achieve complete relaxation, an ethyleneglycoldiaminotetraacetate (EGTA) concentration of 1 mM is sufficient, even when there is a large degree of contamination by Ca2+ as long as the pH stays above 6.5. 相似文献
4.
Comparative measurements of bacterial total counts and volumes of flow cytometry (FCM), transmission electron (TEM), and epifluorescence microscopy (EFM), were undertaken during a four week mesocosm experiment. Total counts of bacteria measured by TEM, EFM, and FCM were in the range of 1 · 10 6−6 cells ml −1, 1 · 10 6−3 · 10 16 cells ml −1, and 5 · 10 5 cells ml −1 respectively. The mean volume of the bacterial community, measured by means of EFM and TEM, increased from 0.12–0.15 μm 3 at the start of the experiment to 0.39–0.53 μm 3 at the end. Generally, there was good agreement between the two methods and regression analyses gave r = 0.87 ( p < < 0.01) for cell volume and r = 0.97 ( p < < 0.01) for cell number. DAPI stained bacteria with volumes less than 0.2 μm 3 were not detected by flow cytometry and these were generally an order of magnitude lower than counts made by TEM and EFM. For samples where the mean bacterial cell volume was longer than 0.3 μm 3, all three methods were in agreement both with respect to counts and volume estimates. 相似文献
5.
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
6.
An improved and standardized procedure for isolation of chloroplast particles from the unicellular green alga, Scenedesmus obliquus, D 3, is described. The method is generally applicable to heterotrophically- and autotrophically-grown cells of Scenedesmus as well as to Chlamydomonas reinhardti and Chlorella sorokiniana (7-11-05) cultures. Chloroplast particles with high NADP + photoreducing capacity are obtained from heterotrophic cultures only when the cell types are random and the culture is in the logarithmic growth phase; maximal rates of 240–260 μmoles NADP + reduced/h per mg chlorophyll are achieved. Optimal conditions for separation of such chloroplast particles require the use of Tricine buffer (20 mM, pH 7.5), 50 nM EDTA, 10 mM KCl and 0.5 mM dithiothreitol in the breaking medium; for the maintenance of high photochemical activity it is necessary to store particles in a solution consisting of 0.4 M sucrose, 30 mM KCl and 1% bovine serum albumin. Optimum reaction conditions were developed and the properties of the isolated particles investigated. Maximal activities are obtained when the sucrose concentration is maintained below 0.4 M; the pH optimum with Tricine buffer is between 7.8–8.1; and at least 30 mM Cl− is required. Red actinic light (wavelength >620 nm) with an intensity of 106 ergs/cm2 per s is required for saturation. Ferrodoxin and ferredoxin-NADP+ oxidoreductase are lost from the particles during the preparatory procedures and maximum photochemical activity is attained only when they are added back in balanced amounts. Stimulatory effects of added plastocyanin and cytochrome c-553 are noted only with particles having an initially low photochemical activity. 相似文献
7.
Nitrogen dioxide (NO 2•) is a key biological oxidant. It can be derived from peroxynitrite via the interaction of nitric oxide with superoxide, from nitrite with peroxidases, or from autoxidation of nitric oxide. In this study, submicromolar concentrations of NO 2• were generated in < 1 μs using pulse radiolysis, and the kinetics of scavenging NO 2• by glutathione, cysteine, or uric acid were monitored by spectrophotometry. The formation of the urate radical was observed directly, while the production of the oxidizing radical obtained on reaction of NO 2• with the thiols (the thiyl radical) was monitored via oxidation of 2,2′-azino-bis-(3-ethylthiazoline-6-sulfonic acid). At pH 7.4, rate constants for reaction of NO 2• with glutathione, cysteine, and urate were estimated as 2 × 10 7, 5 × 10 7, and 2 × 10 7 M −1 s −1, respectively. The variation of these rate constants with pH indicated that thiolate reacted much faster than undissociated thiol. The dissociation of urate also accelerated reaction with NO 2• at pH > 8. The thiyl radical from GSH reacted with urate with a rate constant of 3 × 10 7 M −1 s −1. The implications of these values are: (i) the lifetime of NO 2• in cytosol is < 10 μs; (ii) thiols are the dominant ‘sink’ for NO 2• in cells/tissue, whereas urate is also a major scavenger in plasma; (iii) the diffusion distance of NO 2• is 0.2 μm in the cytoplasm and < 0.8 μm in plasma; (iv) urate protects GSH against depletion on oxidative challenge from NO 2•; and (v) reactions between NO 2• and thiols/urate severely limit the likelihood of reaction of NO 2• with NO• to form N 2O 3 in the cytoplasm. 相似文献
8.
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
9.
The formation of nitrite reductase and cytochrome c in Micrococcus denitrificans was repressed by O 2. The purified nitrite reductase utilized reduced forms of cytochrome c, phenazine methosulphate, benzyl viologen and methyl viologen, respectively, as electron donors. The enzyme was inhibited by KCN, NaN 3 and NH 2OH each at 1 mM, whereas CO and bathocuproin, diethyl dithiocarbamate, o-phenanthroline and ,'-dipyridyl at 1 mM concentrations were relatively ineffective. The purified enzyme contains cytochromes, probably of the c and a2 types, in one complex. A Km of 46 μM for NO 2− and a pH optimum of 6.7 were recorded for the enzyme. The molecular weight of the enzyme was estimated to be around 130000, and its anodic mobility was 6.8·10 −6 cm 2·sec −1·V −1 at pH 4.55. The most highly purified nitrite reductase still exhibited cytochrome c oxidase activity with a Km of 27 μM for O2. This activity was also inhibited by KCN, NaN3 and NH2OH and by NO2−. A constitutive cytochrome oxidase associated with membranes was also isolated from cells grown anaerobically with NO2−. It was inhibited by smaller amounts of KCN, NaN3 and NH2OH than the cytochrome oxidase activity of the nitrite reductase enzyme and also differed in having a pH optimum of about 8 and a Km for O2 of less than 0.1 μM. Spectroscopically, cytochromes b and c were found to be associated with the constitutive oxidase in the particulate preparation. Its activity was also inhibited by NO2−. The physiological role of the cytochrome oxidase activity associated with the purified nitrite reductase is likely to be of secondary importance for the following reasons: (a) it accounts for less than 10% of total cytochrome c oxidase activity of cell extracts; (b) the constitutive cytochrome c oxidase has a smaller Km for O2 and would therefore be expected to function more efficiently especially at low concentrations of O2. 相似文献
10.
1. Single reduced methyl viologen (MV .+) acts as an electron donor in a number of enzyme systems. The large changes in extinction coefficient upon oxidation (λ max 600 nm; MV .+, = 1.3 · 10 4 M −1 · cm −1; oxidised form of methyl viologen (MV 2+), = 0.0) make it ideally suited to kinetic studies of electron transfer reactions using stopped-flow and standard spectrophotometric techniques. 2. A convenient electrochemical preparation of large amounts of MV.+ has been developed. 3. A commercial stopped-flow apparatus was modified in order to obtain a high degree of anaerobicity. 4. The reaction of MV.+ with O2 produced H2O2 (k > 5 · 106 M−1 · s−1, pH 7.5, 25 °C). H2O2 subsequently reacted with excess MV.+ (k = 2.3 · 103 M−1 · s−1, pH 7.5, 25 °C) to produce water. The kinetics of this reaction were complex and have only been interpreted over a limited range of concentrations. 5. The results support the theory that the herbicidal action of methyl viologen (Paraquat, Gramoxone) is due to H2O2 (or radicals derived from H2O2) induced damage of plant cell membrane. 相似文献
11.
The perchlorate (ClO 4−)-respiring organism, strain perc1ace, can grow using nitrate (NO 3−) as a terminal electron acceptor. In resting cell suspensions, NO 3− grown cells reduced ClO 4−, and ClO 4− grown cells reduced NO 3−. Activity assays showed that nitrate reductase (NR) activity was 1.31 μmol min −1 (mg protein) −1 in ClO 4− grown cells, and perchlorate reductase (PR) activity was 4.24 μmol min −1 (mg protein) −1 in NO 3− grown cells. PR activity was detected within the periplasmic space, with activities as high as 14 μmol min −1 (mg protein) −1. The NR had a pH optimum of 9.0 while the PR had an optimum of 8.0. This study suggests that separate terminal reductases are present in strain perclace to reduce NO 3− and ClO 4−. 相似文献
12.
Biological properties of amino-terminal PTHrP analogues modified in the region 11–13 were examined using ROS 17/2.8 cells. [Leu 11,D-Trp 12,Arg 13,Tyr 36]PTHrP(1–36)amide had a 17-fold lower binding affinity for the receptor (apparent K d: 5 × 10 −8 M) than [Tyr 36]PTHrP(1–36)amide or [Arg 11,13,Tyr 36]PTHrP(1–36)amide (apparent K d for both: 2 × 10 −9 M). Moreover, it is only a weak partial agonist despite completely inhibiting radioligand binding. [Leu 11,D-Trp 12,Arg 13,Tyr 36,Cys 38]PTHrP(7–38) and PTHrP(7–34)amide had similar receptor affinities (apparent K ds: 5 × 10 −8 M and 8 × 10 −8 M), while that of [Nle 8,18,Tyr 34]bPTH(7–34)amide was more than 10-fold lower (apparent K d: 2 × 10 −6 M). These changes in biological properties suggest that high affinity receptor binding requires both amino- and carboxyl-terminal domains of the PTHrP(1–36) sequence and/or intramolecular interactions which are impaired by the D-Trp substitution for Gly 12. 相似文献
13.
When cultivated in Murashige & Skoog medium supplemented with 0.2 mg l −1 2,4-dichlorophenoxy acetic acid and 0.5 mg l −1 6-benzyladenine, Perilla frutescens cells in suspension culture grew rapidly reaching about 13.6 g dry wt l −1 after 12 days. The cell line produced both anthocyanin 0.9 g l −1 and triterpenoids: 16 mg l −1 oleanolic acid (OA), 25 mg l −1 ursolic acid (UA) and 14 mg l −1 tormentic acid (TA). When P. frutescens cells of 7-day-old cultures were exposed to a yeast elicitor at 0.5–5% (v/v) for 7 days, it was found that anthocyanin content peaked at 10.2% of dry weight with yeast elicitor at 1% (v/v) whereas the maximum production of oleanolic acid and ursolic acid in cultures treated with 2% (v/v) yeast elicitor was 19 and 27 mg l −1, a 46 and 24% increase over the control, respectively. This is the first report of simultaneous production of both anthocyanin and triterpenoids in a single culture system. 相似文献
14.
The interaction between sialosyl cholesterol (- or
neuraminyl cholesterol, - or β-SC) and the plasma membrane of astrocytes was investigated by the use of 14C-labeled - or β-SC. Both - and β-SC were dose-dependently and time-dependently bound to rat astrocytes. The Scatchard plot analyses showed that rat astrocytes bound apparently 9.69 × 10 9 molecules of both -SC/cell (apparent Kd = 2.29 × 10 −5 M) and β-SC/cell (apparent Kd = 5.39 × 10 −5 M) at 37°C. Both the binding of -SC to astrocytes and the subsequent inhibition of DNA synthesis were decreased at the low temperature (4°C), and also suppressed by serum proteins including albumin. One molecule of bovine serum albumin (BSA) bound 2.3 molecules of -SC with the slightly lower K d- value (8.03 × 10 −6 M) than that for the binding site on astrocytes. BSA not only suppressed the -SC-binding to astrocytes but also increased its release from the cells to the culture media. Gangliosides such as GM1 and GM3 unaffected the -SC-binding, promoted the small release of -SC from the cell surface, and inhibited the morphological changes of astrocytes induced by -SC. The mechanism of -SC-binding to cultured astrocytes with reference to the effects of serum or gangliosides is discussed. 相似文献
15.
1. Difference spectra, at room and liquid N 2 temperatures, of S 2O 42−-, and NO 2−-reduced intact cells and cell-free preparations of Nitrobacter agilis demonstrated the presence of cytochromes of the c- and a-types. Reduction of cytochromes by succinate, and to a limited extent, by NADPH also occurred, provided KCN (0.1 mM) was also present. 2. A particulate, heat-labile nitrite oxidase having an absolute requirement for O2 was prepared from N. agilis cells using sonic oscillation and differential centrifugation. The particles also possessed NADH oxidase, succinoxidase, formate oxidase and traces of NADPH oxidase activity. The stoichiometry of the nitrite oxidase reaction approached the theoretical value of 2 moles of NO2− consumed per mole of O2 consumed. The pH optimum of the nitrite oxidase system shifted to progressively more alkaline values as the NO2− concentration was increased, changing from a pH value of 6.8 at 0.6 mM KNO2 to pH 8.0 at 0.01 M KNO2 with apparent Km's of 0.2 and 1.2 mM NO2−, respectively. Computations of the HNO2 concentrations present under the above conditions showed an approx. 500-fold greater affinity for HNO2 which was independent of pH, suggesting the involvement of HNO2 as both a substrate and an inhibitor (at higher concentrations) of the nitrite oxidase system. The marked inhibition by NaN3, NaCN and Na2S, as well the light-reversible inhibition by CO, indicated the presence of cytochrome oxidase which was subsequently characterized. NO2− proved to be a competitive inhibitor of the nitrite oxidase system. 3. The particulate preparation also possessed a heat-labile nitrite-cytochrome c reductase activity which was energy independent and routinely measured under anaerobic conditions. As in the case of nitrite oxidase, the affinity of the enzyme for NO3− increased as the pH was lowered, but the pH optimum remained unaffected. In terms of calculated HNO2 concentration an approximately constant Km of about 0.2 μM was estimated at the several pH's examined. The inhibition by NO3− was shown to be competitive. The marked sensitivity of the reductase to several metal-binding agents implicated a metal component in the electron transport chain at the site prior to cytochrome c. 4. The membrane-like composition of the nitrite oxidase system is indicated. 相似文献
16.
Crab shell particles were used as a biosorbent to remove lead from aqueous solutions. The equilibrium isotherm showed that crab shell particles took up lead to the extent of 1300 mg Pb g −1 crab shell. The optimum pH range for maximum lead removal was increased to 5·5–11·0 compared to the shell-free control pH of 8·5–11·0. pH values of solutions with crab shell material added were increased spontaneously to about 10 as a result of the CaCO 3 present, which formed complexes with lead according to pH. Electron spectroscopy, Fourier transform infrared spectrometry, scanning electron microscopy and X-ray diffraction results confirmed that -NHCOCH 3 and CO 32 were involved in binding of lead. In addition, the removal of lead occurred mainly through dissolution of CaCO 3 followed by precipitation of Pb 3(CO 3) 2(OH) 2 and PbCO 3 near the surface of crab shell. Micro precipitates formed were then adsorbed to the chitin on the surface of the crab shell particles. 相似文献
17.
Amperometric choline biosensors were fabricated by the covalent immobilization of an enzyme of choline oxidase (ChO) and a bi-enzyme of ChO/horseradish peroxidase (ChO/HRP) onto poly-5,2′:5′,2″-terthiophene-3′-carboxylic acid (poly-TTCA) modified electrodes (CPMEs). A sensor modified with ChO utilized the oxidation process of enzymatically generated H 2O 2 in a choline solution at +0.6 V. The other one modified with ChO/HRP utilized the reduction process of H 2O 2 in a choline solution at −0.2 V. Experimental parameters affecting the sensitivity of sensors, such as pH, applied potential, and temperature were optimized. A performance comparison of two sensors showed that one based on ChO/HRP/CPME had a linear range from 1.0×10 −6 to 8.0×10 −5 M and the other based on ChO/CPME from 1.0×10 −6 to 5.0×10 −5 M. The detection limits for choline employing ChO/HRP/CPME and ChO/CPME were determined to be about 1.0×10 −7 and 4.0×10 −7 M, respectively. The response time of sensors was less than 5 s. Sensors showed good selectivity to interfering species. The long-term storage stability of the sensor based on ChO/HRP/CPME was longer than that based on ChO/CPME. 相似文献
18.
A new functional macrocyclic ligand, 2,4-dinitrophenylcyclen (= 1-(2,4-dinitrophenyl)-1,4,7,10-tetraazacyclododecane), has been synthesized and isolated as its trihydrochloric acid salt (L·3HCl). The protonation constants (log Kn) for three secondary nitrogens of L were determined by potentiometric pH titration to be 10.10, 7.33 and <2 with I = 0.10 (NaNO 3) at 25°C. The 2,4-dinitrophenylaniline chromophore was proven to be a good reporter signaling proton- and metal-binding events in the macrocyclic cavity. The UV absorption band (λ max 370 nm, 8200) of the 2,4-dinitrophenylaniline moiety at pH ≥ 9 becomes quenched as pH is lowered (to pH 3.1, where the major species is L·2H +), due to the strong protonation effect extended to the aniline moiety within the macrocyclic cavity. This is in sharp contrast to the pH-independent UV absorption (λ max 390 nm, 14 000) of a reference compound, N,N-diethyl-2,4-dinitroaniline. The UV absorption band of L is shifted to lower wavelengths with Zn 2+ (λ max 320 nm), Cd 2+ (λ max 316 nm) and Pb 2+ (λ max 317 nm), while it almost disappears with Cu 2+ and Ni 2+. The 1:1 Zn 2+ and Cu 2+ complexes with L were isolated and characterized. The Zn 2+ complex recognizes 1-methylthymine anion (MT −) in aqueous solution at physiological pH to yield a stable ternary complex ZnL-MT −. The X-ray crystal structure of ZnL-MT − showed that Zn 2+ is four-coordinate with three secondary nitrogens of L and the deprotonated imide anion that is cofacial to the 2,4-dinitrophenyl ring. 相似文献
19.
1. Rate constants for reduction of paraquat ion (1,1′-dimethyl-4,4′-bipyridy-lium, PQ 2+) to paraquat radical (PQ +·) by e−aq and CO 2−· have been measured by pulse radiolysis. Reduction by e−aq is diffusion controlled ( k = 8.4·10 10 M −1·s −1) and reduction by CO 2−· is also very fast k = 1.5·10 10 M −1·s −1). 2. The reaction of paraquat radical with oxygen has been analysed to give rate constants of 7.7·108 M−1·s−1 and 6.5·108 M−1·s−1 for the reactions of paraquat radical with O2 and O2−·, respectively. The similarity in these rate constants is in marked contrast to the difference in redox potentials of O2 and O2−· (− 0.59 V and + 1.12 V, respectively). 3. These rate constants, together with that for the self-reaction of O2−·, have been used to calculate the steady-state concentration of O2−· under conditions thought to apply at the site of reduction of paraquat in the plant cell. On the basis of these calculations the decay of O2−· appears to be governed almost entirely by its self-reaction, and the concentration 5 μm away from the thylakoid is still 90% of that at the thylakoid itself. Thus, O2−· persists long enough to diffuse as far as the chloroplast envelope and tonoplast, which are the first structures to be damaged by paraquat treatment. O2−· is therefore sufficiently long-lived to be a candidate for the phytotoxic product formed by paraquat in plants. 相似文献
20.
The growth of the freshwater microalga Scenedesmus obliquus was studied at 30°C in a mineral culture medium with phosphorus concentrations of between 0 and 372 μ
. The values for the specific growth rates, between
and
, fitted a semistructured substrate-limitation model with μ m1 = 0·0466 h −1, μ m2 = 0·0256 h −1 and
. The specific uptake rate of phosphorus reached a maximum value of qSm1 = 658·01 × 10 −4 μmol P mg −1 biomass h −1. 相似文献
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