首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 31 毫秒
1.
The conditions of dissolution of freshly precipitated niobium (V) oxide in α-hydroxy carboxylic acids glycolic, lactic, malic and tartaric were investigated. The dissolution is a function of the molar ratio α-hydroxy carboxylic acid/hydrated niobium(V) oxide, pH of the solution, temperature and time. From solutions of α-hydroxy monocarboxylic acids at 2 < pH < 3 the binuclear complexes H3O[Nb2O4(C2H2O3)(C2H3O3)]·H2O and H3O[Nb2O4(C3H4O3)(C3H5O3)]·H2O were isolated. Colourless, poorly-crystalline complexes are 1:1 electrolytes and, according to i.r. spectral evidence, the binuclearity in their structures is achieved through oxygen bridges. With α-hydroxy dicarboxylic acids crystalline M[Nb2O3(C4H3O5)(C4H4O5)]·nH2O and poorly crystalline complexes, M2[Nb2O2(C4H2O6)2]·nH2O, M = H3O+, NH4+ were prepared as 1:1 electrolytes for the former and 1:2 electrolytes for the latter. Analytical, spectral, conductometric and potentiometric titration data give evidence for binuclear malatoniobate(V) and tartratoniobate(V) anions with bridging complex-forming agents.  相似文献   

2.
Phosphonium zwitterions of a known type were obtained in high yield via a 1:1 reaction of p-benzoquinone or methoxy-p-benzoquinone with the tertiary phosphines R3P [R = (CH2)3OH, Ph, Et, Me] and Ph2MeP, in acetone or benzene at room temperature. In all cases, attack of the P-atom occurs at a C-atom rather than at an O-atom. The products were characterized to various degrees by elemental analysis, 31P{1H}, 1H and 13C NMR spectroscopies, and mass spectrometry, and two of the zwitterions, the new [HO(CH2)3]3P+C6H2(O)(OH)(MeO) and the known Ph3P+C6H3(O)(OH), were structurally characterized by X-ray analysis. The PEt3 reaction also produces small amounts of the ‘dimeric’, μ-oxo co-product Et3P+C6H2(O)(OH)-O-C6H3(O)P+Et3 that is tentatively characterized by 1D- and 2D-NMR data. 2,5-Di-tert-butyl- and 2,3,5,6-tetramethyl-p-benzoquinone do not react with [HO(CH2)3]3P under the conditions noted above. Heating D2O solutions of the water-soluble zwitterions R3P+C6H3(O)(OH) [R = (CH2)3OH, Et] at 90 °C for 72 h leads to complete H/D exchange of the H-atom in the position ortho to the phosphonium center.  相似文献   

3.
In this report, chiral dirhodium (II) with ortho-metalated phosphane ligands, namely (M)-Rh2(O2CR) 2(PC)2 [PC = ortho-metalated aryl phosphane, O2CR = carboxylate bridging ligands) (1a-g), have been used for the intramolecular cyclopropanation of racemic1-diazo-6-methyl-3-(2-propenyl)-5-hepten-2-one (2), containing both a tri- and monosubstituted carbon-carbon double bond, in pentane. The highest level of regiocontrol has been obtained with chiral catalyst Rh2(O2CCH3)2[(p-MeC6H3)P(p-MeC6H4)2]2 (M)-1c, affording favorably trisubstituted cyclopropane 3 versus monosubstituted cyclopropane 4 in 74:26 ratio. An exceptional diastereoselectivity was obtained with the entire catalyst series, leading to the unique formation of the syn products. Excellent enantiocontrol values (80-90% ee) have been achieved with catalysts 1a [(PC = (C6H3)P(C6H4)2, R = C(CH3)3)], 1c [PC = p-MeC6H3)P(p-MeC6H4)2, R = CH3], 1f [PC = m-CH3C6H3)P(m-CH3C6H4)2, R = CF3] and 1g [PC = 3,5-(CH3)2C6H3)P(3,5-(CH3)2C6H4)2, R = CF3] at room temperature. Pentane is found to be a convenient solvent for high enantiocontrol in the cyclopropanation of α-diazo ketone 2.  相似文献   

4.
The main sesquiterpene lactone found in collections of Helenium autumnale L. from Pennsylvania was not helenalin, but tenulin. Minor constituents of the flowers were mexicanin I, flexuosin A, a new lactone C15H20O4 and the flavone hispidulin.  相似文献   

5.
This study aimed to assess the impact of Moringa oleifera (M. oleifera) leaf extract against the poisoning of lead acetate; therefore, sixty mice were allocated into 4 groups with 15 in each, as G1) blank control, G2) supplied with 300 mg/kg body weight (BWT). M. oleifera extract, G3) supplied with 60 mg/kg BWT of lead acetate [Pb(C2H3O2)2], and G4) supplied with extract of M. oleifera + lead acetate. The liver enzymes were elevated post-treatment with Pb(C2H3O2)2, which then lowered to almost the normal level when M. oleifera was supplied to mice previously treated with Pb(C2H3O2)2. The values in (G3) decreased when compared with G1 (92.33 ± 12.99, 21.67 ± 2.91 and 98.00 ± 13.20 U/L, respectively. Also, the cholesterol and low-density lipoprotein levels were elevated post-supplementation with M. oleifera and Pb(C2H3O2)2. Pb(C2H3O2)2 improves the lipid profile, whereas M. oleifera pretreatment reduced cholesterol (CHOL), high density low cholesterol (HDL-c), and low-density low cholesterol (LDL-c) levels in animals fed Pb(C2H3O2)2. Pb(C2H3O2)2 elevates the total protein but lowers the total bilirubin and triglycerides post M. oleifera treatment and Pb(C2H3O2)2 when contrasted with G1. The protective effect of M. oleifera was caused by the fact that it lowered triglycerides (TG) and total bilirubin (TBIL) and raised total protein (TP). After administration of Pb(C2H3O2)2, the histological examination revealed alterations in the hepatocytes and kidneys of G3. Also, the liver and kidney cells in mice supplied with M. oleifera after Pb(C2H3O2)2 poisoning recovered. In conclusion, Pb is toxic, and the usage of M. oleifera partially enhances the negative impacts induced by Pb(C2H3O2)2.  相似文献   

6.
In Saccharomyces cerevisiae, the diffusion rate of hydrogen peroxide (H2O2) through the plasma membrane decreases during adaptation to H2O2 by means of a mechanism that is still unknown. Here, evidence is presented that during adaptation to H2O2 the anisotropy of the plasma membrane increases. Adaptation to H2O2 was studied at several times (15min up to 90min) by applying the steady-state H2O2 delivery model. For wild-type cells, the steady-state fluorescence anisotropy increased after 30min, or 60min, when using 2-(9-anthroyloxy) stearic acid (2-AS), or diphenylhexatriene (DPH) membrane probe, respectively. Moreover, a 40% decrease in plasma membrane permeability to H2O2 was observed at 15min with a concomitant two-fold increase in catalase activity. Disruption of the ergosterol pathway, by knocking out either ERG3 or ERG6, prevents the changes in anisotropy during H2O2 adaptation. H2O2 diffusion through the plasma membrane in S. cerevisiae cells is not mediated by aquaporins since the H2O2 permeability constant is not altered in the presence of the aquaporin inhibitor mercuric chloride. Altogether, these results indicate that the regulation of the plasma membrane permeability towards H2O2 is mediated by modulation of the biophysical properties of the plasma membrane.  相似文献   

7.

Background and Aims

The hormone auxin and reactive oxygen species (ROS) regulate root elongation, but the interactions between the two pathways are not well understood. The aim of this study was to investigate how auxin interacts with ROS in regulating root elongation in tomato, Solanum lycopersicum.

Methods

Wild-type and auxin-resistant mutant, diageotropica (dgt), of tomato (S. lycopersicum ‘Ailsa Craig’) were characterized in terms of root apical meristem and elongation zone histology, expression of the cell-cycle marker gene Sl-CycB1;1, accumulation of ROS, response to auxin and hydrogen peroxide (H2O2), and expression of ROS-related mRNAs.

Key Results

The dgt mutant exhibited histological defects in the root apical meristem and elongation zone and displayed a constitutively increased level of hydrogen peroxide (H2O2) in the root tip, part of which was detected in the apoplast. Treatments of wild-type with auxin increased the H2O2 concentration in the root tip in a dose-dependent manner. Auxin and H2O2 elicited similar inhibition of cell elongation while bringing forth differential responses in terms of meristem length and number of cells in the elongation zone. Auxin treatments affected the expression of mRNAs of ROS-scavenging enzymes and less significantly mRNAs related to antioxidant level. The dgt mutation resulted in resistance to both auxin and H2O2 and affected profoundly the expression of mRNAs related to antioxidant level.

Conclusions

The results indicate that auxin regulates the level of H2O2 in the root tip, so increasing the auxin level triggers accumulation of H2O2 leading to inhibition of root cell elongation and root growth. The dgt mutation affects this pathway by reducing the auxin responsiveness of tissues and by disrupting the H2O2 homeostasis in the root tip.  相似文献   

8.
Background and Aims In photosynthetic organisms exposure to high light induces the production of reactive oxygen species (ROS), such as hydrogen peroxide (H2O2), which in part is prevented by non-photochemical quenching (NPQ). As one of the most stable and longest-lived ROS, H2O2 is involved in key signalling pathways in development and stress responses, although in excess it can induce damage. A ubiquitous response to high light is the induction of the xanthophyll cycle, but its role in algae is unclear as it is not always associated with NPQ induction. The aim of this study was to reveal how diurnal changes in the level of H2O2 are regulated in a freshwater algal community.Methods A natural freshwater community of algae in a temporary rainwater pool was studied, comprising photosynthetic Euglena species, benthic Navicula diatoms, Chlamydomonas and Chlorella species. Diurnal measurements were made of photosynthetic performance, concentrations of photosynthetic pigments and H2O2. The frequently studied model organisms Chlamydomonas and Chlorella species were isolated to study photosynthesis-related H2O2 responses to high light.Key Results NPQ was shown to prevent H2O2 release in Chlamydomonas and Chlorella species under high light; in addition, dissolved organic carbon excited by UV-B radiation was probably responsible for a part of the H2O2 produced in the water column. Concentrations of H2O2 peaked at 2 µm at midday and algae rapidly scavenged H2O2 rather than releasing it. A vertical H2O2 gradient was observed that was lowest next to diatom-rich benthic algal mats. The diurnal changes in photosynthetic pigments included the violaxanthin and diadinoxanthin cycles; the former was induced prior to the latter, but neither was strictly correlated with NPQ.Conclusions The diurnal cycling of H2O2 was apparently modulated by the organisms in this freshwater algal community. Although the community showed flexibility in its levels of NPQ, the diurnal changes in xanthophylls correlated with H2O2 concentrations. Alternative NPQ mechanisms in algae involving proteins of the light-harvesting complex type and antioxidant protection of the thylakoid membrane by de-epoxidized carotenoids are discussed.  相似文献   

9.
Mesembryathemum crystallinum plants performing C3 or CAM (crassulacean acid metabolism) appear to be highly resistant to Botrytis cinerea as well as to Pseudomonas syringae. Fungal hyphae growth was restricted to 48 h post-inoculation (hpi) in both metabolic types and morphology of hyphae differed between those growing in C3 and CAM plants. Growth of bacteria was inhibited significantly 24 hpi in both C3 and CAM plants. B. cinerea and P. syringae infection led to an increase in the concentration of H2O2 in C3 plants 3 hpi, while a decrease in H2O2 content was observed in CAM performing plants. The concentration of H2O2 returned to the control level 24 and 48 hpi. Changes in H2O2 content corresponded with the activity of guaiacol peroxidase (POD), mostly 3 hpi. We noted that its activity decreased significantly in C3 plants and increased in CAM plants in response to inoculation with both pathogens. On the contrary, changes in the activity of CAT did not correlate with H2O2 level. It increased significantly after interaction of C3 plants with B. cinerea or P. syringae, but in CAM performing plants, the activity of this enzyme was unchanged. Inoculation with B. cinerea or P. syringae led to an increase in the total SOD activity in C3 plants while CAM plants did not exhibit changes in the total SOD activity after interaction with both pathogens. In conclusion, the pathogen-induced changes in H2O2 content and in SOD, POD and CAT activities in M. crystallinum leaves, were related to the photosynthetic metabolism type of the stressed plants rather than to the lifestyle of the invading pathogen.  相似文献   

10.
The time course profiles of C2H2 reduction by intact Scirpus olneyi (bulrush), Oryza sativa (rice) and Spartina alterniflora (cordgrass) with roots in atmospheres of N2 and 30-day-old Glycine max (soybean) in air were all immediately linear. This is the first report of immediately linear rates of C2H2 reduction by grass roots removed from soil. The immediately linear profile of C2H2 reduction by soil-free grass roots was achieved by preventing contact between the roots and air. Roots of soybeans and S. olneyi receiving pretreatments of O2 above normal environmental levels for 15 min before assay exhibited a short delay in C2H2 reduction. These initially nonlinear rates of C2H2 reduction are attributable to transient O2 inhibition of nitrogenase. Initial nonlinear rates of C2H2 reduction were also observed with immature soybean plants and with intact plant assays of O. sativa and S. olneyi in which C2H2 was injected into cylinders surrounding the plant tops. These results indicate that, apart from O2 inhibition of nitrogenase, the diffusion of C2H2 and C2H4 between the nitrogen-fixing sites and the sampling ports may cause initial nonlinear rates of C2H2 reduction. We conclude that in situ plant-associated nitrogenase activity should result in immediate reduction of C2H2 and that linear rates are observed when the proper assay conditions are used. Our data suggest that nitrogen fixation is closely associated with the roots of S. olneyi, O. sativa, and S. alterniflora growing in salt marsh sediment.  相似文献   

11.
The X-ray diffraction and i.r. absorption conformational analysis of N-tert-butyloxycarbonyl-l-phenylalanine has showed the absence of intramolecularly hydrogen-bonded peptide conformations in the solid state. The molecules are held together in rows of ‘cyclic dimer’ motifs through intermolecular NHOC (acid) and OHOC {urethane} hydrogen bonds, the secondary amide-like group of the urethane moiety being in the unusual cis conformation, whereas the carboxylic acid group in the common syn conformation. The two molecules in the unit cell present a centrosymmetric set of ?, ψ1, and ψ2 values. In polar solvents solvated species largely predominate. In saturated hydrocarbon solution non-associated and associated (mostly involving the carboxylic acid CO as the proton acceptor) species simultaneously occur. The extent of association decreases with dilution. The amount of intramolecularly hydrogen-bonded oxy-C7 and C5 forms if any, should be extremely small. The type of association at saturation seems to differ from that found in the crystalline compound obtained by precipitation with saturated aliphatic hydrocarbons (from a diethyl ether solution).  相似文献   

12.
Asclepias eriocarpa and A. labriformis contain three new cardenolides, the structures of which have been partially assigned by their spectral properties and comparison with the known cardenolides of A. curassavica. They include labriformin (C31H39O10NS), labriformidin (C29H31O11) and eriocarpin.  相似文献   

13.
FabF elongation condensing enzyme is a critical factor in determining the spectrum of products produced by the FASII pathway. Its active site contains a critical cysteine-thiol residue, which is a plausible target for oxidation by H2O2. Streptococcus pneumoniae produces exceptionally high levels of H2O2, mainly through the conversion of pyruvate to acetyl-P via pyruvate oxidase (SpxB). We present evidence showing that endogenous H2O2 inhibits FabF activity by specifically oxidizing its active site cysteine-thiol residue. Thiol trapping methods revealed that one of the three FabF cysteines in the wild-type strain was oxidized, whereas in an spxB mutant, defective in H2O2 production, none of the cysteines was oxidized, indicating that the difference in FabF redox state originated from endogenous H2O2. In vitro exposure of the spxB mutant to various H2O2 concentrations further confirmed that only one cysteine residue was susceptible to oxidation. By blocking FabF active site cysteine with cerulenin we show that the oxidized cysteine was the catalytic one. Inhibition of FabF activity by either H2O2 or cerulenin resulted in altered membrane fatty acid composition. We conclude that FabF activity is inhibited by H2O2 produced by S. pneumoniae.  相似文献   

14.
The kinetics of the formation and relaxation of transmembrane electric potential (Δψ) during the complete single turnover of CcO was studied in the bovine heart mitochondrial and the aa3-type Paracoccus denitrificans enzymes incorporated into proteoliposome membrane. The real-time Δψ kinetics was followed by the direct electrometry technique. The prompt oxidation of CcO and formation of the activated, oxidized (OH) state of the enzyme leaves the enzyme trapped in the open state that provides an internal leak for protons and thus facilitates dissipation of Δψ (τapp ≤ 0.5-0.8 s). By contrast, when the enzyme in the OH state is rapidly re-reduced by sequential electron delivery, Δψ dissipates much slower (τapp > 3 s). In P. denitrificans CcO proteoliposomes the accelerated Δψ dissipation is slowed down by a mutational block of the proton conductance through the D-, but not K-channel. We concluded that in contrast to the other intermediates the OH state of CcO is vulnerable to the elevated internal proton leak that proceeds via the D-channel.  相似文献   

15.
To ascertain the effect of exogenously applied hydrogen peroxide (H2O2) on drought stress, we examined whether the spraying of soybean leaves with H2O2 would alleviate the symptoms of drought stress. Pre-treatment by spraying leaves with H2O2 delayed foliar wilting caused by drought stress compared to leaves sprayed with distilled water (DW). Additionally, the relative water content of drought-stressed leaves pre-treated with H2O2 was higher than that of leaves pre-treated with DW. Therefore, we analyzed the effect of H2O2 spraying on photosynthetic parameters and on the biosynthesis of oligosaccharides related to water retention in leaves during drought stress. Under conditions of drought stress, the net photosynthetic rate and stomatal conductance of leaves pre-treated with H2O2 were higher than those of leaves pre-treated with DW. In contrast to DW spraying, H2O2 spraying immediately caused an increase in the mRNA levels of d-myo-inositol 3-phosphate synthase 2 (GmMIPS2) and galactinol synthase (GolS), which encode key enzymes for the biosynthesis of oligosaccharides known to help plants tolerate drought stress. In addition, the levels of myo-inositol and galactinol were higher in H2O2-treated leaves than in DW-treated leaves. These results indicated that H2O2 spraying enabled the soybean plant to avoid drought stress through the maintenance of leaf water content, and that this water retention was caused by the promotion of oligosaccharide biosynthesis rather than by rapid stomatal closure.  相似文献   

16.
A series of palladium complexes, trans-[1-(R)-pz3,5-Me2]2PdCl2 {R = CH2CONH(2,6-i-Pr2-C6H3) (1b) and 2-(OH)-C6H10 (2b)}, supported over N/O-functionalized pyrazole derived ligands effectively catalyzed the more challenging Suzuki-Miyaura cross-coupling of a variety of activated aryl chlorides with phenyl boronic acid in air in a mixed-aqueous medium (DMF:H2O, v/v = 9:1) in moderate to excellent yields. Besides the commonly encountered Csp2-Csp2 coupling, the 1b and 2b precatalysts also catalyzed the relatively difficult Csp2-Csp3 coupling of benzyl chloride with phenyl boronic acid. The 1b and 2b complexes were synthesized by the direct reaction of the respective N/O-functionalized pyrazolyl ligands, 1a and 2a, with (COD)PdCl2 in 62-66% yields. The stability of the pyrazole-palladium interaction in the 1b and 2b complexes has been attributed to the deeply buried Npyrazole-Pd interaction as evidenced from the density functional theory (DFT) studies.  相似文献   

17.
Application of 1-aminocyclopropane-1-carboxylic acid (ACC) to rib segments excised from flowers of Ipomoea tricolor Cav. resulted in the formation of C2H4 in greater quantities than produced under natural conditions. The ability of ACC to enhance C2H4 production was independent of the physiological age of the tissue and its capacity to synthesize C2H4 without applied ACC. When ACC was fed to rib segments that had been treated with [14C]methionine, incorporation of radioactivity into C2H4 was reduced by 80%. Aminoethoxyvinylglycine and aminooxyacetic acid inhibited C2H4 production in rib segments of I. tricolor but had no effect on ACC-enhanced C2H4 production. Protoplasts obtained from flower tissue of I. tricolor did not form C2H4, even when incubated with methionine or selenomethionine. They produced C2H4 upon incubation with ACC, however. ACC-dependent C2H4 production in protoplasts was inhibited by n-propyl gallate, AgCl, CoCl2, KCN, Na2S, and NaN3. ACC-dependent C2H4 synthesis in rib segments and protoplasts was dependent on O2, the Km for O2 being 1.0 to 1.4% (v/v). These results confirm the following pathway for C2H4 biosynthesis in I. tricolor. methionine [selenomethionine] → S-adenosylmethionine [selenoadenosylmethionine] → ACC → C2H4.  相似文献   

18.
The reaction of the racemic and resolved tetrahydrosalen derivative LH2 (LH2 = N,N’-bis(3,5-dichloro-2-hydroxybenzyl)-trans-1,2-diaminocyclohexane) with the resolved titanium(IV) sec-butoxides Ti(OR-2Bu)4 or Ti(OS-2Bu)4 yielded a series of four compounds, LTi(O2Bu)2 (1-4), which have been characterized by IR, elemental analysis, 1H and 13C NMR and X-ray crystallography. X-ray crystallography revealed the co-crystallization of two pseudo-C2-symmetric products from racemic LH2, whereas a perfect chiral induction of the ligand to the metal occurred when resolved (R,R)-LH2 was used, resulting in a Δ fac-fac wrapping mode of the tetradentate ligand about the titanium center. Ab initio electronic structure calculations (DFT) are in agreement that the lowest energy isomer is that which is experimentally observed. Catalysis screenings show that Ti(OS-2Bu)4, in conjunction with (R,R)-LH2, forms a matched pair that catalyzes the addition of dimethyl zinc to benzaldehyde with higher enantioselectivity than that observed for resolved (R,R)-LH2 with Ti(OR-2Bu)4 or achiral Ti(OiPr)4. Increasing the temperature of the system results in slightly increased enantiomeric excess.  相似文献   

19.
Aquaspirillum magnetotacticum strain MS-1 and two nonmagnetic mutants derived from it reduced C2H2 microaerobically but not anaerobically even with NO3 ?. This organism apparently is not capable of NO3 ?-dependent nitrogen fixation. Cells ofA. magnetotacticum reduced C2H2 at rates comparable to those ofAzospirillum lipoferum grown under similar conditions, but much lower than that ofAzotobacter vinelandii grown aerobically. Cells ofA. magnetotacticum in anaerobic cultures lacking NO3 ? did not reduce C2H2 until O2 was introduced. Optimum rates of C2H2 reduction byA. magnetotacticum were obtained at 200 Pa O2. C2H2 reduction was inhibited by more than 1 kPa O2 or 0.2 mM NO3 ? or NH4 +. These results suggest thatA. magnetotacticum fixes N2 only under microaerobic, N-limited conditions.  相似文献   

20.
Four analogs with 3′-O-alkyl groups (9a: CH3, 9b: C2H5, 9c: C13H27 or 9d: CH2Ph) instead of the 3′-O-sulfate anion in salacinol (1), a naturally occurring potent α-glucosidase inhibitor, were synthesized by the coupling reaction of 1,4-dideoxy-1,4-epithio-d-arabinitols (18a and 18b) with appropriate epoxides (10a-10d). These analogs showed equal or considerably higher inhibitory activity against rat small intestinal α-glucosidases than the original sulfate (1), and one of them (9d) was found more potent than currently used α-glucosidase inhibitors as antidiabetics. Thus, introduction of a hydrophobic moiety at the C3′ position of this new class of inhibitor was found beneficial for onset of stronger inhibition against these enzymes.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号