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 15 毫秒
1.
Five red shikonin pigments, deoxyshikonin, shikonin, acetylshikonin, isobutylshikonin, and beta-hydroxyisovalerylshikonin, were isolated from the roots of Lithospermum erythrorhizon cultivated in Korea. The purified pigments were red, purple, and blue at acidic, neutral, and alkaline pH values, respectively. Physical stability of the purified pigments against heat and light in an aqueous solution was examined for possible value-added food colorants. The thermal degradation reactions were carried out at pH 3.0 (50 mM glycine buffer) in 50% EtOH/H(2)O. Deoxyshikonin (t(1/2) = 14.6 h, 60 degrees C) and isobutylshikinin (t(1/2) = 19.3 h, 60 degrees C) are relatively less stable than other shikonin derivatives (t(1/2) = 40-50 h, 60 degrees C). Activation energies of thermal degradation of the isolated pigments were calculated. The activation energy of deoxyshikonin was the highest (12.5 kcal mol(-)(1)) and that of beta-hydroxyisovalerylshikonin was the lowest (1.71 kcal mol(-)(1)) value. Light stabilities of the pigments were similar to each other in that the half-life values of photodegradation for 20000 lx light intensity were 4.2-5.1 h.  相似文献   

2.
It is reported that 2-chlorobenzamide, one of the chief degradation products of CCU (1-(2-chlorobenzoyl)-3-(4-chlorophenyl) urea), a new insect growth regulator, is a potential carcinogen, but few studies about its environmental stability have been found. This paper is concerned with the hydrolysis of 2-chlorobenzamide as part of the environmental study of CCU. The results showed that 2-chlorobenzamide is relatively stable in solutions of pH = 6 and 8, for which the rate constants are 0.00286 h(-)(1) (R = 99.13%, SD = 0. 0095) and 0.00109 h(-)(1) (R = 96.70%, SD = 0.0072), respectively. Hydrolysis was more rapid in acidic (pH = 5), alkaline (pH = 10), and neutral (pH = 7) environments, with hydrolytic rate constants of 0.00417h(-)(1) (R = 95.76%, SD = 0.0390), 0.00411h(-)(1) (R = 99.89%, SD = 0.0162) and 0.00408h(-)(1) (R = 98.29%, SD = 0.0237), respectively. The change of the rate of hydrolysis with pH showed two minima at 25 degrees C. Temperature has some impact on the hydrolysis, showing at higher temperature the larger rate of reaction.  相似文献   

3.
The upper horizons of old vineyard soils have substantial copper contents due to the traditional use of copper-based fungicides. Total copper levels in eight vineyard soils in the Rías Baixas area of Galicia (northwestern Spain) ranged from 60 to 560 mg kg(-1) (mean +/- SD = 206 +/- 170 mg kg(-1)). The adsorption of the fungicides metalaxyl (pK(a) = 1.41) and penconazole (pK(a) = 2.83) by these soils was determined using fungicide solutions of pH 2.5 and 5.5, and desorption of fungicide adsorbed at pH 5.5 was also determined. In all cases, Freundlich equations were fitted to the data with R (2) > 0.96. Penconazole was adsorbed and retained more strongly than metalaxyl, with K(F) values more than an order of magnitude greater. In the desorption experiments, both fungicides exhibited hysteresis. Soil copper content hardly affected the adsorption of metalaxyl, but K(F) values for adsorption of penconazole increased at a rate of about 0.1 mL(n) (microg of penconazole)(1-n) (microg of Cu)(-1), which is attributed to the formation of Cu(2+)-penconazole complexes with greater affinity for soil colloids than penconazole itself. Because the dependence of K(F) for penconazole adsorption on copper content was the same at both pH values, complex formation appears not to have been affected by the solubilization of 6-17% of soil copper at pH 2.5. A similar copper dependence, or lack of dependence, was observed when 100-1000 mg kg(-1) of copper was added as Cu(NO(3))(2).2H(2)O to the solutions from which the fungicides were adsorbed.  相似文献   

4.
Invertase (EC 3.2.1.26) catalyzes the hydrolysis of sucrose into D-glucose and D-fructose. Insoluble acid invertase (INAC-INV) was purified from pea (Pisum sativum L.) by sequential procedures entailing ammonium sulfate precipitation, ion exchange chromatography, absorption chromatography, reactive green-19 affinity chromatography, and gel filtration. The purified INAC-INV had a pH optimum of 4.0 and a temperature optimum of 45 °C. The effects of various concentrations of Tris-HCl, HgCl(2), and CuSO(4) on the activities of the purified invertase were examined. INAC-INV was not affected by Tris-HCl and HgCl(2). INAC-INV activity was inhibited by 6.2 mM CuSO(4) up to 50%. The enzymes display typical hyperbolic saturation kinetics for sucrose hydrolysis. The K(m) and V(max) values of INAC-INV were determined to be 4.41 mM and 8.41 U (mg protein)(-1) min(-1), respectively. INAC-INV is a true member of the β-fructofuranosidases, which can react with sucrose and raffinose as substrates. SDS-PAGE and immunoblotting were used to determine the molecular mass of INAC-INV to be 69 kDa. The isoelectric point of INAC-INV was estimated to be about pH 8.0. Taken together, INAC-INV is a pea seedling invertase with a stable and optimum activity at lower acid pH and at higher temperature than other invertases.  相似文献   

5.
The browning of glucose-fructose-glycine mixtures involves parallel glucose-glycine and fructose-glycine reactions, which share a common intermediate, the immediate precursor of melanoidins in the kinetic model. At pH 5.5, 55 degrees C glucose is converted into this intermediate in a two step process where k(1) = (7.8 +/- 1.1) x 10(-)(4) mol L(-)(1) h(-)(1) and k(2) = (1.84 +/- 0.31) x 10(-)(3) h(-)(1) according to established kinetics, whereas fructose is converted into this intermediate in a single step where k(4) = 5.32 x 10(-)(5)()()mol L(-)(1) h(-)(1). The intermediate is converted to melanoidins in a single rate limiting process where k(mix) = 0.0177 h(-)(1) and the molar extinction coefficient (based on the concentration of sugar converted) of the melanoidins so formed is 1073 +/- 4 mol(-)(1) L cm(-)(1). Whereas the value of k(mix) is the same when the individual sugars undergo browning, the value of the molar extinction coefficient is similar to that for melanoidins from the glucose-glycine reaction (955 +/- 45 mol(-)(1) L cm(-)(1)) but it is approximately double the value for melanoidins from the fructose-glycine reaction (478 +/- 18 mol(-)(1) L cm(-)(1)). This is the reason that the effects of glucose and fructose on the rate of browning are synergistic.  相似文献   

6.
Aqueous crude extracts of a series of plant wastes (agricultural, wild plants, residues from sports activities (grass), ornamental residues (gardens)) from 17 different plant species representative of the typical biodiversity of the Iberian peninsula were investigated as new sources of peroxidases (EC 1.11.1.7). Of these, lentil (Lens culinaris L.) stubble crude extract was seen to provide one of the highest specific peroxidase activities, catalyzing the oxidation of guaiacol in the presence of hydrogen peroxide to tetraguaiacol, and was used for further studies. For the optimum extraction conditions found, the peroxidase activity in this crude extract (110 U mL(-1)) did not vary for at least 15 months when stored at 4 °C (k(inact) = 0.146 year(-1), t(1/2 inact) = 4.75 year), whereas, for comparative purposes, the peroxidase activity (60 U mL(-1)) of horseradish (Armoracia rusticana L.) root crude extract, obtained and stored under the same conditions, showed much faster inactivation kinetics (k(inact) = 2.2 × 10(-3) day(-1), t(1/2 inact) = 315 days). Using guaiacol as an H donor and a universal buffer (see above), all crude extract samples exhibited the highest peroxidase activity in the pH range between 4 and 7. Once semipurified by passing the crude extract through hydrophobic chromatography on phenyl-Sepharose CL-4B, the novel peroxidase (LSP) was characterized as having a purity number (RZ) of 2.5 and three SDS-PAGE electrophoretic bands corresponding to molecular masses of 52, 35, and 18 kDa. The steady-state kinetic study carried out on the H(2)O(2)-mediated oxidation of guaiacol by the catalytic action of this partially purified peroxidase pointed to apparent Michaelian kinetic behavior (K(m)(appH(2)O(2)) = 1.87 mM; V(max)(appH(2)O(2)) = 6.4 mM min(-1); K(m)(app guaicol) = 32 mM; V(max)(app guaicol) = 9.1 mM min(-1)), compatible with the two-substrate ping-pong mechanism generally accepted for peroxidases. Finally, after the effectiveness of the crude extracts of LSP in oxidizing and removing from solution a series of last-generation dyes present in effluents from textile industries (1) had been checked, a steady-state kinetic study of the H(2)O(2)-mediated oxidation and decolorization of Green Domalan BL by the catalytic action of the lentil stubble extract was carried out, with the observation of the same apparent Michaelian kinetic behavior (K(m)(appGD) = 471 μM; V(max)(appGD)= 23 μM min(-1)). Further studies are currently under way to address the application of this LSP crude extract for the clinical and biochemical analysis of biomarkers.  相似文献   

7.
The effects of the chloride salts LiCl, CaCl(2), MgCl(2), AlCl(3), EuCl(3), and FeCl(3) and the iron salts FeCl(2), FeCl(3), Fe(NO(3))(3), FeSO(4), and Fe(2)(SO(4))(3) on water conductance of exocarp segments (ES) and rates of water uptake into detached sweet cherry fruit (Prunus avium L. cv. Adriana, Early Rivers, Namare, Namosa, and Sam) were studied. ES were excised from the cheek of mature fruit and mounted in stainless steel diffusion cell; water penetration was monitored gravimetrically from donor solutions containing the above mineral salts into a PEG 6000 (osmolality = 1.14 osM, pH 4.8, 25 degrees C) receiver solution. Conductance of ES was calculated from the amount of water taken up per unit of surface area and time by dividing by the gradient in water activity across ES. LiCl, CaCl(2), MgCl(2), FeCl(2), and FeSO(4) had no significant effect on conductance, but AlCl(3), FeCl(3), Fe(NO(3))(3), and Fe(2)(SO(4))(3) significantly reduced conductance compared to water only as a donor. Also, EuCl(3) lowered conductance; however, this effect was not always significant. Effects of salts on water conductance of ES and rates of water uptake into detached fruit were closely related (R 2 = 0.97***). Upon application of an FeCl(3)-containing donor conductance decreased instantaneously. FeCl(3) concentrations of <6.6 x 10(-)(4) M had no effect on conductance, but concentrations at or above this threshold decreased conductance. FeCl(3) lowered water conductance at a receiver pH of 4.8, but not at pH < or =2.6. The effect of FeCl(3) on conductance was largest in cv. Namare and smallest in cv. Adriana. There was no significant effect of FeCl(3) on conductance for transpiration. Formation of aluminum and iron oxides and hydroxides in the exocarp as a result of a pH gradient between donor and receiver solution is discussed as the potential mechanism for Fe(3+) and Al(3+) reducing conductance for water uptake.  相似文献   

8.
Precarthamin, a yellow precursor of carthamin, was efficiently isolated from the yellow petals of safflower (Carthamus tinctorius L. ) with Sephadex LH-20 column chromatography and preparative HPLC, and identified with UV-vis and NMR spectrometry. The UV-vis spectrum of precarthamin showed lambda(max) of 238 and 406 nm in MeOH. The molar extinction coefficients of precarthamin at 406 nm in MeOH and 50 mM citrate buffer (pH 5.0) were 59 000 M(-)(1) cm(-)(1) and 45 400 M(-)(1) cm(-)(1), respectively. The isolated and structurally identified precarthamin was converted to a red pigment by a homogeneously purified enzyme from the immature petals of safflower in 50 mM citrate buffer (pH 5.0). The enzymatically converted red pigment was identified as carthamin, a red pigment of safflower by TLC, HPLC, and UV-vis spectral analysis.  相似文献   

9.
Kinetic study of the oxidation of quercetin by mushroom tyrosinase   总被引：1，自引：0，他引：1  
The kinetic behavior of mushroom tyrosinase in the presence of the flavonol quercetin was studied. This flavonol was oxidized by mushroom tyrosinase and the reaction was followed by recording spectral changes over time. The spectra obtained during the reaction showed two isosbectic points, indicating a stable o-quinone. When quercetin was oxidized by tyrosinase in the presence of cysteine and 3-methyl-2-benzothiazolone hydrazone (Besthorn's hydrazone, MBTH) isosbestic points were also observed indicating a definite stoichiometry. From the data analysis of the initial rate in the presence of MBTH, the kinetic parameters: = (16.2 +/- 0.6) microM/min, = (0.12 +/- 0.01) mM, (/) = (V(max)/K(S)(')()) = (13.5 +/- 1.4) x 10(-)(2) min(-)(1), = (6.2 +/- 0.6) s(-)(1) were determined. We propose that quercetin acts simultaneously as a substrate and a rapid reversible inhibitor of mushroom tyrosinase, depending on how it binds to the copper atom of the enzyme active site. Thus, if the binding occurs through the hydroxylic groups at the C3' and C4' positions, quercetin acts as a substrate, while if it occurs through the hydroxylic group at the C3 position of the pyrone ring, quercetin acts as an inhibitor.  相似文献   

10.
Water uptake through the fruit surface is considered as an important factor in cracking of sweet cherry (Prunus avium L.) fruit. Uptake may occur by diffusion and/or viscous flow along a polar pathway. To establish the mechanism of water uptake, the effects of viscosity and molecular weight of selected osmotica on water uptake into detached sweet cherry fruit were investigated. In addition we investigated the effect of temperature on penetration of 2-(1-naphthyl)[1-(14)C]acetic acid ([(14)C]NAA; pK(a) = 4.2) as a molecular probe in the non-dissociated (pH 2.2) and dissociated (pH 6.2) forms. Rates of water uptake were linearly related to the inverse viscosity of gum arabic solutions (range of concentrations and dynamic viscosities 10-300 g L(-1) and 1.3 x 10(-3) to 115.9 x 10(-3) Pa s, respectively). When fruit was incubated in solutions of osmotica of differing molecular weight that were isotonic to the fruit's water potential, water uptake depended on the molecular weight of the osmoticum [range 58-6000 for NaCl to poly(ethylene glycol) 6000 (PEG 6000)]. There was no uptake from PEG 6000 solutions, but rates of water uptake increased as the molecular weight of the osmotica decreased. Apparent water potentials of sweet cherry fruit, determined by incubating fruit in concentration series of selected osmotica, increased as the molecular weight of the osmotica increased up to 1500 and remained constant between 1500 and 6000. Reflection coefficients (sigma) estimated from this relationship were closely related to hydrodynamic radii (r) of the osmotica [sigma = 1.0(+/-0.0) - [10.9(+/-0.9) x 10(-11)][r(-1) (m(-1))], R(2) = 0.97, P < 0.0001]. The permeability of the sweet cherry fruit exocarp to NAA (pK(a) = 4.2) and temperature dependence of NAA permeability (P(d)) as indexed by the energy of activation (E(a), temperature range 5-35 degrees C) were significantly higher for the non-dissociated NAA (pH 2.2, P(d) = 10.2(+/-0.8) x 10(-8) m s(-1), E(a) = 67.0 +/- 1.7 kJ mol(-)(1)) than for the dissociated NAA (pH 6.2, P(d) = 1.1(+/-0.2) x 10(-8) m s(-1), E(a) = 51.8 +/- 1.9 kJ mol(-)(1)). The activation energy for penetration of the dissociated NAA was closely related to the stomatal density (R( 2) = 0.84, P < 0.0001) but less so for the non-dissociated NAA (R(2) = 0.30, P < 0.03). These data provide evidence for the presence of polar pathways through the sweet cherry fruit exocarp that allow water uptake by viscous flow. These pathways offer a potentially useful target for strategies to reduce water uptake and fruit cracking, provided that a technique is identified that selectively "plugs" these pathways.  相似文献   

11.
A kinetic study of the alkylating potential of potassium sorbate (S)-a food preservative used worldwide-in 7:3 water/dioxane medium was performed. The following conclusions were drawn: (i) Potassium sorbate shows alkylating activity on the nucleophile 4-(p-nitrobenzyl)pyridine (NBP), a trap for alkylating agents with nucleophilic characteristics similar to those of DNA bases, (ii) The NBP alkylation reaction complies with the rate equation r = k(alk)[H+][S][NBP]/(K(a) + [H+]), K(a) being the sorbic acid dissociation constant and k(alk) the rate constant of NBP alkylation by the undissociated acid. In the range of pH 5-6, the alkylation time ranges between 18 days (pH 5.2) and >1 month (pH > or = 6). (iii) NBP alkylation occurs through a reaction with deltaH# = 78 kJ mol(-1), which is much higher than those of NBP alkylation by stronger alkylating agents. (iv) The absorption coefficient of the sorbate-NBP adduct was determined to be epsilon = 204 M(-1) cm(-1) (lambda = 580 nm), this value being rationalized in terms of the adduct structure. (v) The results can help to establish suitable expiration times for products preserved with potassium sorbate.  相似文献   

12.
Apparent binding constants of aroma compounds limonene, alpha- and beta-ionone, and terpenyl acetate, with beta-lactoglobulin (BLG), were determined, using dynamic coupled column liquid chromatography, for pH values varying from 3 to 11. K(a) values varied from 2.61 to 3.21 x 10(3) M(-1) for limonene, indicating a strong interaction with BLG. Similarly, significant and close apparent binding constants were obtained for alpha- and beta-ionone, 1.7 x 10(2) and 4.5 to 5.4 x 10(2) M(-1), respectively. These data indicated that a similar mechanism is involved for the binding of these two molecules. The weaker values obtained at low pH, for alpha-ionone relative to beta-ionone, can be explained by the existence of steric hindrance. An increase of the apparent binding constant was observed, for all the compounds studied, when the pH was increased from 3 to 9. At this pH, an apparent binding constant was obtained for terpenyl acetate (1.04 x 10(2) M(-1)), whereas this determination was not possible at pH 3 and 6. The apparent binding constant increase was in agreement with the decrease of aroma compound relative activity coefficient in the presence of BLG, previously observed at this pH. It indicated a best accessibility to the same binding site. The binding constants of all the aroma compounds studied decreased at pH 11 as a result of the important release of the BLG structure previously reported.  相似文献   

13.
For the authenticity assessment of (E)-methyl cinnamate from different origins, combustion/pyrolysis-isotope ratio mass spectrometry (C/P-IRMS) was used by an elemental analyzer (EA) and on-line capillary gas chromatography coupling (HRGC-C/P-IRMS). For that reason, (E)-methyl cinnamate self-prepared from synthetic, natural, and semisynthetic educts was analyzed in comparison to the commercial synthetic and natural ester. In addition, (E)-methyl cinnamate from basil extract and a number of commercial natural aromas was investigated. The data of self-synthesized synthetic (E)-methyl cinnamate, i.e., delta(13)C(V)(-)(PDB) = -33.8 per thousand and delta(2)H(V)(-)(SMOW) = +349 per thousand, corresponded with that found for the commercial synthetic samples (-29.5 to -31.4 per thousand and +328 to +360 per thousand for delta(13)C(V)(-)(PDB) and delta(2)H(V)(-)(SMOW), respectively). The ester produced from natural educts by acid as well as Candida antarctica catalysis revealed delta(13)C(V)(-)(PDB) = -25.6 and -30.1 per thousand as well as delta(2)H(V)(-)(SMOW) = -162 and -169 per thousand, respectively. Acid-catalyzed semisynthetic products differed in their delta(13)C(V)(-)(PDB) and delta(2)H(V)(-)(SMOW) values depending on the origin of their educts. For the ester from synthetic methanol and natural cinnamic acid, -27.3 and -126 per thousand were determined for delta(13)C(V)(-)(PDB) and delta(2)H(V)(-)(SMOW), respectively, whereas for the ester produced from natural methanol and synthetic acid delta(13)C(V)(-)(PDB) = -30.6 per thousand and delta(2)H(V)(-)(SMOW) = +287 per thousand were found. Basil extract showed -28.9 and -133 per thousand for delta(13)C(V)(-)(PDB) and delta(2)H(V)(-)(SMOW), respectively. Commercial aromas declared to be natural revealed delta(13)C(V)(-)(PDB) and delta(2)H(V)(-)(SMOW) data ranging from -25.7 to -28.5 per thousand as well as -85 to -191 per thousand, respectively, indicating, in part, incorrect declaration.  相似文献   

14.
Oxidation of the flavonol quercetin by polyphenol oxidase   总被引：5，自引：0，他引：5  
Because direct oxidation of flavonols by polyphenol oxidase (PPO) has not previously been reported and, given the importance of flavonols, the ability of broad bean seed PPO to oxidize the flavonol quercetin was studied. The reaction was followed by recording spectral changes with time. Maximal spectral changes were observed at 291 nm (increase) and at 372 nm (decrease). The presence of two isosbectic points (at 272 and 342 nm) suggested the formation of only one absorbent product. These spectral changes were not observed in the absence of PPO. The oxidation rate, which varied with pH, was highest at pH 5.0. The following kinetic parameters were also determined: V(m) = 11 microM/min, K(m) = 646 microM, V(m)/K(m) = 17 x 10(-)(2) min(-)(1). Flavonol oxidation was efficiently inhibited (K(I) = 3.5 microM) by specific PPO inhibitors such as 4-hexylresorcinol. The results obtained showed that quercetin oxidation was strictly dependent on the presence of PPO.  相似文献   

15.
Metamitron (1) does not undergo hydrolysis at pH 1-8 and up to 5 M H(2)SO(4). The product of its two-electron reduction, 1, 6-dihydrometamitron (2), on the other hand, undergoes at pH <3 relatively fast hydrolysis. The dependence of the measured rate constant on acidity indicates that the completely protonated form (AH(2)(2+)) predominating in strongly acidic media undergoes hydrolysis slower than the species bearing one less proton (AH(+)). The latter most reactive species is present in highest concentration in solutions of pH between 0 and 2. This species is protonated on the 2,3-azomethine bond and yields as final products 2-hydrazino-2-phenylacetic acid (4) and acethydrazide (5). Kinetic, polarographic, and spectrophotometric measurements indicated for the first dissociation an average value pK(a) = -0.8, for the second pK(a) = 0.95. These observations together with the easy reduction of the 1,6-bond in metamitron (1) indicate that in nature the cleavage of metamitron may be preceded by its reduction to 1, 6-dihydrometamitron (2), which is then hydrolyzed. Thus, anaerobic, reductive conditions are likely preferable for the total microbial degradation of metamitron.  相似文献   

16.
Sorption behavior of prochloraz in different soils.   总被引：6，自引：0，他引：6  
The sorption behavior of the imidazole fungicide prochloraz [PCZ; N-propyl-N-[2-(2,4,6-trichlorophenoxy)ethyl]imidazole-1-carboxamide] was studied in batch experiments with different soils. The soil organic matter content was found to control the amount sorbed by different soils. K(d) values ranged from 56 +/- 0 to 552 +/- 10 (mean = 221 +/- 5) and K(OC) values from 7273 +/- 0 to 16250 +/- 1300 (mean = 11829 +/- 303). As calculated from a linear regression of K(d) versus %OC, K(OC) was 12900 +/- 1300. Additionally, the pH value of the soil had considerable influence on the sorption of the weakly basic PCZ (pK(a) = 3.8), giving rise to stronger sorption at lower pH. K(d) values determined on pH-modified soils confirmed the pH dependency. Sorption isotherms on two soils were recorded, initial concentrations ranging from 0.09 to 5.71 mg L(-)(1). The Freundlich isotherm was fitted to the values measured. The Freundlich exponents calculated were significantly smaller than unity, indicating nonlinear sorption. Sorption experiments with two metabolites of PCZ (PCZ-formylurea and PCZ-urea) revealed K(d) values one-fourth to one-third those for PCZ on two soils.  相似文献   

17.
Phytochelatins (PCs) are a family of thiol-rich peptides, with the general structure (gamma-Glu-Cys)(n)()-Gly, with n = 2-11, induced in plants upon exposure to excessive amounts of heavy metals and some metalloids, such as arsenic. Two types of PC analyses are currently used, i.e., acid extraction and separation on HPLC with either precolumn derivatization (pH 8.2) with monobromobimane (mBBr) or postcolumn derivatization (pH 7.8) with Ellman's reagent [5, 5'-dithiobis(2-nitrobenzoic acid), DTNB]. Although both methods were satisfactory for analysis of Cd-induced PCs, formation of (RS)(3)-As complexes during extraction of As-induced PCs rendered the DTNB method useless. This paper shows that precolumn derivatization with mBBr, during which the (RS)(3)-As complexes are disrupted, provides a qualitative and quantitative analysis of both Cd- and As-induced PCs. In addition, derivatization efficiencies of both methods for the oligomers with n = 2-4 (PC(2)(-)(4)) are compared. Derivatization efficiency decreased from 71.8% and 81.4% for mBBr and DTNB derivatization, respectively, for PC(2) to 27.4% and 50.2% for PC(4). This decrease is most likely due to steric hindrance. Correction of measured thiol concentration is therefore advised for better quantification of PC concentrations in plant material.  相似文献   

18.
Hydrolyses of fenamiphos, fipronil, and trifluralin were studied in aqueous buffer solutions of pH 4.1, 7.1, and 9.1 at different temperatures, 5, 22 +/- 1, 32 +/- 1, and 50 +/- 1 degrees C. Fenamiphos, fipronil, and trifluralin were found to be more stable in acidic and neutral buffer solutions at temperatures of 5 and 22 +/- 1, and dissipation is rapid at 50 +/- 1 degrees C. In basic buffer and at higher temperature, degradation of fenamiphos was found to be very rapid when compared with fipronil and trifluralin. The rate constants calculated at 32 degrees C for fenamiphos were 2349.4 x 10(-)(8) (pH 4.1), 225.2 x 10(-)(8) (pH 7.1), and 30476.0 x 10(-)(8) (pH 9.1); for fipronil 1750.0 x 10(-)(8) (pH 4.1), 3103.0 x 10(-)(8) (pH 7.1), and 3883.0 x 10(-)(8) (pH 9.1); and for trifluralin 2331.0 x 10(-)(8) (pH 4.1), 2360.0 x 10(-)(8) (pH 7.1), and 3188.0 x 10(-)(8) (pH 9.1). On the basis of rate constant values, these pesticides appeared to be more susceptible to hydrolysis than synthetic organophosphorus compounds such as chlorpyriphos, diazinon, malathion, and ronnel. DT(50) values calculated at 32 degrees C were 228 (pH 4.1), 5310.24 (pH 7.1), and 37.68 (pH 9.1) h for fenamiphos; 608.6 (pH 4.1), 373.9 (pH 7.1), and 270.2 (pH 9.1) h for fipronil; and 502.1 (pH 4.1), 496.8 (pH 7.1), and 355.7 (pH 9.1) h for trifluralin.  相似文献   

19.
Lipoxygenase (LOX) from eggplant (Solanum melongena L. cv. Belleza negra) was partially purified, and the products and kinetics of the enzyme were studied. Linoleic acid (LA) was the best substrate for this enzyme. Product analysis by HPLC and GC/MS revealed that, at its pH optimum (pH 7.0), the enzyme converted LA almost totally into the 9-hydroperoxy isomer, whereas the 13-hydroperoxy isomer was only a minor product. At this pH, the enzyme had K(m) and V(max) values for LA of 1.4 microM and 2.2 micromol min(-1) (mg of protein)(-1), respectively, when the monomeric form of LA was used as substrate. The dependence of eggplant LOX activity on the physicochemical properties of LA was also studied. Experiments revealed that LA aggregates were used more efficiently than monomeric LA as substrate. The apparent substrate cooperativity observed may be due to the different activities exhibited toward monomers and aggregates. This result can be interpreted as a substrate-aggregation dependent activity.  相似文献   

20.
  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