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1.
This study focuses on the hydrolysis of terbufos, an organophosphorus pesticide. Combining GC-MS and wet chemistry methods, di-tert-butyl disulfide and formaldehyde were identified and quantified as major degradation products. Diethyl dithiophosphate was also indirectly identified as a degradation product under alkaline conditions. Hydrolysis rate constants of terbufos under homogeneous conditions were comparable to those of phorate and show relative insensitivity to pH under slightly acidic to neutral pH conditions, as the observed rate constants varied only in the range of (4.5-5.0) x 10(-6) s(-1) between pH 5.7 and 9.4; neutral hydrolysis is thus the most dominant hydrolysis pathway of terbufos in ambient waters. The mechanisms for terbufos hydrolysis and the formation of the major products and their temporal profiles are discussed. To assess the environmental impact of degradation products of this widely used pesticide, Microtox was used to analyze the toxicity of terbufos and two of its degradation products: diethyl dithiophosphate and di-tert-butyl disulfide; the EC(50) of terbufos was found to be >17 microM, whereas the EC(50) of di-tert-butyl disulfide was 1.3 microM.  相似文献   

2.
The pro-oxidative activity of trout hemoglobin was significantly increased at low pH (2.5-3.5) in a washed fish muscle (WFM) system. It was found that the more unfolded the hemoglobin was the more exposed its heme group was, which increased its pro-oxidative activity. The amount of oxidation products produced (TBARS) were, however, lower at low pH vs neutral pH. At pH 10.5-11, the pro-oxidative activity of hemoglobin was greatly suppressed. The conformation of hemoglobin was significantly more stable at high pH as compared to pH 7 as judged by its visible absorption spectrum. Hemoglobin readjusted from low pH to pH 7 had a higher pro-oxidative activity (i.e., more rapid oxidation) in WFM than native hemoglobin at pH 7, even though TBARS values were lower than in the untreated sample at pH 7. The results suggest that the WFM becomes slightly more susceptible to oxidation after low pH treatment but also produces less TBARS. The increased pro-oxidative activity after pH readjustment correlated well with an incomplete recovery in the native structure on pH readjustment. A longer unfolding time and a lower pH led to a less refolded hemoglobin with increased pro-oxidative activity. Hemoglobin was less pro-oxidative at low pH in the presence of 500 mM NaCl. The presence of salt did, however, increase the pro-oxidative properties of hemoglobin after readjustment to pH 7. The treatment of washed fish muscle at alkaline pH followed by adjustment to pH 7 led to a slight delay in hemoglobin-mediated lipid oxidation in WFM as compared to native hemoglobin at pH 7. The results suggest that WFM becomes less susceptible toward oxidation after pH readjustment from alkaline pH. These results clearly show that for muscle protein extraction/isolation processes requiring highly alkaline or acidic conditions, alkaline conditions are preferred if the lipid oxidation originating from hemoglobin is to be minimized.  相似文献   

3.
A pigment derived from the acetaldehyde-mediated condensation between (+)-catechin and malvidin 3-O-glucoside has been prepared and isolated by semipreparative HPLC, and its characteristics of color and stability have been studied and compared with that of malvidin glucoside in aqueous solutions. When the pH was increased from 2.2 to 5.5, the solution of the pigment became progressively more violet (lambda(max) = 560 nm at pH 5.5), whereas similar solutions of the anthocyanin were almost colorless at pH 4.0. This behavior indicated that the anthocyanin moiety of the pigment was more protected against water attack, and thus the formation of its quinonoidal forms was favored. The color of the pigment also showed more stability with regard to bleaching by SO(2) than that of malvidin glucoside. Nevertheless, the pigment was more sensitive to degradation in aqueous solution than the anthocyanin. The cleavage of the ethyl bridge that links the anthocyanin and the catechin constituted the first step in its degradation, as demonstrated by the formation of malvidin glucoside as a major product.  相似文献   

4.
The oxidation kinetics and mechanisms of diazinon, an organophosphorus pesticide, by aqueous chlorine were studied under different conditions. The oxidation is of first order with respect to both diazinon and chlorine. The oxidation rate is found to increase with decreasing pH. The second-order rate constants at pH 9. 5, 10.0, 10.5, and 11.0 are determined to be 1.6, 0.64, 0.43, and 0. 32 M(-)(1) s(-)(1), respectively. Based on the rate constants at different temperatures, the activation energy is calculated to be 30 kJ/mol at pH 10.0 with a chlorine-to-diazinon ratio of 11:1, 33 kJ/mol at pH 11.0 with a 11:1 ratio, and 36 kJ/mol at pH 11.0 with a 5:1 ratio, respectively. Diazoxon is identified as the oxidation product by GC-MS. Ion chromatography analysis shows an increase of sulfate concentration as the reaction proceeds, indicating that sulfur is being oxidized to sulfate. This study indicates that oxidation by aqueous chlorine can significantly affect the fate of diazinon in the environment.  相似文献   

5.
Theaflavin and its galloyl esters are polyphenolic pigments of black tea. In the course of studies on the oxidation mechanism of tea polyphenols, two theaflavin oxidation products named bistheaflavins A and B were isolated, and their structures were elucidated on the basis of MS and NMR spectroscopic analyses. Treatment of a mixture of (-)-epicatechin and (-)-epigallocatechin with banana fruit homogenate yielded bistheaflavin A together with theaflavin and theanaphthoquinone. The symmetrical structure of bistheaflavin A suggested that this compound was formed by oxidative C [bond] C coupling of two theaflavin molecules. In contrast, theaflavin in phosphate buffer (pH 7.3) was gradually oxidized to give bistheaflavin B and theanaphthoquinone. Bistheaflavin B possesses a bicyclooctane skeleton probably formed by intermolecular cyclization between dehydrotheaflavin and dihydrotheanaphthoquinone.  相似文献   

6.
This study investigates the hydrolysis kinetics of fenthion and its five oxidation metabolites in pH 7 and pH 9 buffered aqueous media at 25, 50, and 65 degrees C. Five metabolites and three hydrolysis products were synthesized and purified. The reactant and the corresponding hydrolysis products were determined by HPLC. Rate constant and half-life studies revealed that fenthion and its metabolites were relatively stable in neutral media, and their stability decreased as pH increased. The half-lives at 25 degrees C ranged from 59.0 days for fenthion to 16.5 days for fenoxon sulfone at pH 7, and from 55.5 days for fenthion to 9.50 days for fenoxon sulfone at pH 9; half-lives were greatly reduced at elevated temperatures. The activation energy (E(a)) was found to range from 16.7 to 22.1 kcal/mol for the compounds investigated. The phenol hydrolysis product of fenthion and fenoxon, 3-methyl-4-methylthiophenol was not stable in pH 7 and pH 9 buffered solutions at 50 degrees C, whereas 3-methyl-4-methylsulfonylphenol and 3-methyl-4-methylsulfinylphenol were relatively stable under the same conditions. At pH 9, the primary hydrolysis mechanisms of fenthion and its oxidation metabolites were combination of hydroxide ion and neutral water molecule attacking on the P atom to form corresponding phenol derivatives. Under neutral conditions, the primary hydrolysis mechanisms of fenthion and its oxidation metabolites were assumed to be the combination of water molecule attacking on the P atom to form phenol derivatives and on the C atom to yield dealkylation products.  相似文献   

7.
The present study was conducted to investigate the ability of black tea theaflavins to inhibit the off-odor formation from citral under acidic aqueous conditions. Acidic buffer solutions (pH 3.0) containing citral (10 mg/L) and an inhibitor (0-5 mg/L) were stored at 40 degrees C for 2 weeks. The formation of possible off-odorants p-cresol and p-methylacetophenone in the citral solutions was monitored by high-performance liquid chromatography. A mixture of the theaflavins showed inhibitory effects on the formation of both p-cresol and p-methylacetophenone with 50% inhibitory concentrations (IC(50)) of 0.18 and 0.10 mg/L, respectively. Individual theaflavins and a structurally related compound, purpurogallin, also inhibited the formation of both off-odorants, with the lowest IC(50) values for theaflavin 3,3'-digallate (0.17 and 0.06 mg/L for p-cresol and p-methylacetophenone, respectively). On the other hand, a mixture of green tea catechins and its major constituent, (-)-epigallocatechin gallate, showed relatively high IC(50) values for the formation of p-methylacetophenone (1.29 and 1.28 mg/L, respectively) and showed no inhibitory effect on the formation of p-cresol. The results of the sensory evaluation showed that the off-odor intensity of the stored citral solution was significantly decreased by the addition of the theaflavin mixture at concentrations of 0.5 mg/L and above. In addition, the calculation of the odor activity values (OAVs) for the volatile compounds detected by a gas chromatographic analysis indicated that the total OAVs of the major volatile compounds in the citral solution were significantly decreased by the addition of the theaflavins.  相似文献   

8.
(-)-Epigallocatechin-3-gallate (EGCG), the most abundant and biologically active compound in tea, has been extensively studied for its activities related to disease prevention in animal models and in vitro. However, its stability under different experimental conditions has not been well-characterized. In the present study, the stability of EGCG in animal drinking fluid and under cell culture conditions and the factors that affect its stability under these conditions were investigated. Our results demonstrated that auto-oxidation and epimerization are the two major reactions causing the instability of EGCG. The structures of the major oxidation products, EGCG dimers, were identified. The rates of these reactions were affected by the temperature, pH, the partial pressure of oxygen, the level of antioxidants, the concentration of EGCG, and other components of tea. In future studies with EGCG, its stability should be considered in order to avoid possible artifacts.  相似文献   

9.
The changes in allomerized chlorophyll during the growth and development of the olive fruit as well as during the main operations of its processing as green table olive (alkaline treatment and lactic fermentation) were investigated to study their influence in the color alteration known as green staining (GS). Chlorophyll synthesis coincided in time with the maximum content in allomerized intermediates, weeks before the fruits were harvested for processing. The alkaline treatment caused a subsequent chlorophyll oxidation independent of the high or low initial content of allomerized chlorophylls. However, this oxidation was directly related with the oxidizing capacity of the alkaline solution and the cell deterioration caused. The later maintenance of the fruits in osmotic solutions at different pHs that reproduce the pH reduction caused by the lactic fermentation showed that at pH below 4.5 the insertion of Cu into the chlorophyll molecule was produced in certain fruits; the extent of this reaction was greatest when the prior formation of oxidized chlorophylls exceeded 23%. This apparent relationship between chlorophyll oxidation and copper chlorophyll complexes was investigated in table olives with GS alteration.  相似文献   

10.
Secondary reactions occurring in pyrite‒containing sediments under aerobic conditions are complex and are not fully understood. Objectives were to (i) study the formation of secondary minerals using x‒ray diffraction (XRD) and ion activity product (IAP) calculations; (ii) to obtain a budget of acidity producing and consuming processes; and (iii) to study the performance of a chemical equilibrium model (including kinetic reactions) using sequential batch experiments with varying input solutions on samples of different pyrite oxidation states. A sediment sample from the open pit coal mine Garzweiler, Germany, was oxidised in the laboratory to obtain four different pyrite oxidation states. Sequential batch experiments were carried out using H2O, 100 mM CaCl2 and 10 mM NaOH as input solutions. A coupled equilibria model was used to describe the experiments. The model (PHREEQC) included inorganic complexation, redox reactions, precipitation/dissolution of sparingly soluble salts, multiple cation exchange and pyrite oxidation using a simple input function. IAP calculations and XRD showed the formation of large amounts of gypsum with increasing pyrite oxidation and for the highly oxidised sample also the formation of hydroniumjarosite. The budget of acidity consuming processes followed the order (i) release of Fe(III) into the solution phase (51% of produced acidity); (ii) Al release into solution and exchangeable phases (probably mainly due to silicate weathering, 22% of produced acidity); and (iii) CEC reducing processes (11% of produced acidity). Modeling of the sequential equilibration experiments with water and CaCl2 gave satisfactory agreements between modeled and measured pH and sorption values, indicating that the main processes governing pH and ion sorption were quite well understood. However, model results of the alkaline additions at larger pyrite oxidation states differed considerably from the experimental results.  相似文献   

11.
The pH stability of urease, acid phosphatase, alkaline phosphatase and phosphodiesterase in soils was investigated by first incubating a soil sample at the indicated pH (1–13) for 24 h and then measuring the activity at its optimal pH under standardized conditions. Generally, the decline in enzyme activity in a pH-profile near the optimum pH range was due to a reversible reaction that involved ionization or deionization of acidic or basic groups in the active centre of the enzyme-protein. Irreversible inactivation of the enzyme was particularly evident at the lower and higher ranges of acidic and alkaline conditions. Results showed that the pH stability of soil enzymes was highly dependent on the soils being assayed. The variation among soils may be attributed to the diversity of vegetation, micro-organisms and soil fauna as sources contributing to the enzyme activity and to the protective sites which allowed entrapment of the enzyme within colloidal humus and organo-mineral complexes. Adherence of the enzyme-protein to the humic-clay fractions would allow some resistance to pH denaturation.  相似文献   

12.
Changes in the specific activities of polyphenol oxidase (PPO), peroxidase (POD), and protease and in the relative amounts of flavan-3-ols for eight genetically derived cultivated teas at various stages of leaf maturity and in four succescive seasons were examined. A series of investigations were carried out to study the cross-reactivity of complex polyphenols and PPO-generated orange-yellow theaflavins, as well as of POD oxidized substrates, producing brown so-called thearubigins during fermented tea processing. From the estimation of five major catechins, PPO activities in young shoots, and theaflavin and thearubigin contents of crushed, torn, and curled (CTC) black teas, the superior variety and flavorful flush characteristics were refined. Notable protein hydrolysis by endogenous protease as measured from free amino acids and formation of tannin-protein complex (browning products) was obtained for cultivar character and product quality. Results showed that process optimization with respect to time, temperature, moisture, and pH maximizes PPO-catalyzed desirable theaflavin pigments, whereas POD-mediated chemical reaction produces dull color.  相似文献   

13.
The crude extract of the polyphenol oxidase (PPO) enzyme from the Manzanilla cultivar (Olea europaea pomiformis) was obtained, and its properties were characterized. The browning reaction followed a zero-order kinetic model. Its maximum activity was at pH 6.0. This activity was completely inhibited at a pH below 3.0 regardless of temperature; however, in alkaline conditions, pH inhibition depended on temperature and was observed at values above 9.0 and 11.0 at 8 and 25 degrees C, respectively. The thermodynamic parameters of substrate oxidation depended on pH within the range in which activity was observed. The reaction occurred according to an isokinetic system because pH affected the enzymatic reaction rate but not the energy required to carry out the reaction. In the alkaline pH region, browning was due to a combination of enzymatic and nonenzymatic reactions that occurred in parallel. These results correlated well with the browning behavior observed in intentionally bruised fruits at different temperatures and in different storage solutions. The use of a low temperature ( approximately 8 degrees C) was very effective for preventing browning regardless of the cover solution used.  相似文献   

14.
The behavior of antioxidants in emulsions is influenced by several factors such as pH and emulsifier type. This study aimed to evaluate the interaction between selected food emulsifiers, phenolic compounds, iron, and pH and their effect on the oxidative stability of n-3 polyunsaturated lipids in a 10% oil-in-water emulsion. The emulsifiers tested were Tween 80 and Citrem, and the phenolic compounds were naringenin, rutin, caffeic acid, and coumaric acid. Lipid oxidation was evaluated at all levels, that is, formation of radicals (ESR), hydroperoxides (PV), and secondary volatile oxidation products. When iron was present, the pH was crucial for the formation of lipid oxidation products. At pH 3 some phenolic compounds, especially caffeic acid, reduced Fe(3+) to Fe(2+), and Fe(2+) increased lipid oxidation at this pH compared to pH 6. Among the evaluated phenols, caffeic acid had the most significant effects, as caffeic acid was found to be prooxidative irrespective of pH, emulsifier type, and presence of iron, although the degrees of lipid oxidation were different at the different experimental conditions. The other evaluated phenols were prooxidative at pH 3 in Citrem-stabilized emulsions and had no significant effect at pH 6 in Citrem- or Tween-stabilized emulsions on the basis of the formation of volatiles. The results indicated that phenol-iron complexes/nanoparticles were formed at pH 6.  相似文献   

15.
It has been reported earlier that when macerated tea leaf is fermented at lower pH, the resultant black tea contains increased levels of theaflavin, an important quality marker in black tea. In an attempt to investigate the biochemistry and chemistry underlying this observation, in vitro oxidation experiments using polyphenol oxidase (PPO) from fresh tea leaves, horseradish peroxidase (POD), and tea catechins, precursors for theaflavins, were carried out. In vitro oxidation experiments using crude tea PPO resulted in higher content of theaflavins at pH 4.5 in comparison with pH 5.5, the normal pH of the macerated tea leaf. When purified PPO was used in the in vitro system, surprisingly a reversal of this trend was observed, with more theaflavins being formed at the higher pH. A combination of pure tea PPO and POD led to an observation similar to that with the crude enzyme preparation, suggesting a possible role for POD in the formation or degradation of theaflavin. POD was observed to oxidize theaflavins in the presence of H(2)O(2), leading to the formation of thearubigin, another black tea pigment. This paper demonstrates that tea PPO, while oxidizing catechins, generates H(2)O(2). The amount of H(2)O(2) produced is greater at pH 5.5, the optimum pH for PPO activity, than at pH 4.5. Hence, an observed increase of theaflavins in black teas fermented at pH 4.5 appears to be due to lower turnover of formed theaflavins into thearubigins.  相似文献   

16.
Proanthocyanidins constitute an important class of polyphenols ubiquitously found in plants. They have been extensively studied for their antioxidant capacity and bioactivity in vitro and in animal models. However, their stability under different pH conditions and in cell culture medium has not been well documented. In the present study, it was observed that proanthocyanidin A2 (PA2) was relatively more stable in acidic condition than in weak alkaline condition. PA2 was also quite unstable in basal-Dulbecco's Modified Eagle medium (b-DMEM medium) at 37 °C. The addition of PA2 to the cell culture medium accelerated its epimerization with a half-life of <15 min, and ethylenediaminetetraacetic acid (EDTA) could not stop the reaction. The results also demonstrated that the major isomers transformed in the weak alkaline condition or cell culture medium at 37 °C were identified as epicatechin-(4β→8; 2β→O→7)-ent-catechin (proanthocyanidin A4) and epicatechin-(4β→6; 2β→O→7)-ent-catechin. The rates of transformation were dependent on the pH or the components of the medium. Therefore, the results obtained for PA2 in the cell culture bioassays, which were usually carried out for 24 h, might not represent the true activity of the original PA2. The stability and transformation of PA2 should be considered when the bioactivity of PA2 is evaluated in a given cell culture system.  相似文献   

17.
Degradation of green tea catechins in tea drinks   总被引：9，自引：0，他引：9  
Green tea cateachins (GTC). namely (-) epicatechin (EC), (-) epicatechin gallate (ECG), (-) epigallocatechin (EGC), and (-) epigallocatechin gallate (EGCG), have been studied extensively for their wide-ranging biological activities. The goal of the present study was to examine the stability of GTC as a mixture under various processing conditions. The stability study demonstrated that GTC was stable in water at room temperature. When it was brewed at 98 degrees C for 7 h, longjing GTC degraded by 20%. When longjing GTC and pure EGCG were autoclaved at 120 degrees C for 20 min, the epimerization of EGCG to (-) gallocatechin gallate (GCG) was observed. The relatively high amount of GCG found in some tea drinks was most likely the epimerization product of EGCG during autoclaving. If other ingredients were absent, the GTC in aqueous solutions was pH-sensitive: the lower the pH, the more stable the GTC during storage. When it was added into commercially available soft drinks or sucrose solutions containing citric acid and ascorbic acid, longjing GTC exhibited varying stability irrespective of low pH value. This suggested that other ingredients used in production of tea drinks might interact with GTC and affect its stability. When canned and bottled tea drinks are produced, stored, and transported, the degradation of GTC must be taken into consideration.  相似文献   

18.
A 10-month study of surface waters in Canadian Creek (Ballarat, Victoria, Australia) showed the significant influence of historic gold mining waste material. The investigation focussed on the hydrogeochemistry of the surface waters and soils in order to: (1) document the levels and seasonal trends in major, minor and trace elements in the creek, (2) identify the process by which As is released from the soil/waste mining material to surface waters. For most dissolved major and trace elements (Na, Ca, Mg, K, and As) in surface waters, the concentrations decreased with the increasing rainfall and flow conditions except for Al and Fe. Two sites selected along the creek (< 1 km apart) allowed evaluation of the possibility that mining waste material is contributing to the elevated As concentrations (up to 145 μg/l) in downstream surface water. Arsenic concentration varied more than 28 fold seasonally and was highest in autumn and lowest in spring. Elevated concentrations of As (up to 1946 mg/kg) at the downstream site indicated the presence of a source of As concentration in both surface and subsurface soils. Oxidation of arsenic sulphides under aerobic conditions with redox fluctuations (7 to 201 mV) could cause elevated As levels in the creek. Significant statistical correlations among the major cations (Ca, Na and Mg) point to a common source(s) resulting in neutral to slightly alkaline (pH ～ 6.5 to 7.8) surface water. Fe and Al secondary phases under oxidising conditions are a significant controlling mechanism for the mobilization of As in highly contaminated soils (> 1500 mg/kg) in the study area. The large As adsorption capacity of Fe and Al could be limiting extreme mobilization into the water. Rainfall with relatively low pH is possibly causing mobilisation of Al, Fe and As from highly alkaline soils (pH ≈ 9.0) into the nearby creek.  相似文献   

19.
Adsorption behaviour of cadmium (Cd) in soils is an important process which exerts a major influence on its uptake by plant roots. Thirteen soils from various parts of India (tropical region), their pH ranging from 4.2 to 8.4, were subjected to Cd treatment at various concentrations (1 to 100 μg ml-1) and equilibrated at room temperature (25 ± 1 °C). The Cd adsorbed by each soil was calculated as the difference between the amount of Cd present in the solution initially and that remaining after equilibration. Results indicated that the adsorption capacity of the soils for Cd increased with an increase in the pH or alkalinity of the soils. The rate of adsorption was, however, found to decrease with increased pH. All the 13 soils used in this study followed linear and Freundlich adsorption isotherms with highly significant positive correlations (r). The neutral and alkaline soils also followed the Langmuir adsorption isotherm, the adsorption maxima being lowest for the neutral soil and highest for the alkaline soil. The adsorption data, in general, indicated that Cd was in a fixed form at higher pH levels. The results are generally similar to those of the temperate regions; however, Cd adsorption capacity of tropical vertisols was comparatively higher than those of temperate vertisols.  相似文献   

20.
Phenolics are phytochemicals extensively distributed among plants that have been receiving great deal of attention for their functionality. Rice bran is a good source of phenolics, especially hydroxycinnamates. Although chemicals are commonly employed to isolate phenolics, the use of physical treatments such as sonication is still limited. This study was conducted to optimize a procedure to isolate phenolics from rice bran using sonication as a preextraction treatment. Sonication was optimized by varying output, time, and temperature. Extraction was optimized by varying solvent, extraction time, temperature, and sample-to-solvent ratio. After an optimum procedure was established, HPLC analyses were conducted to identify and quantify the major individual phenolic acids extracted. The optimum conditions for extracting phenolics from rice bran were sonication with water (1:100 sample-to-solvent ratio) for 1 min; output intensity of 10; holding at ambient temperature; and autoclave treatment for 20 min at 121°C. This procedure extracted 8.85 ± 0.18 mg of phenolics/g of dried rice bran. Benzoic, p-coumaric, and trans-ferulic acids were the major phenolics identified in the extract. The proposed procedure will be valuable in obtaining phenolics for increasing product functionality such as lipid oxidation inhibitors in food systems.  相似文献   

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