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.
Tocopherols (alpha, beta, gamma, and delta) and Trolox were found to deactivate triplet-excited riboflavin in homogeneous aqueous solution (7:3 v/v tert-butanol/water) with second-order reaction rates close to diffusion control [k2 between 4.8 x 10(8) (delta-tocopherol) and 6.2 x 10(8) L mol(-1) s(-1) (Trolox) at 24.0 +/- 0.2 degrees C] as determined by laser flash photolysis transient absorption spectroscopy. In aqueous buffer (pH 6.4) the rate constant for Trolox was 2.6 x 10(9) L mol(-1) s1 and comparable to the rate constant found for ascorbate (2.0 x 10(9) L mol(-1) s(-1)). The deactivation rate constant was found to be inferior in heterogeneous systems as shown for alpha-tocopherol and Trolox in aqueous Tween-20 emulsion (approximately by a factor of 4 compared to 7:3 v/v tert-butanol/water). Neither beta-carotene (7:3 v/v tert-butanol/water and Tween-20 emulsion), lycopene (7:3 v/v tert-butanol/water), nor crocin (aqueous buffer at pH 6.4, 7:3 v/v tert-butanol/water, and Tween-20 emulsion) showed any quenching on the triplet excited state of riboflavin. Therefore, all carotenoids seem to reduce the formation of triplet-excited riboflavin through an inner-filter effect. Activation parameters were based on the temperature dependence of the triplet-excited deactivation between 15 and 35 degrees C, and the isokinetic behavior, which was found to include purine derivatives previously studied, confirms a common deactivation mechanism with a bimolecular diffusion-controlled encounter with electron (or hydrogen atom) transfer as rate-determining step. DeltaH for deactivation by ascorbic acid, Trolox, and homologue tocopherols (ranging from 18 kJ mol(-1) for Trolox in Tween-20 emulsion to 184 kJ mol(-1) for ascorbic acid in aqueous buffer at pH 6.4) showed a linear dependence on DeltaS (ranging from -19 J mol(-1) K(-1) for Trolox in aqueous buffer at pH 6.4 to +550 J mol(-1) K(-1) for ascorbic acid in aqueous buffer pH 6.4). Among photooxidation products from the chemical quenching, lumicrome, alpha-tocopherol quinones and epoxyquinones, and alpha-tocopherol dimers were identified by ESI-QqTOF-MS.  相似文献   

2.
The effects of 0, 0.3, 0.6, and 0.9 mM Trolox and ascorbic acid on the singlet oxygen oxidation of tryptophan and tyrosine containing 25 ppm of riboflavin were determined by measuring tryptophan and tyrosine concentration by high-performance liquid chromatography analysis. The samples were stored in the a 1000 lx light storage box for 4 h at 30 degrees C. As the concentration of Trolox and ascorbic acid increased, the degradation of tryptophan and tyrosine decreased significantly at p < 0.05. Trolox reduced tryptophan and tyrosine degradation by quenching both singlet oxygen and excited triplet riboflavin, whereas ascorbic acid quenched singlet oxygen only. The total singlet oxygen quenchings of Trolox in the presence of tryptophan and tyrosine were 1.55 x 10(7) and 1.32 x 10(7) M(-1) s(-1), respectively. The total singlet oxygen quenchings of ascorbic acid in the presence of tryptophan and tyrosine were 1.16 x 10(7) and 1.10 x 10(7) M(-1) s(-1), respectively. Trolox was more effective than ascorbic acid in preventing the degradation of tryptophan and tyrosine.  相似文献   

3.
The reactivity of purine derivatives (uric acid, xanthine, hypoxanthine, and purine) toward triplet-excited riboflavin in aqueous solution at pH 6.4 is described on the basis of kinetic (laser flash photolysis), electrochemical (square-wave voltammetry), and theoretical data (density functional theory, DFT). Direct deactivation of triplet-excited riboflavin in aqueous solution, pH 6.4 at 24 degrees C, in the presence of uric acid, xanthine, and hypoxanthine strongly suggests a direct electron transfer from the purine to the triplet-excited riboflavin with k = 2.9 x 10(9) M(-1) s(-1) (DeltaH(++) = 14.7 kJ mol(-1), DeltaS(++) = -15.6 J mol(-1) K(-1)), 1.2 x 10(9) M(-1) s(-1) (DeltaH(++) = 34.3 kJ mol(-1), DeltaS(++) = +45.3 J mol(-1) K(-1)), and 1.7 x10(8) M(-1) s(-1) (DeltaH(++) = 122 kJ mol(-1), DeltaS(++) = +319 J mol(-1) K(-1)), respectively. From the respective one-electron oxidation potentials collected in aqueous solution at pH 6.4 for uric acid (E = +0.686 vs normal hydrogen electrode, NHE), xanthine (E = +1.106 vs NHE), and hypoxanthine (E = +1.654 vs NHE), the overall free energy changes for electron transfer from the quencher to the triplet-excited riboflavin are as follows: uric acid (DeltaG(o) = -114 kJ mol(-1)), xanthine (DeltaG(o) = -73.5 kJ mol(-1)), hypoxanthine (DeltaG(o) = -20.6 kJ mol(-1)), and purine (DeltaG(o) > 0). The inertness observed for purine toward triplet-excited riboflavin corroborates with its electrochemical inactivity in the potential range from 0 up to 2 V vs NHE. These data are in agreement with the DFT results, which show that the energy of the purine highest occupied molecular orbital (HOMO) (-0.2685 arbitrary unit) is lower than the energy of the semioccupied molecular orbital (SOMO) (-0.2557 a.u.) of triplet-excited riboflavin, indicating an endergonic process for the electron-transfer process. The rate-determining step for deactivation by purine derivatives can be assigned to an electron transfer from the purine derivative to the SOMO orbital of the triplet-excited riboflavin. The results show that uric acid may compete with oxygen and other antioxidants to deactivate triplet-excited riboflavin in milk serum and other biological fluids leading to a free radical process.  相似文献   

4.
Pulse and steady state radiolysis techniques have been used to determine the bimolecular rate constants and to investigate the spectral nature of the intermediates and the degradation induced by hydroxyl radicals ((*)OH) with 1,3,5-triazine (T), 2,4, 6-trimethoxy-1,3,5-triazine (TMT), and 2,4-dioxohexahydro-1,3, 5-triazine (DHT) in aqueous medium. A competitive kinetic method with KSCN as the (*)OH scavenger was used to determine the rate constants for the reaction of (*)OH with T, TMT, and DHT. The bimolecular rate constants are 3.4 x 10(9), 2.06 x 10(8), and 1.61 x 10(9) dm(3) mol(-)(1) s(-)(1) respectively, for T, TMT, and DHT at pH approximately 6. The transient absorption spectra obtained from the reaction of (*)OH with T, TMT, and DHT have single absorption maxima at 320, 300, and 300 nm, respectively, and were found to undergo a second-order decay. The formation of TOH(*) [C(6)OH-N(5)-yl radical], TMTOH(*) [N(5)OH-C(6)-yl radical], and DHT(*) [C(6)-yl radical] is proposed from the initial attack of (*)OH with T, TMT, and DHT, respectively. A complete degradation of TMT (10(-3) mol dm(-3)) was obtained after absorbed doses of 5 kGy in N(2)O-saturated solutions and 16 kGy in aerated solutions. A similar degradation pattern was obtained with DHT in N(2)O-saturated solutions. Complete degradation was observed with an absorbed dose of 7 kGy. On the basis of the results from both pulse and steady state radiolysis, a possible reaction mechanism involving (*)OH-mediated oxidative degradation is proposed. A complete photodecomposition of DHT was also observed in the presence of ferric perchlorate using ultraviolet light at low pH. Photoinduced electron transfer between Fe(III) and DHT in the Fe(III)-DHT complex and subsequent formation of DHT(*) are proposed to be the major processes that lead to the complete degradation of DHT at pH 3.  相似文献   

5.
Phenolic compounds present in beer were shown by fluorescence spectroscopy and laser flash photolysis to deactivate both singlet- and triplet-excited states of riboflavin with bimolecular rate constants close to the diffusion control ranging from 2.8x10(9) to 1.1x10(10) M-1 s-1 and from 1.1x10(9) to 2.6x10(9) M-1 s-1, respectively. Enthalpies of activation were low (up to 33.2 kJ mol-1), and entropies of activation were positive, ranging from 17 to 92 J mol-1 K-1, as derived from temperature dependence, indicating a compensation effect. From a Stern-Volmer analysis of the singlet-excited riboflavin quenching by phenols it was found that high amounts of phenolic compounds (>0.3 M) would be needed to hinder triplet-excited riboflavin generation. On the other hand, a phenolic content of 0.36 mM is likely to quench 90% of the triplet-excited state. Phenol photodegradation was found to be complex, and using ESI-MS analysis it was not possible to identify specific photooxidation products of the phenolic compounds; only the photoproducts of riboflavin could be detected and structurally assigned. The rate of reaction of triplet-excited riboflavin with phenolic compounds in acetonitrile/citrate buffer (pH 4.6, 10 mM) is 550 times faster than the reaction with iso-alpha-acids from hops, indicating that triplet-excited quenchers such as phenols may be involved in the early steps in light-struck flavor formation in beer through radical formation. Terpenes present in herb-flavored beers were found to be nonreactive toward singlet- and triplet-excited-state riboflavin, and any protection depends on other mechanisms.  相似文献   

6.
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   

7.
Regeneration of β-carotene from the β-carotene radical cation by the 4'-propylpuerarin anion (second-order rate constant=1.5×10(9) L mol(-1) s(-1) in methanol/chloroform=1:9 (v/v) solution at 25 °C as determined by laser flash photolysis) was found to be marginally slower than regeneration by the 7-propylpuerarin anion (2.3×10(9) L mol(-1) s(-1)), in agreement with the 7-propylpuerarin anion being more reducing (E'=0.56 V vs NHE) than the 4'-propylpuerarin anion (E'=1.01 V vs NHE). The potentials were calculated from E°=1.12 and 1.44 V (vs NHE) as determined by cyclic voltametry in aqueous solution and pKa=9.51 and 7.23 obtained previously for 7-propylpuerarin and 4'-propylpuerarin, respectively. The less reducing but more acidic 4'-propylpuerarin showed less antioxidant activity in liposome of pH 7.4, but more significant antioxidant synergism with β-carotene than the more reducing but less acidic 7-propylpuerarin for oxidation initiated in the liposome lipid phase. Electrostatic effects are concluded to be important in the regeneration of β-carotene from the radical cation in the water/lipid interface because approximately 50% of 4'-propylpuerarin is present as the anion, whereas only 0.5% of 7-propylpuerarin is present as the anion. In contrast, penetration of the undissociated phenolic group into the lipid phase, more significant for 7-propylpuerarin than for 4'-propylpuerarin according to the calculated water/lipid partition coefficients, becomes important for the chain-breaking action in lipid oxidation of the puerarin derivatives as models for (iso)flavonoids and their glycosides.  相似文献   

8.
Vanillin was found to be efficient as a deactivator of ferrylmyoglobin with a second-order rate constant of k(2) = 57 ± 1 L mol(-1) s(-1) for reduction to metmyoglobin with ΔH(?) = 58.3 ± 0.3 kJ mol(-1) and ΔS(?) = -14 ± 1 J mol(-1) K(-1) in aqueous pH 7.4 solution at 25 °C. Binding to β-lactoglobulin (βLG) was found to affect the reactivity of vanillin at 25 °C only slightly to k(2) = 48 ± 2 L mol(-1) s(-1) (ΔH(?) = 68.4 ± 0.4 kJ mol(-1) and ΔS(?) = 17 ± 1 J mol(-1) K(-1)) for deactivation of ferrylmyoglobin. Binding of vanillin to βLG was found to have a binding stoichiometry vanillin/βLG > 10 with K(A) = 6 × 10(2) L mol(-1) and an apparent total ΔH° of approximately -38 kJ mol(-1) and ΔS° = -55.4 ± 4 J mol(-1) K(-1) at 25 °C and ΔC(p, obs) = -1.02 kJ mol(-1) K(-1) indicative of increasing ordering in the complex, as determined by isothermal titration microcalorimetry. From tryptophan fluorescence quenching for βLG by vanillin, approximately one vanillin was found to bind to each βLG far stronger with K(A) = 5 × 10(4) L mol(-1) and a ΔH° = -10.2 kJ mol(-1) and ΔS° = 55 J mol(-1) K(-1) at 25 °C. The kinetic entropy/enthalpy compensation effect seen for vanillin reactivity by binding to βLG is concluded to relate to the weakly bound vanillin oriented through hydrogen bonds on the βLG surface with the phenolic group pointing toward the solvent, in effect making both ΔH(?) and ΔS(?) more positive. The more strongly bound vanillin capable of tryptophan quenching in the βLG calyx seems less or nonreactive.  相似文献   

9.
Lumichrome and lumiflavin were formed from riboflavin under light. pH had a significant influence on the formation of lumichrome and lumiflavin from riboflavin. Lumichrome was the only major product from riboflavin under neutral or acidic pH values. Lumiflavin was also formed from riboflavin in basic pH. The maximum concentration of lumiflavin from 100 microM riboflavin at pH 8.5 was 30.9 microM, and it was reached after 2 h of exposure at 1500 lux. The maximum concentration of lumichrome formed from 100 microM riboflavin at pH 4.5, 6.5, or 8.5 was 79.9, 58.7, and 73.1 microM, respectively, after 8, 6, or 2 h of light exposure. The formation of lumichrome and lumiflavin from riboflavin was due to the type I mechanism of the riboflavin photosensitized reaction. Singlet oxygen was also involved in the photosensitized degradation of lumiflavin and lumichrome. The reaction rates of riboflavin, lumiflavin, and lumichrome with singlet oxygen were 9.66 x 10(8), 8.58 x 10(8), and 8.21 x 10(8) M(-1) s(-1), respectively. The headspace oxygen depletion and headspace volatile formation were significant in soy milk containing lumichrome or lumiflavin under light (p < 0.05) and were insignificant (p > 0.05) in the dark. Ascorbic acid could inhibit the total volatile changes of soy milk under light. Soy milk should be protected from light to prevent the photodegradation of riboflavin and the oxidation of soy milk.  相似文献   

10.
A flow injection spectrophotometric procedure with enzymatic hydrolysis was developed for determination of orthophosphate, phytate and total phosphorus in cereal samples. Phosphorus species were extracted from cereals with 0.05 mol L(-1) potassium hydrogen phthalate buffer solution at pH 5.7. Orthophosphate was directly determined in the extracts by molybdenum blue spectrophotometric method. The phytate was hydrolyzed by the enzyme phytase coupled to a solid phase packed into an enzymatic reactor, and the resulting hydrolyzed orthophosphate was also determined by spectrophotometry at 650 nm. After optimization for phosphorus species extraction and enzymatic hydrolysis, a linear calibration graph was obtained up to 196 x 10(-6) mol L(-1) orthophosphate (P conc = -2.67 + 0.52x, r = 0.9998). Measurements are characterized by relative standard deviation of 1.6% for a standard of 72 x 10(-6) mol L(-1) orthophosphate and no baseline drift was observed during 4 h operation periods. It provides 72 measurements per hour, with 2.4 x 10(-)6) mol L(-1) and 7.9 x 10(-6) mol L(-1) as detection and quantification limits, respectively.  相似文献   

11.
The hydrolysis of triazophos was studied in buffered solutions in the range of pH 4-10 and in sodium hydroxide solutions with pH values up to 12. The results showed that the degradation of triazophos in the above solutions followed simple pseudo-first-order kinetics. At 35 degrees C, the rate constants in buffered solutions ranged from 0.0222 d(-1) at pH 4 to 0.5357 d(-1) at pH 10, and increased to 0.6251 h(-1) in 0.01 mol/L sodium hydroxide solution. The results also indicated that the base-catalysis was more important than acid-catalysis in the hydrolysis of triazophos. On the basis of the Arrhenius plot, the calculated activation energy (E(a)) and the frequency factor (A) for the hydrolysis of triazophos in buffered solution of pH 10 were 78.6 kJ/mol and 1.13 x 10(13) d(-1), respectively. Hydrolytic products of triazophos in buffered solutions of pH 4 and 10, as well as in sodium hydroxide solution of pH 11, were identified as their corresponding trimethylsilyl derivatives with a gas chromatography-mass spectrometer (GC-MS). The possible hydrolytic pathways of triazophos were also proposed.  相似文献   

12.
Using 2,2-azino-bis(3-ethylbenzthiazoline-6-sulfonate) (ABTS) as substrate, it has been shown that the increased peroxidase activity for decreasing pH of myoglobin activated by hydrogen peroxide is due to a protonization of ferrylmyoglobin, MbFe(IV)=O, facilitating electron transfer from the substrate and corresponding to pK(a) approximately 5.2 at 25.0 degrees C and ionic strength 0.16, rather than due to specific acid catalysis. On the basis of stopped flow absorption spectroscopy with detection of the radical cation ABTS(.+), the second-order rate constant and activation parameters for the reaction between MbFe(IV)=O and ABTS were found to have the values k = 698 +/- 32 M(-1) s(-1), DeltaH# = 66 +/- 4 kJ mol(-1), and DeltaS# = 30 +/- 15 J mol(-1) K(-1) at 25.0 degrees C and physiological pH (7.4) and ionic strength (= 0.16 M NaCl). At a lower pH (5.8) corresponding to the conditions in meat, values were found as follows: k = 3.5 +/- 0.3 x 10(4) M(-1) s(-1), DeltaH# = 31 +/- 6 kJ mol(-1), and DeltaS# = -53 +/- 19 J mol(-1) K(-1), indicative of a shift from outersphere electron transfer to an innersphere mechanism. For steady state assay conditions, this shift is paralleled by a shift from saturation kinetics at pH 7.4 to first-order kinetics for H2O2 as substrate at pH 5.8. In contrast, the activation reaction between myoglobin and hydrogen peroxide was found at 25.0 degrees C to be slow and independent of pH with values of 171 +/- 7 and 196 +/- 19 M(-1) s(-1) found at physiological and meat pH, respectively, as determined by sequential stopped flow spectroscopy, from which a lower limit of k = 6 x 10(5) M(-1) s(-1) for the reaction between perferrylmyoglobin, .MbFe(IV)=O, and ABTS could be estimated. As compared to the traditional peroxidase assay, a better characterization of pseudoperoxidase activity of heme pigments and their denatured or proteolyzed forms is thus becoming possible, and specific kinetic effects on activation, substrate oxidation, or shift in rate determining steps may be detected.  相似文献   

13.
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   

14.
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   

15.
The hypervalent heme pigment ferrylmyoglobin, a potential prooxidant in muscle tissue and meat, is efficiently reduced by epigallocatechin gallate (EGCG) from green tea and by green tea polyphenol extract (GTP) in neutral or moderately acidic aqueous solution (0.16 M NaCl) to yield metmyoglobin in two parallel processes. The second-order rate constant for direct reduction at pH 7.4 and 25 degrees C was found to have the value 1170 +/- 83 M(-1).s(-1) and activation parameters DeltaH(#) = 70.6 +/- 7.2 kJ.mol(-1) and DeltaS(#) = 50.7 +/- 24.1 J.mol(-1).K(-1) for EGCG and the value 2300 +/- 77 M(-1).s(-1) and parameters DeltaH(#) = 60.6 +/- 2.6 kJ.mol(-1) and DeltaS(#) = 23 +/- 9 J.mol(-1).K(-1) for GTP (based on EGCG concentration). For decreasing pH, the rate increased moderately due to a parallel reduction of protonated ferrylmyoglobin. At physiological pH, EGCG is more efficient in deactivating ferrylmyoglobin than other plant phenols investigated, and the relatively high enthalpy and positive entropy of activation suggest an outer-sphere electron transfer mechanism. The interaction between EGCG and other tea catechins in GTP could be responsible for the even stronger ability for GTP to deactivate ferrylmyoglobin.  相似文献   

16.
The lactosylation of alpha-lactalbumin in aqueous solution was followed at pH(c) = 6.0, 6.3, 7.0, 7.3, and 7.9 and constant ionic strength (I = 0.080) at 50-60 degrees C by reversed-phase high-performance liquid chromatography (RP-HPLC) and electrospray mass spectrometry (MS). The rate of the lactosylation reaction increased with increasing pH and with temperature most significantly at lower pH. The rate of lactosylation could be described by an acid dissociation curve corresponding to pK(a) of the epsilon-amino group of lysine in alpha-lactalbumin. From initial rates for conditions of excess of lactose, pseudo-first-order rate constants were calculated and further transferred into second-order rate constants by dividing with the lactose concentration. Second-order rate constants for protonated and unprotonated lysine in alpha-lactalbumin both showed Arrhenius behavior, and using transition-state theory, DeltaH# = 31 +/- 2 kJ/mol and DeltaS# = -266 +/- 48 J/(mol . K) were determined for the unprotonated form and DeltaH# = 158 +/- 49 kJ/mol and DeltaS# = 80 +/- 150 J/(mol . K) for the protonated form, respectively. On the basis of the marked differences in activation parameters, initial formation of a lactosylamine is suggested as rate-determining for reaction between lactose and a protonated lysine in alpha-lactalbumin, while subsequent water elimination to form a Schiff base becomes rate-determining for the unprotonated form.  相似文献   

17.
A study is made of the kinetics and mechanism of the reaction of radiolytically produced hydrated electron (e-(aq)) with some triazine derivatives [1,3,5-triazine (T), 2,4,6-trimethoxy-1,3,5-triazine (TMT), 2,4-dioxohexahydro-1,3,5-triazine (DHT), 6-chloro N-ethyl N-(1-methylethyl)-1,3,5-triazine 2,4-diamine (atrazine, AT), and cyanuric acid (CA)] in aqueous medium using pulse and steady-state radiolysis techniques. The second-order rate constants were determined from the pseudo first-order decay of e(-)(aq) in the presence of triazines at 720 nm, and the values obtained with T, TMT, AT, and CA are in the order of 10(9) dm(3) mol(-1) s(-1) and that of DHT was 10(8) dm(3) mol(-1) s(-1) at pH 6. The transient absorption spectra from the reaction of e(-)(aq) with T and TMT are characterized by their lambda(max) at 310 nm, and those of DHT and CA are around 280 and 290 nm, respectively. However, a very weak and featureless absorption spectrum is obtained from AT. On the basis of the spectral evidence and on the quantitative electron transfer from the transient intermediates to the oxidant, methyl viologen (MV(2+)), the intermediate radicals are assigned to N-protonated electron adducts (with the unpaired spin density at carbon) of triazines. The degradation profiles, monitored as the disappearance of parent triazine concentrations as a function of dose, obtained with AT, TMT, CA, and DHT, highlight the potential use of e-(aq) in the degradation of triazines.  相似文献   

18.
A simple flow injection potentiometric (FIP) system, which uses a tubular cobalt electrode, has been developed for phosphorus nutritional evaluation of seeds and grains. Inorganic phosphorus, P(i), is determined using a 1 x 10(-2) mol.L(-1) potassium phthalate buffer solution adjusted at pH 4. A sensitivity of 47 mV/decade and an operating range from 10 to 1000 mg.L(-1) (1 x 10(-4)-1 x 10(-2) M) of dihydrogen phosphate are obtained. The inositol phosphates amount, which is referred to the organic phosphorus, P(org), is directly determined from extracts using a 1 x 10(-2) mol.L(-1) Tris-HCl buffer solution adjusted at pH 8. A sensitivity of 127 mV/decade and an operating range of 10-1000 mg.L(-1) (2.5 x 10(-4)-5 x 10(-3) M) of P(org) (expressed as inositol hexakisphosphoric acid monocalcium) are achieved. Some samples of seed and grain are analyzed by an ICP-OES and a spectrophotometric method to compare results to the developed flow system; no significant differences at the 95% confidence level are observed using a paired t test. Other samples such as animal nursing feed, soybean meal, and corn are also analyzed with the proposed FIP system, showing a good correlation to the ICP-OES values.  相似文献   

19.
The kinetics of the reactions of chlorpyrifos-methyl, an organophosphorus insecticide, with hydrogen sulfide (H(2)S) and bisulfide (HS(-)) were determined in well-defined aqueous solutions. The resulting pseudo-first-order rate constant for chlorpyrifos-methyl with bisulfide yielded a second-order rate constant of (2.1 +/- 0.3) x 10(-3) M(-1) s(-1). The second-order rate constant for chlorpyrifos-methyl with hydrogen sulfide is significantly slower than the second-order rate constant with bisulfide. The contribution of H(2)S to the observed degradation rate constant of chlorpyrifos-methyl at concentrations of up to 4 mM H(2)S is not significant. The second-order rate constant of chlorpyrifos-methyl with H(2)S was too low to be measured in this study. The results indicate that HS(-) present at environmentally relevant concentrations may represent an important sink for phosphorothionate triesters in a coastal marine environment, while H(2)S reacts too slowly to be environmentally relevant (pH 6-9). Trichloropyridinol, the major product of hydrolysis of chlorpyrifos-methyl, is only a minor product of the reaction of chlorpyrifos-methyl with bisulfide; however, trichloropyridinol was found to be stable under the experimental conditions.  相似文献   

20.
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   

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