共查询到20条相似文献,搜索用时 218 毫秒
1.
A UV spectrometric method was developed to determine the molar absorptivity ( C) and formation constant ( Kc) of the association complex of unsubstituted chalcone in cyclohexane, in the concentration range from 4.00·10 −4 to 2.00·10 −2 mol dm −3. The thermodynamic and spectroscopic magnitudes such as Kc and C contribute to the understanding of the physicochemical behavior of several , β-unsaturated carbonylic compounds, of low solubility in water, as it is the case of numerous flavonoids of chemical and biological importance. The studied association complex, formed by two chalcone molecules, is characterized by the constants C (300.8 nm)=4.98·10 4 dm 3 mol −1 cm −1 and Kc=5.58·10 3. The method proposed is convenient for the study of solute–solute molecular associations particularly those due to dipole–dipole interactions. 相似文献
2.
The extraction equilibrium of the ion-associate of periodate with 1-(3,5-diamino-6-chloropyrazinecarboxyl) guanidine hydrochloride (DPG +Cl −) was investigated spectrophotometrically. The optimum conditions for the extraction of the ion associate DPG +·IO 4− with cyclohexanone have been established. The ion association constants, β, KD and Kex and the molar absorpativity of the formed ion associate were determined. These values enable a convenient application of the investigated system for the extraction spectrophotometric determination of periodate ions in the aqueous media. The molar ratio of the ion associate was found to be 1:1 of periodate to the reagent DPG +Cl − at pH 4–5. 相似文献
3.
Formation constants for recrystallized thymol blue were determined in water, using the SQUAD and SUPERQUAD programs. The best model correlating spectrophotometric, potentiometric and conductimetric data was fitted with the dissociation of HL −=L 2−+H +−log K=8.918±0.070 and H 3L 2−=2L 2−+3H +−log K=29.806±0.133 with the SUPERQUAD program at variable low ionic strength (1.5×10 −4–3.0×10 −4 M); and HL=L 2−+H +−log K=8.9±0.000, H 3L 2− =2L 2−+3H +−log K=30.730±0.032, H 4L 2=2L 2−+4H +−log K=32.106±0.033 with SQUAD at 1.1 M ionic strength. 相似文献
4.
A voltammetric study on the binding properties of the commercially available rabbit liver metallothionein (MT) Sigma M 7641 for cadmium ions was performed. The capacity of metallothionein to complex cadmium and the stability constants of Cd–T complex (T denotes the apoprotein or thionein molecule) have been determined from the direct titration of the defined MT concentration with the standard CdCl 2 solution in 0.59 M sodium chloride medium at pH 7.9 and 2.0. At pH 7.9 the formation of the Cd–T complex has been followed measuring the specific anodic signal height of the complex. Stability constants of Cd–T have been evaluated from the experimental data using three different procedures. The calculated apparent stability constants K=(7.6±0.2)·10 8 dm 3 mol −1 of Cd–T complex in 0.59 M NaCl at pH 7.9 and 25°C indicate that under the given experimental conditions all the three procedures are equally applicable. 相似文献
5.
The oxidation reaction of 2-aminophenol (OAP) to 2-aminophenoxazin-3-one (APX) initiated by 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) has been investigated in methanol at ambient temperature. The oxidation of OAP was followed by electronic spectroscopy and the rate constants were determined according to the rate law −d[OAP]/d t= kobs[OAP][TEMPO]. The rate constant, activation enthalpy and entropy at 298 K are as follows: kobs (dm 3 mol −1 s −1)=(1.49±0.02)×10 −4, Ea=18±5 kJ mol −1, Δ H‡=15±4 kJ mol −1, Δ S‡=−82±17 J mol −1 K −1. The results of oxidation of OAP show that the formation of 2-aminophenoxyl radical is the key step in the activation process of the substrate. 相似文献
6.
From extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium H 3O +(aq) + 1· Na +(nb) 1·H 3O +(nb) + Na +(aq) taking place in the two-phase water–nitrobenzene system (1 = hexaethyl p-tert-butylcalix[6]arene hexaacetate; aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log Kex (H 3O +, 1·Na +) = −0.6 ± 0.1. Further, the stability constant of the 1·H 3O + complex in water saturated nitrobenzene was calculated for a temperature of 25 °C as log βnb (1·H 3O +) = 6.8 ± 0. 2. By using quantum mechanical DFT calculations, the most probable structure of the 1·H 3O + complex species was predicted. In this complex, the hydroxonium ion H 3O + is bound partly to three carbonyl oxygen atoms by strong hydrogen bonds and partly to three alternate phenoxy oxygens by somewhat weaker hydrogen bonds. 相似文献
7.
The one-electron oxidation of Mitomycin C (MMC) as well as the formation of the corresponding peroxyl radicals were investigated by both steady-state and pulse radiolysis. The steady-state MMC-radiolysis by OH-attack followed at both absorption bands showed different yields: at 218 nm G i (-MMC) = 3.0 and at 364 nm G i (-MMC) = 3.9, indicating the formation of various not yet identified products, among which ammonia was determined, G(NH 3) = 0.81. By means of pulse radiolysis it was established a total κ (OH + MMC) = (5.8 ± 0.2) × 10 9 dm 3 mol −1 s −1. The transient absorption spectrum from the one-electron oxidized MMC showed absorption maxima at 295 nm (ε = 9950 dm 3 mol −1 cm t-1), 410 nm (ε = 1450 dm 3 mol −1 cm −1) and 505 nm ( ε = 5420 dm 3 mol −1 cm −1). At 280–320 and 505 nm and above they exhibit in the first 150 μs a first order decay, κ 1 = (0.85 ± 0.1) × 10 3 s −1, and followed upto ms time range, by a second order decay, 2κ = (1.3 ± 0.3) × 10 8 dm 3 mol -1 s −1. Around 410 nm the kinetics are rather mixed and could not be resolved. The steady-state MMC-radiolysis in the presence of oxygen featured a proportionality towards the absorbed dose for both MMC-absorption bands, resulting in a Gi (-MMC) = 1.5. Among several products ammonia-yield was determined G(NH3) = 0.52. The formation of MMC-peroxyl radicals was studied by pulse radiolysis, likewise in neutral aqueous solution, but saturated with a gas mixture of 80% N2O and 20% O2. The maxima of the observed transient spectrum are slightly shifted compared to that of the one-electron oxidized MMC-species, namely: 290 nm (ε = 10100 dm3 mol−1 cm−1), 410 nm (ε = 2900 dm3 mol−1 cm−1) and 520 nm (ε = 5500 dm3 mol−1 cm−1). The O2-addition to the MMC-one-electron oxidized transients was found to be at 290 to 410 nm gk(MMC·OH + O2) = 5 × 107 dm3 mol−1 s−1, around 480 nm κ = 1.6 × 108 dm3 mol−1 s−1 and at 510 nm and above, κ = 3 × 108 dm3 mol−1 s−1. The decay kinetics of the MMC-peroxyl radicals were also found to be different at the various absorption bands, but predominantly of first order; at 290–420 nm κ1 = 1.5 × 103 s−1 and at 500 nm and above, κ = 7.0 × 103 s−1. The presented results are of interest for the radiation behaviour of MMC as well as for its application as an antitumor drug in the combined radiation-chemotherapy of patients. 相似文献
8.
Gaussian-2 ab initio calculations were performed to examine the six modes of unimolecular dissociation of cis-CH 3CHSH + (1 +), trans-CH 3CHSH + (2 +), and CH 3SCH 2+ (3 +): 1 +→CH 3++ trans-HCSH (1); 1 +→CH 3+ trans-HCSH + (2); 1 +→CH 4+HCS + (3); 1 +→H 2+ c-CH 2CHS + (4); 2 +→H 2+CH 3CS + (5); and 3 +→H 2+ c-CH 2CHS + (6). Reactions (1) and (2) have endothermicities of 584 and 496 kJ mol −1, respectively. Loss of CH 4 from 1 + (reaction (3)) proceeds through proton transfer from the S atom to the methyl group, followed by cleavage of the C–C bond. The reaction pathway has an energy barrier of 292 kJ mol −1 and a transition state with a wide spectrum of nonclassical structures. Reaction (4) has a critical energy of 296 kJ mol −1 and it also proceeds through the same proton transfer step as reaction (3), followed by elimination of H 2. Formation of CH 3CS + from 2 + (reaction (5)) by loss of H 2 proceeds through protonation of the methine (C H) group, followed by dissociation of the H 2 moiety. Its energy barrier is 276 kJ mol −1. On both the MP2/6-31G* and QCISD/6-31G* potential-energy surfaces, the H 2 1,1-elimination from 3 + (reaction (6)) proceeds via a nonclassical intermediate resembling c-CH 3SCH 2+ and has a critical energy of 269 kJ mol −1. 相似文献
9.
Using N •3 species as specific electron acceptor a defined ascorbate radical: AH •↔A •−+H + (λ max=360 nm, =3400 dm 3 mol −1 cm −1) is observed. The attack of DMSO •+ on vit.E results in a vit.E • radical ( k=1×10 9 dm 3 mol −1 s −1; λ max=425 nm, =2400 dm 3 mol −1 cm −1; 2 k=4.7×10 8 dm 3 mol −1 s −1). Vit.E-acetate leads to the formation of a radical cation (vit.E-ac •+). β-carotene reacts also with DMSO •+ forming a radical cation, β-car •+ ( k=1.75×10 8 dm 3 mol −1 s −1; λ max=942 nm, =14 600 dm 3 mol −1 cm −1), which probably leads to the formation of a dimer radical cation, (β-car) •+2 ( k=2.5×10 7 dm 3 mol −1 s −1). Using E.coli bacteria (AB1157) as a model system in vitro it was found that all three vitamins are rather efficient radiation protecting agents. They can also increase the activity of cytostatica, e.g., mitomycin C (MMC), by electron transfer process. The mixture of vit.E-ac and β-car acts contradictory, but adding vit.C to it a strong cooperative enhancement of the MMC activity is observed once again. A relationship between the pulse radiolysis and the radiation biological data is found and discussed. A possible explanation of the previously reported trials concerning the role of vit.E and β-car on the increased occurence of lung and other types of cancer in smokers and drinkers is presented. 相似文献
10.
The acidity constants of benzidine (Bz) in aqueous solutions determined potentiometrically at 25° were Ka1 = (1.11 ± 0.08) × 10 −5, Ka2 = (1.45 ± 0.12) × 10 −4. The apparent mixed constants in 0.1 M sodium nitrate are Ka1 = (5.37 ± 0.28) × 10 −6 and Ka2 = (1.14 ± 0.09) × 10 −4. The ultraviolet spectra were recorded as a function of pH and analysed with these constants to obtain the absorption spectra of H 2Bz 2+, HBz + and Bz; the corresponding wavelengths of maximal absorption are 247, 273 and 278 nm, and molar absorptivities 1.63 × 10 4, 1.76 × 10 4 and 2.26 × 10 4 1.mole −1.cm −1. 相似文献
11.
The self-termination rates of the benzyl radical (C 6H 5---CH 2√) and para-substituted benzyl radicals (X---C 6H 4---CH 2√) were studied in aqueous solutions. The Arrhenius parameters and activation energies were determined in the temperature range 275.5–328 K. The kinetic activation energies of these radicals were close to the dynamic activation energy of the solvent, indicating that the termination rate is controlled by diffusion. The values for the rate constants (2 kt (10 9 dm 3 mol −1 s −1)) and the activation energies ( E (kJ mol −1)) were 5.94±0.52 and 14.69±0.61 for CH 3O---C 6H 4---CH 2√, 4.52±0.2 and 17.65±1.16 for CH 37z.sbnd;C 6H 4---CH 2√, 3.07±0.45 and 17.58±0.97 for H---C 6H 4---CH 2√, 4.13±0.81 and 19.10±1.20 for Cl---C 6H 4---CH 2√ and 4.17±0.44 and 14.62±0.52 for NO 2---C 6H 4---CH 2√. 相似文献
12.
Cd II complexes with glycine (gly) and sarcosine (sar) were studied by glass electrode potentiometry, direct current polarography, virtual potentiometry, and molecular modelling. The electrochemically reversible Cd II–glycine–OH labile system was best described by a model consisting of M(HL), ML, ML 2, ML 3, ML(OH) and ML 2(OH) (M = Cd II, L = gly) with the overall stability constants, as log β, determined to be 10.30 ± 0.05, 4.21 ± 0.03, 7.30 ± 0.05, 9.84 ± 0.04, 8.9 ± 0.1, and 10.75 ± 0.10, respectively. In case of the electrochemically quasi-reversible Cd II–sarcosine–OH labile system, only ML, ML 2 and ML 3 (M = Cd II, L = sar) were found and their stability constants, as log β, were determined to be 3.80 ± 0.03, 6.91 ± 0.07, and 8.9 ± 0.4, respectively. Stability constants for the ML complexes, the prime focus of this work, were thus established with an uncertainty smaller than 0.05 log units. The observed departure from electrochemical reversibility for the Cd–sarcosine–OH system was attributed mainly to the decrease in the transfer coefficient . The MM2 force field, supplemented by additional parameters, reproduced the reported crystal structures of diaqua-bis(glycinato-O,N)nickel(II) and fac-tri(glycinato)-nickelate(II) very well. These parameters were used to predict structures of all possible isomers of (i) [Ni(H 2O) 4(gly)] + and [Ni(H 2O) 4(sar)] +; and (ii) [Ni(H 2O) 3(IDA)] and [Ni(H 2O) 3(MIDA)] (IDA = iminodiacetic acid, MIDA = N-methyl iminodiacetic acid) by molecular mechanics/simulated annealing methods. The change in strain energy, Δ Ustr, that accompanies the substitution of one ligand by another (ML + L′ → ML′ + L), was computed and a strain energy Δ Ustr = +0.28 kcal mol −1 for the reaction [Ni(H 2O) 4(gly)] + + sar → [Ni(H 2O) 4(sar)] + + gly was found. This predicts the monoglycine complex to be marginally more stable. By contrast, for the reaction [Ni(H 2O) 3IDA] + MIDA → [Ni(H 2O) 3MIDA] + IDA, Δ Ustr = −0.64 kcal mol −1, and the monoMIDA complex is predicted to be more stable. This correlates well with (i) stability constants for Cd–gly and Cd–sar reported here; and (ii) known stability constants of ML complex for glycine, sarcosine, IDA, and MIDA. 相似文献
13.
The highly neutralized ethylenediaminetetraacetate (EDTA) titrant (95–99% as Y 4− anion) precipitates with Ag + cations to form the Ag 4Y species, in aqueous medium, which is well characterized from conductometric titration, thermal analysis and potentiometric titration of the silver content of the solid. The precipitate dissolves in excess Y 4− to form a complex, AgY 3−. Equilibrium studies at 25°C and ionic strength 0.50 M (NaNO 3) have shown from solubility and potentiometric measurements that the formation constant (95% confidence level) β 1 = (1.93 ± 0.07) × 10 5 M −1 and the solubility products are KS0 = [Ag +] 4[Y 4−] = (9.0 ± 0.4) × 10 −18 M 5 and KS1 = [Ag +] 3[AgY 3−] = (1.74 ± 0.08) × 10 −12 M4. The presence of Na +, rather than ionic strength, markedly affects the equilibrium; the data at ionic strength 0.10 M are: β 1 = (1.19 ± 0.03) × 10 6 M −1, KS0 = (1.6 ± 0.4) × 10 −19 M 5 and KS1 = (1.9 ± 0.5) × 10 −13 M 4; at ionic strength tending to zero; β 1 = (1.82 ± 0.05) × 10 7 M −1, KS0 = (2.6 ± 0.8) × 10 −22 M 5 and KS1 = (5 ± 1) × 10 −15 M 4. The intrinsic solubility is 2.03 mM silver (I) in 0.50 M NaNO 3. Well-defined potentiometric titration curves can be taken in the range 1–2 mM with the Ag indicator electrode. Thermal analysis revealed from differential scanning calorimetry a sharp exothermic peak at 142°C; thermal gravimetry/differential thermal gravimetry has shown mass loss due to silver formation and a brown residue, a water-soluble polymeric acid (decomposition range 135–157°C), tending to pure silver at 600°C, consistent with the original Ag 4Y salt. 相似文献
14.
The activity of enzyme I (EI), the first protein in the bacterial PEP:sugar phosphotransferase system, is regulated by a monomer–dimer equilibrium where a Mg 2+-dependent autophosphorylation by PEP requires the homodimer. Using inactive EI(H189A), in which alanine is substituted for the active-site His189, substrate binding effects can be separated from those of phosphorylation. Whereas 1 mM PEP (with 2 mM Mg 2+) strongly promotes dimerization of EI(H189A) at pH 7.5 and 20 °C, 5 mM pyruvate (with 2 mM Mg 2+) has the opposite effect. A correlation between the coupling of N- and C-terminal domain unfolding, measured by differential scanning calorimetry, and the dimerization constant for EI, determined by sedimentation equilibrium, is observed. That is, when the coupling between N- and C-terminal domain unfolding produced by 0.2 or 1.0 mM PEP and 2 mM Mg 2+ is inhibited by 5 mM pyruvate, the dimerization constant for EI(H189A) decreases from >10 8 to <5 × 10 5 or 3 × 10 7 M −1, respectively. With 2 mM Mg 2+ at 15–25 °C and pH 7.5, PEP has been found to bind to one site/monomer of EI(H189A) with KA′10 6 M −1 (Δ G′=−33.7±0.2 kJ mol −1 and Δ H=+16.3 kJ mol −1 at 20 °C with Δ Cp=−1.4 kJ K −1 mol −1). The binding of PEP to EI(H189A) is synergistic with that of Mg 2+. Thus, physiological concentrations of PEP and Mg 2+ increase, whereas pyruvate and Mg 2+ decrease the amount of dimeric, active, dephospho-enzyme I. 相似文献
15.
The reactions of hydroxyl radical, hydrogen atom and hydrated electron intermediates of water radiolysis with N-isopropylacrylamide (NIPAAm) were studied by pulse radiolysis in dilute aqueous solutions. OH, H and e aq− react with NIPAAm with rate coefficient of (6.9±1.2)×10 9, (6.6±1)×10 9, and (1.0±0.2)×10 10 mol −1 dm 3 s −1. In OH and H radical addition to the double bond mainly -carboxyalkyl type radicals form, (OHCH 2CH √C(N- i-C 3H 7)O and CH 3CH √C(N- i-C 3H 7)O). In reaction of e aq− oxygen atom centered radical anion is produced (CH 2CHC √(N- i-C 3H 7)O −), the anion undergoes reversible protonation with p Ka=8.7. There is also an irreversible protonation on the β-carbon atom that produces the same radical as forms in H atom reaction (CH 3CH √C(N- i-C 3H 7)O). The -carboxyalkyl type radicals at low NIPAAm concentration (0.1–1 mmol dm −3) mainly disappear in self-termination reactions, 2 kt,m=8.4×10 8 mol −1 dm 3 s −1. At higher concentrations the decay curves reflect the competition of the self-termination and radical addition to monomer (propagation). The termination rate coefficient of oligomer radicals containing a few monomer units is 2 kt≈2×10 8 mol −1 dm 3 s 1. 相似文献
16.
The enthalpy and entropy of sublimation of N-ethylthiourea were obtained from the temperature dependence of its vapour pressure measured by both the torsion–effusion and the Knudsen effusion method in the temperature range 360–380 K. The compound undergoes no solid-to-solid phase transition or decomposition below 380 K. The pressure against reciprocal temperature resulted in lg( p, kPa) = (13.40 ± 0.27) − (6067 ± 102) / T(K). The molar sublimation enthalpy and entropy at the mid interval temperature were Δ subHm(370 K) = (116.1 ± 2.0) kJ mol −1 and Δ subSm(370 K) = (218.0 ± 5.2) J mol −1 K −1, respectively. The same quantities derived at 298.15 K were (118.8 ± 2.1) kJ mol −1 and (226.1 ± 5.5) J mol −1 K −1, respectively. 相似文献
17.
At 25°C, I = 1.0 M (CF 3SO 3−Li ++CF 3SO 3H), [H +] = 0.034–0.274 M and λ = 453 nm, the rate equation for the oxidation of Ti(H 2O), 63+ by bromine was found to be: −d/[Br 2] T/d t= kK/[Br 2][Ti III]/[H +]+ K+ kK/[Br 3−][Ti III]/[H ++ K, where k = 9.2 × 10 −3 M −1 s −1 and K = 4.5 × 10 −3 M. At [H +] = 1.0 M, [Br −] = 0.05–0.4 M, the apparent second-order rate constant decreases as [Br −] increases. The pH-dependence of the oxidation of TiIII-edta by bromine is interpreted in terms of the change in identity of the TiIII-edta species as the pH of the reaction medium changes. The second-order rate constants were fitted using a non-linear least-square computer program with (1/k0edta)2 weighting into an equation of the form: k0edta =k1+k2K1[H+]−1+k3K1K2[H+]−2/1+K1[H+[H+−1+K1K2[H+]−2, with K1 and K2 fixed as earlier determined at 9.55 × 10−3 and 2.29 × 10−9 M, respectively, for the oxidation of bromine. k1=k2=(3.1±0.32)×103M−1s−1 k3=(2.3±0.45)×106N−1s−1. It is proposed that these electron transfer reactions proceed by univalent changes with the production of Br2.− as a transient intermediate. An outer-sphere mechanism is proposed for these reactions. The homonuclear exchange rate for TiIII-edta+TiIV-edta is estimated at 32 M−1 s−1. 相似文献
18.
Pulse radiolysis of epicatechin in aqueous solution has been done to investigate the reactions of epicatechin derived phenoxy radical (EpO) at neutral pH. EpO was generated by N 3 reacting toward EpOH, the rate constant was measured to be 3 × 10 8 dm 3 mol −1 s −1. The biomolecular termination of EpO is rather slow ((2 k < × 10 6 dm 3 mol −1 s −1) and results in products exhibiting strong visible absorption around 450 nm. No reactions have been observed for EpO with O 2 and O 2 in the time scale of pulse radiolysis (0.01 s), suggesting the bimolecular rate constant are less than 10 4 and 5 × 10 6 dm 3 mol −1 s −1, respectively. 相似文献
19.
A new bulk acoustic wave (BAW) cellulase sensing technique, which is based on the enzymatic hydrolysis process of sodium carboxymethylcellulose (CMC) by cellulase, was established. The frequency shift curves of BAW sensor indicated that the viscosity of the tested solutions decreased during the hydrolysis process. The hydrolysis rate of CMC by cellulase was calculated from the frequency shift curves. The hydrolysis rate of CMC under different pH conditions at 30°C showed that cellulase had high hydrolysis ability approximately at pH 5.0. Kinetic parameters (the Michaelis constant Km and the maximum rate Vmax) of the process were estimated by using a linear method of Lineweaver–Burk plot. Km is 1.95±0.25 mg ml −1 and Vmax is −(4.25±0.58)×10 −3 g 1/2 cm −3/2 cP 1/2 min −1. Also the activation energy ( Ea) of the enzymatic hydrolysis, with a value of 51.99±1.26 kJ mol −1, was estimated in this work. 相似文献
20.
Free radical reactions of dehydrozingerone (DZ), a methoxy phenol, were studied at dfferent pHs with a variety of oxidants using nanosecond pulse radiolysis technique. Hydroxyl radical (OH) reaction with the phenolic form at pH 6 led mainly to the formation of an OH-adduct absorbing at 460 nm in addition to a minor oxidation product. On the other hand, at pH 10 with the deprotonated phenoxide ion, the only reaction observable was oxidation generating a phenoxyl radical absorbing at 360 nm. HPLC analysis indicated formation of two different products at pH 6 from addition and oxidation reactions, whereas at pH 10, only the oxidation product was detectable. Reactions of more specific secondary oxidizing radicals, N3√, Br√, Br −2√ and Tl(II) with DZ gave rise to the phenoxyl radical over the entire pH range. DZ in the phenoxide ion form reacted with nitrogen dioxide and trichloromethyl peroxyl radicals with rate constants 6×10 8 and 8.8×10 8 dm 3 mol −1 s −1 respectively leading to the phenoxyl radicals. The DZ phenoxyl radical reacted with trolox C (an analogue of -tocopherol) with a rate constant of 8.3×10 7 dm 3 mol −1 s −1. One electron reduction potential of the DZ phenoxyl radical at pH 6 was determined to be +1.1 V vs NHE using N3√/N −3 as the standard couple. 相似文献
|