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1.
The pKas of 3‐pyridylboronic acid and its derivatives were determined spectrophotometrically. Most of them had two pKas assignable to the boron center and pyridine moiety. The pKa assignment performed by 11B nuclear magnetic resonance spectroscopy revealed that both boron centers in 3‐pyridylboronic acid [3‐PyB(OH)2] and the N‐methylated derivative [3‐(N‐Me)Py+B(OH)2] have strong acidities (pKa = 4.4 for both). It was found that introduction of a substituent to pyridine‐C atom in 3‐pyridylboronic acid drastically increased the acidity of the pyridinium moiety, but decreased the acidity of the boron center, whereas the introduction to pyridine‐N atom had no influence on the acidity of the boron center. Kinetic studies on the complexation reactions of 3‐pyridinium boronic acid [3‐HPy+B(OH)2] with 4‐isopropyltropolone (Hipt) carried out in strongly acidic aqueous solution indicated that the positive charge on the boronic acid influenced little on its reactivity; 3‐HPy+B(OH)2 reacts with Hipt and protonated H2ipt+, and its reactivity was in line with those of a series of boronic acids. Kinetics in weakly acidic aqueous solution revealed that 3‐HPy+B(OH)2 reacts with Hipt faster than its conjugate boronate [3‐HPy+B(OH)3], which is consistent with our recent results. The reactivity of 3‐(N‐Me)Py+B(OH)2 towards Hipt was also examined kinetically; the reactivities of 3‐(N‐Me)Py+B(OH)2 and 3‐(N‐Me)Py+B(OH)3 are almost the same as those of their original 3‐HPy+B(OH)2 and 3‐HPy+B(OH)3, respectively. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

2.
Poly (n-alkyl itaconates) containing long alkyl side chains (comb polymers) are eluted through PS gel in THF at 298 K. Their retention volumes are higher than those predicted by universal calibration, indicating affinity of the polymers for the gel. The distribution coefficient for solute-gel interactions, K p(K p 1), increases with polymer molecular weight. This increase is larger for the polymer bearing the longest n-alkyl side chain and is scarcely sensitive to the presence or absence of free carboxylic groups in the polymer (monoitaconates). This might be due to a balance of the solute-gel and solute-solvent interactions.  相似文献   

3.
The reactions of pyridines and secondary alicyclic (SA) amines with phenyl and 4‐nitrophenyl chlorodithioformates (PClDTF and NPClDTF, respectively) are subjected to a kinetic study in aqueous ethanol (44 wt% ethanol) solution, at 25.0 °C, and an ionic strength of 0.2 M (KCl). The reactions are studied spectrophotometrically. Under amine excess, pseudo‐first‐order rate coefficients (kobs) are found. Plots of kobs versus [amine] are linear and pH independent, with slope kN. The Brønsted‐type plots (log kN vs. pKa of aminium ions) are linear for the reactions of PClDTF with SA amines (slope β of 0.3) and pyridines (β = 0.26) and those of NPClDTF with pyridines (β = 0.30). For the reaction of NPClDTF with SA amines the Brønsted‐type plot is biphasic, with slopes β1 = 0.2 (at high pKa) and β2 = 1.1 (at low pKa). The pKa value at the center of curvature (pK) is 7.7. The magnitude of the slopes indicates that the mechanisms of these reactions are stepwise, with the formation of a zwitterionic tetrahedral intermediate as the rate‐determining step, except for the reaction of NPClDTF with SA amines where there is a change in the rate‐determining step, from formation to breakdown of the tetrahedral intermediate, as the amine basicity decreases. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

4.
The reactions of bis(4‐nitrophenyl), 3‐chlorophenyl 4‐nitrophenyl, and 3‐methoxyphenyl 4‐nitrophenyl thionocarbonates ( 1 , 2 , and 3 , respectively) with a series of anilines are subjected to a kinetic investigation in 44 wt.% ethanol–water, at 25.0 °C and an ionic strength of 0.2 M. Under aniline excess, pseudo‐first‐order rate coefficients (kobs) are found. Plots of kobs versus aniline concentration are linear, with the slopes (kN) pH independent, kN being the rate coefficient for the anilinolysis of the thionocarbonates. The Brønsted plot (log kN vs. pKa of anilinium ions) for thionocarbonate 1 is linear, with slope (β) 0.62, which is consistent with a concerted mechanism. The Brønsted plots for thionocarbonates 2 and 3 are curved, with slopes 0.1 at high pKa for both reaction series and slopes 0.84 and 0.79 at low pKa for the reactions of 2 and 3 , respectively. The latter plots are in accordance to stepwise mechanisms, through a zwitterionic tetrahedral intermediate (T±) and its anionic analogue (T?), the latter being formed by deprotonation of T± by the basic form of the buffer (HPO). The Brønsted curves are explained by a change in the rate‐limiting step, from deprotonation of T± at low pKa, to its formation at high pKa. The influence of the amine nature and the non‐leaving and electrophilic groups of the substrate on the kinetics and mechanism is also discussed. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

5.
A scheme for the pKa estimation of organic acids in dimethylsulfoxide (DMSO) solution based on quantum chemical calculations is proposed. The procedure of pKa calculation requires several steps. The first is the calculation of the gas phase acidity of the compound. The G3MP2B3, G4MP2 as well as CBS‐QB3 composite methods made it possible to estimate values of gas phase acidities of an extensive set of structures with a high confidence level (standard deviations equal to 1.15, 1.13 and 1.29 kcal mol?1, respectively; the test set included 91 compounds). The second step is the computation of the solvation correction with the integral equation formalism version of polarizable continuum model (IEF‐PCM)–B3LYP/6‐311+G(d,p) approximation. Within the bounds of our approach, the medium properties were covered only by the PCM model, i.e. the proposed procedure neglects specific interactions between DMSO and the solute. It was determined that the approach to pKa estimation mentioned above is the most balanced in terms of accuracy, resource intensity and computation time cost. In the third step, the error of the pKa calculation was decreased by correlation allowances. Correlation allowances were determined for each acid class (62 С―Н, 55 N―Н, 24 O―Н and 5 S―Н acids) in the range of 50 units in terms of logarithmic scale using the test set including 146 compounds. Seven O―H acids showing the ability to form cyclic dimers were separated into a discrete group. The proposed methodology was applied to the estimation of pKa for trans‐ and cis‐dimethyl‐4,5‐dihydro‐3H‐pyrazol‐3,5‐dicarboxylates as well as for 5‐fluorouracil subject to competitive dissociation, the latter by N1―H or N3―H bonds. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

6.
A new computational procedure for the protonation model building of a multiwavelength and multivariate spectra treatment is proposed for the special case of small changes in spectra. The absorbance change Δi for the ith spectrum divided with the instrumental standard deviation sinst(A) represents the signal‐to‐error ratio SER of the spectra studied. The determination of the number of chemical components in a mixture is the first important step for further quantitative analysis in all forms of spectral data treatment. Most index‐based methods of the factor analysis can always predict the correct number of components, and even the presence of a minor one, when the SER is higher than 10. The Wernimont–Kankare procedure in the program INDICES performs reliable determinations of the instrumental standard deviation of the spectrophotometer used sinst(A), correctly predicts the number of light‐absorbing components present, and also solves ill‐defined problems with severe collinearity in spectra or very small changes in spectra. The mixed dissociation constants of three drugs, haemanthamine, lisuride, and losartan, including diprotic molecules at ionic strengths of I = 0.5 and 0.01 and at 25°C were determined using two different multiwavelength and multivariate treatments of the spectral data, SPECFIT32 and SQUAD(84) non‐linear regression analyses and INDICES factor analysis, even in the case of small absorbance changes in spectra. The dissociation constant pKa was estimated by non‐linear regression of {pKa, I} data at 25°C: for haemanthamine pKa = 7.28(1) at I = 0.50, for lisuride pKa = 7.86(1) and for losartan pKa,1 = 3.60(1), pKa,2 = 4.73(1) at I = 0.01. Goodness‐of‐fit tests for the various regression diagnostics enabled the reliability of the parameter estimates found to be proven. PALLAS and MARVIN predict pKa being based on the structural formulae of the drug compounds in agreement with the experimental value. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

7.
The α1‐adrenoceptor is a target for the treatment of several conditions from hypertension to benign prostatic hyperplasia. In this paper, we describe a new analysis approach to explore the conformational space of several ligands of the α1‐adrenoceptor and we also present the calculation of their proton affinity and basicity. For each compound a conformational search followed by a semi‐empirical optimisation was performed and a selection of conformations for each ligand was subjected to further optimisation using density functional theory methods. Different positions were explored to determine the favoured site of protonation, and then, the proton affinity (in the gas phase) and basicity (using the polarisable continuum model for the aqueous solution) were calculated for each of them. In addition, an alternative method using one explicit water molecule in combination with the polarisable continuum model for aqueous solvent was explored. Moreover, the acid dissociation constant (pKa) in water of these 26 compounds was calculated because this is an important parameter for a ligand when binding to its receptor. The experimental pKa values of six of these ligands and those of two compounds with a very low and a very large pKa were used to validate the theoretical methodology. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

8.
Basicity constants, pKa, for a wide range of mono‐protonated diaminobenzenes and diaminonaphthalenes, including dimethylamino derivatives were for the first time uniformly measured in 20% aqueous ethanol (29 compounds) and 80% aqueous dioxane (39 compounds) spanning from aniline to 1,8‐bis(dimethylamino)naphthalene (‘proton sponge’). The dioxane system proved to be more versatile and because of better solubility of N‐alkylated polyaminoarenes allowed to add to the same scale some superbasic bis(dialkylamino)‐, tetrakis(dialkylamino)‐, and hexakis(dialkylamino)naphthalenes, thus extending the scale for almost 10 pKa units, revealing possible limits of basicity changes in aromatic amines. The basicity of reference bases, pyridine and triethylamine, was also measured in these solvent systems. A group of N‐alkylated compounds was found to be less basic in aqueous dioxane when compared with their NH2‐analogs. This anomaly was not observed in aqueous ethanol. Other basicity trends and correlations between different basicity scales were also discussed. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

9.
K‐edge subtraction computed tomography (KES‐CT) allows simultaneous imaging of both structural features and regional distribution of contrast elements inside an organ. Using this technique, regional lung ventilation and blood volume distributions can be measured experimentally in vivo. In order for this imaging technology to be applicable in humans, it is crucial to minimize exposure to ionizing radiation with little compromise in image quality. The goal of this study was to assess the changes in signal‐to‐noise ratio (SNR) of KES‐CT lung images as a function of radiation dose. The experiments were performed in anesthetized and ventilated rabbits using inhaled xenon gas in O2 at two concentrations: 20% and 70%. Radiation dose, defined as air kerma (Ka), was measured free‐in‐air and in a 16 cm polymethyl methacrylate phantom with a cylindrical ionization chamber. The dose free‐in‐air was varied from 2.7 mGy to 8.0 Gy. SNR in the images of xenon in air spaces was above the Rose criterion (SNR > 5) when Ka was over 400 mGy with 20% xenon, and over 40 mGy with 70% xenon. Although in human thorax attenuation is higher, based on these findings it is estimated that, by optimizing the imaging sequence and reconstruction algorithms, the radiation dose could be further reduced to clinically acceptable levels.  相似文献   

10.
Benzoporphyrin monoacid derivatives, here named B3A and B3B, are promising new drugs for photodynamic therapy. Although both isomers show interesting characteristics as photosensitizing compounds, they have some distinct physicochemical properties such as the tendency to self‐aggregate in water‐rich media. Because pH drives the presence of each species, the pKa of these compounds assumes strategic importance. However, traditional micro‐titration methods and UV–Vis absorption techniques fail to give reliable pKa values due to the characteristics of this highly complex system, such as the precipitation of hydrophobic species, close pKa values, and high absorption band superposition. In the present work, chemometric tools are employed to evaluate pKa, and the kinetic tendency of monomers to undergo self‐aggregation is investigated. In solvent mixtures at low water percentage in ethanol, both B3A and B3B are stabilized in a monomeric state. However, in mixtures with a high water content, self‐aggregation takes place, mainly under a mild pH acid condition (3 < pH < 6), in which the prevalent protolytic species of both isomers is the neutral charged form, compounds with carboxylic and porphyrin free‐base groups. It is demonstrated that both isomers can undergo aggregation following a self‐catalytic mechanism, which is 2000 times slower to B3A than B3B. For B3A, the aggregation is manifested by a decrease in the monomer band with the aggregation band probably superposed to that of the monomer. For B3B, together with the decrease in the monomer band, a new band related to self‐aggregates is observed. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

11.
Substituent effects on the hydration, tautomerization, and isomerization equilibria of flavylium salts can be described by a series of linear free energy relationships (LSER) based on Hammett correlations. The positions on the flavylium rings were classified as either activated (para‐like) or nonactivated (meta‐like) to decide which σ value to employ (σR or σm, respectively), while the steric effects of substituents at C‐3 were included via the ES parameter. Based on these relationships, we then show that it is possible to predict values of the “apparent pKa” (pKap) of flavylium ions that were not included in the original data set, as well as those of several naturally occurring anthocyanins. The value of pKap provides a measure of the thermodynamic stability of the flavylium cation as a function of pH and is directly related to the pH range in which the color of the flavylium cation form of anthocyanins persists in aqueous solution. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

12.
Linear correlations were established between the calculated bond lengths and the pKa or σI values for a series of 4‐substituted bicyclo[2.2.2]octane‐1‐carboxylic acid and 4‐cubane‐1‐carboxylic acid derivatives. The bond lengths have been calculated at a modest computational level, HF/6‐31G(d), both in the gas phase and with the continuum solvation model, polarisable continuum model (PCM). In general, the best correlations are obtained when the PCM model is taken into account, especially when neutral and charged molecules are considered together. The best models in each case show square correlation coefficients (R2) larger than 0.9 and indicate that they can be used as predictive tools. These results expand previous results that indicate the possibility of a relationship between gas phase bond length and pKa values in aqueous solution and indicate that such relationships are more general than hitherto expected. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

13.
The base‐promoted solvolysis of a series of O,O‐dimethyl O‐aryl and O,O‐dimethyl O‐alkyl phosphorothioates (1) as well as O,O‐dimethyl O‐aryl and O,O‐dimethyl O‐alkyl phosphates (2) was studied computationally by density functional theory methods in methanol and water continuum media to determine the transition between concerted and stepwise processes. In addition, an experimental study was undertaken on the solvolysis of these series in basic methanol and water. The computations indicate that the solvolytic mechanism for series 1 involves lyoxide attack anti to the leaving group in a concerted manner with good leaving groups having pKaLg values < 12.3 in methanol and in a stepwise fashion with the formation of a 5‐coordinate thiophosphorane intermediate when the pKaLg > 12.3. A similar transition from concerted to stepwise mechanism occurs with series 2 in methanol as well as with series 1 and 2 in water, although for the aqueous solvolyses with hydroxide nucleophile, the transitions between concerted and stepwise mechanisms occur with better leaving groups than in the case in methanol. The computational data allow the construction of Brønsted plots of log k2?OS versus pKaLg in methanol and water, which are compared with the experimental Brønsted plots determined with these series previously and with new data determined in this work. Both the computational and experimental Brønsted data reveal discontinuities in the plots between substrates bearing O‐aryl and O‐alkyl leaving groups, with the gradients of the plots being far steeper than, and non‐collinear with, the O‐aryl leaving groups for solvolysis of the O‐alkyl‐containing substrates. These discontinuities signify that care should be exercised in interpreting breaks in Brønsted plots in terms of changes in rate‐limiting steps that signify the formation of an intermediate during a solvolytic process. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

14.
Xu  Yanhao  Duan  Ruikang  Liu  Hao  Xia  Chengcai  Duan  Guiyun  Ge  Yanqing 《Journal of fluorescence》2021,31(5):1219-1225

A novel pH-responsive probe based on an imidazo[1,2-a]indole fluorophore architecture is reported. The probe was highly selective to strongly acidic pH (pKa = 3.56) with high sensitivity and a fast response time (within 30 s). The probe did not demonstrate any fluorescence changes in the presence of interfering metal ions, and it featured excellent reversibility under strongly acidic conditions. The mechanism of detection of the probe was determined to be based on intramolecular charge transfer (ICT) at different pH. The probe was also able to be used for imaging for detecting acidic pH in Saccharomyces cerevisiae.

  相似文献   

15.
Nucleophilic reactivity of some functionalized surfactants, i.e. quaternary pyridinium aldoximes towards the hydrolysis of p‐nitrophenyl acetate (PNPA), p‐nitrophenyl benzoate (PNPB), p‐nitrophenyldiphenyl phosphate (PNPDPP) and p‐nitrophenyl p‐toluene sulphonate (PNPTS) has been studied at pH 7.1 and 27 °C. Addition of functionalized surfactant to reaction medium causes progressive increase in the rate of hydrolysis and reaches a maximum and then decreases due to further addition of surfactant. An increase in the alkyl chain length of functionalized surfactants resulted in an increase in the first‐order rate constant. The apparent pKa and CMC of functionalized surfactants have also been determined by spectrophotometric and conductometric methods, respectively. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

16.
Hansen solubility parameters and Lewis acid–base parameters are two groups of parameters that are used to characterize solvents, polymers, and the interactions between polymer matrix and additives. Although their definitions are very different, they can well explain the interactions in polymer composites. Therefore, some relations should exist between them. In this paper, the Lewis acid–base parameters of three polymers (PET, PS, and PVA) were measured by IGC technique. Including five other polymers (PE, PMMA, PC, PVC, and PVDF), the relationship between total Lewis acid–base intensity (K a+K b) and total polar intensity (δ2 p2 h) of the eight kinds of polymers are discussed, where δp and δh are the polar components of the Hansen parameters. It is found that these polymers are all Lewis base polymers according to the values of K b/K a. Generally, a polymer with a larger (K a+K b) value possesses a higher (δ2 p2 h) value.  相似文献   

17.
18.
The reactions of quinuclidines with phenyl, 4‐chlorophenyl, 4‐cyanophenyl, and 4‐nitrophenyl chlorothionoformates ( 1 , 2 , 3 , and 4 , respectively) are subjected to a kinetic study in aqueous solution, at 25.0°C, and an ionic strength of 0.2 M (KCl). The reactions are studied by following spectrophotometrically the release of the corresponding phenoxide anion/phenol generated in the parallel hydrolysis of the substrates. Under amine excess, pseudo‐first‐order rate coefficients (kobs) are found. Plots of kobs versus [amine] are linear, with slope kN. The Brønsted‐type plots (log kN vs. pKa of aminium ions) are linear, with slopes β = 0.26, 0.22, 0.19, and 0.28 for the reactions with 1 , 2 , 3 , and 4 , respectively. The magnitudes of the slopes indicate that these mechanisms are stepwise, with rate‐determining formation of a zwitterionic tetrahedral intermediate (T±). A dual parametric equation with the pKa of the nucleophiles and non‐leaving groups show βN = 0.26 and β nlg = ?0.16, also in accordance with the proposed mechanism. On the other hand, the reactivity of these thiocarbonyl substrates and their carbonyl derivatives was studied using their hardness index and compared with their experimental parameters, confirming the proposed mechanisms. By comparison of the title reactions with similar aminolyses, the following conclusions arise: (i) The mechanism of the reactions under investigation is stepwise with rate‐determining formation of T±. (ii) The reactivity of the substrates toward quinuclidines follows the order 4 > 3 > 2 > 1 . (iii) Quinuclidines are more reactive than isobasic pyridines toward chlorothionoformates. (iv) Chlorothionoformates are less reactive than chloroformates towards quinuclidines in accordance with the HSAB principle. (v) The kN values for phenyl chloroformate and 4 can be correlated with the pKa of quinuclidines and also with the hardness values calculated by the HF/3‐21G level of theory. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

19.
The reactions of 4‐methylphenyl and 4‐chlorophenyl 4‐nitrophenyl carbonates ( 1 and 2 , respectively), phenyl, 4‐methylphenyl, 4‐chlorophenyl, and 4‐nitrophenyl 2,4‐dinitrophenyl carbonates ( 3 , 4 , 5 , and 6 , respectively), and bis(2,4‐dinitrophenyl) carbonate ( 7 ) with a series of pyridines are studied kinetically at 25.0 °C in 44 wt% ethanol–water and an ionic strength of 0.2 M (KCl). The reactions are followed spectrophotometrically and under excess amine pseudo‐first‐order rate coefficients (kobs) are found. For all these reactions, plots of kobs versus free amine concentration at constant pH are linear, the slope (kN) being independent of pH. The Brønsted‐type plots (log kN vs. pKa of the conjugate acids of the pyridines) are all biphasic (linear portions at high and low pKa and a curvature in between). These plots are in accordance with a stepwise mechanism, through a zwitterionic tetrahedral intermediate (T±), and a change in the rate‐determining step from formation of T± to its breakdown to products, as the pyridine basicity decreases. Also studied are the effects of the leaving, non‐leaving, and electrophilic groups of the substrate, and of the amine nature, on the value (value at the center of curvature of the Brønsted‐type plots). Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

20.
The combination of synchrotron X‐ray radiation and metal‐based radiosensitizer is a novel form of photon activation therapy which offers the advantage of treating malignant tumors with greater efficacy and higher precision than conventional radiation therapy. In this study the anticancer cytotoxic efficacy of a new chlorophyll derivative, iodinated chlorin p6 copper complex (ICp6‐Cu), combined with synchrotron X‐ray radiation (8–10 keV) in two human oral cancer cell lines is explored. Pre‐treatment of cells with 20 µM and 30 µM ICp6‐Cu for 3 h was found to enhance the X‐ray‐induced cytotoxicity with sensitization enhancement ratios of 1.8 and 2.8, respectively. ICp6‐Cu localized in cytoplasm, mainly in lysosomes and endoplasmic reticulum, and did not cause any cytotoxicity alone. The radiosensitization effect of ICp6‐Cu accompanied a significant increase in the level of reactive oxygen species, damage to lysosomes, inhibition of repair of radiation‐induced DNA double‐strand breaks, increase in cell death and no significant effect on cell cycle progression. These results demonstrate that ICp6‐Cu is a potential agent for synchrotron photon activation therapy of cancer.  相似文献   

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