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1.
Capillary electrophoresis affords a simple, automated approach for the measurement of pKa values in the range 2-11 at a throughput of less than 1 h per sample per instrument. Agreement with literature values is usually within 0.20 log units with a precision better than 0.07 log units. The attractive features of capillary electrophoresis for pKa measurements are: (1) conventional instrumentation with a high level of automation are suitable for all measurements; (2) because it is a separation method samples need not be of high purity; (3) samples of low water solubility with suitable chromophores are easily handled (detection limits in the microM range); (4) sample consumption per measurement is in the microgram range; and (5) since only mobilities are measured, exact knowledge of concentrations is not needed. The general approach can be extended to pKa measurements in aqueous-organic solvent mixtures and non-aqueous solvents with suitable calibration. The widespread use of absorbance detection in capillary electrophoresis means that the sample must have a suitable chromophore for detection. The main source of controllable error is the accuracy of buffer standardization and their stability in use, and uncontrollable error, the retentive interactions of the sample with the column wall. The latter seems to be a rare problem in practice for typical operating conditions. 相似文献
2.
Yuki Tanikami Takuma Tagami Mayu Sakamoto Yukihiro Arakawa Hitoshi Mizuguchi Yasushi Imada Toshio Takayanagi 《Electrophoresis》2020,41(15):1316-1325
Acid dissociation constants (pKa) of nine kinds of flavin analogues as molecular catalyst candidates were determined by CZE. Although some of the analogues are instable and degradable under the light exposure or in alkaline aqueous solutions, the effective electrophoretic mobility of the flavin analogue of interest has been measured with the residual substance. The pKa values of the flavin analogues were analyzed through the changes in the effective electrophoretic mobility with varying pH of the separation buffer. One or two steps pKa values were determined by the analysis. One of the degraded species from the flavin analogues, lumichrome, was also detected in the CZE analysis, and its pKa values were also determined. While coexisting impurities generated over the storage conditions were found in some analogues, the pKa values of the target analogues were successfully determined with the help of the CZE separations. A pressure-assisted CZE was utilized for the determination or the estimation of the pKa values of such analogues as possessing carboxylic acid moiety. 相似文献
3.
氟试剂(茜素氨羧配合剂,Alizarin Complexan,简称ALC)的离解平衡及吸收光谱变化比较复杂,用经典的电位法或光度法较难获得准确的结果,以致数据差距较大.我们研究了它在不同pH时的吸收光谱,然后在等色点波长以吸光度对pH作图,得到一组等色点摩尔比曲线,由此导出测定离解常数的新方法.方法涉及的试验和计算均较简单,结果比较令人满意. 实验仪器 UV-240分光光度计;pH S-2型酸度计. 试剂氟试剂(A.R.北京化工厂)提纯后,常法配制成1.0×10~(-2)mol·L~(-1)的微酸性溶液. 相似文献
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Ernest Giralt Roser Viladrich Enrique Pedroso 《Magnetic resonance in chemistry : MRC》1983,21(3):208-213
The acid dissociation constant of the imidazolium ring of the decapeptide luliberin has been determined by 1H NMR-followed titration in D2O. The normal procedure for the analysis of the titration curve, i.e. direct use of the Henderson-Haselbalch equation, is still applicable in this case, but for more complex peptides a modified calculation procedure is proposed. Results obtained when both methods were applied to luliberin are compared. The influence of D2O when used as the solvent in this type of determination has been studied using Nα-acetyl-L -histidine methyl ester as a model compound. The difference between the acid dissociation constant of this molecule determined in H2O and in D2O implies that a correction of ?.25 unit is needed for those pKa values calculated by plotting the chemical shifts in D2O vs the apparent pH meter readings. The pKa found for Nα-acetyl-L-histidine methyl ester, 6.30 ± 0.04, can be taken as a standard value for histidine-containing peptides. 相似文献
6.
We propose a method for the determination of acid dissociation constants based on the rapid detection of the equivalence point (EP) by feedback-based flow ratiometry and the subsequent estimation of the half equivalence point (EP1/2). The titrant (e.g., NaOH) flow rate FB was varied in response to a control voltage Vc from a controller, while the titrand (e.g., CH3COOH) flow rate FA was held constant. The pH of the mixed solution was monitored downstream from the confluence point of the solutions following a knotted tubular mixer. Initially, Vc was increased linearly. At the instance the detector sensed EP, the ramp direction of Vc changed downward. The pH increased further because of the lag time between the mixing of solutions and the sensing of pH. Following the pH maximum, the pH decreased. The EP was sensed again in this downward scan. The Vc that gives EP1/2 was computed from the Vc just at the time of the EP detection. The Vc was held constant at this level for 18 s, and the plateau pH value thus obtained was taken to be the pKa of the analyte subject to activity corrections. Studies on the dependence of the pKa on the ionic strength or dielectric constant of the solution were conducted in an automated fashion by delivering NaCl solution or acetonitrile through an additional channel. Satisfactory results were obtained with good throughput (53 s per determination) and precision (R.S.D.≈0.3%) for various acids. 相似文献
7.
R. H. Contreras A. R. Engelmann G. E. Scuseria J. C. Facelli 《Magnetic resonance in chemistry : MRC》1980,13(2):137-142
Alterations have been introduced in the semi-empirical INDO parameters in order to study their influence on the Fermi contact term of the indirect spin–spin coupling constants as calculated by the finite perturbation theory (FPT). For this purpose a set of molecules containing hydrogen, carbon and/or fluorine has been selected. In general, most coupling constants are found to be much more sensitive than other molecular properties to small changes in the INDO parameters. This sensitivity depends strongly on the particular calculated coupling constant. In most cases the uncertainty in the INDO parameters leads to uncertainties in the coupling constants which are much greater than their experimental errors. 相似文献
8.
Wichien Sang‐Aroon Vithaya Ruangpornvisuti 《International journal of quantum chemistry》2008,108(6):1181-1188
Determination of acid‐dissociation constants, pKa, of aspartic acid in aqueous solution, using density functional theory calculations combined with the conductor‐like polarizable continuum model (CPCM) and with integral‐equation‐formalism polarizable continuum model (IEFPCM) based on the UAKS and UAHF radii, was carried out. The computed pKa values derived from the CPCM and IEFPCM with UAKS cavity model of bare structures of the B3LYP/6‐31+G(d,p)‐optimized tetrahydrated structures of aspartic acid species are mostly close to the experimental pKa values. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 相似文献
9.
Ionization constants of some flavanols (catechin and epicatechin) and flavonols (kaempherol, fisetin, morin, and quercetin) are determined by capillary zone electrophoresis (CZE). This technique allows the determination of pK(a) values until about 12. The pK(a) values obtained are compared with those calculated by the SPARC computational program. This program predicts the microscopic and macroscopic pK(a) values and the order of deprotonation of the different -OH groups. While for catechin and epicatechin the first ionizable OH group occurs in ring 1 and the second ionizable group in ring 2, in flavonols the first deprotonation occurs in ring 2 and the second in ring 1. 相似文献
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Sodium salts of 2,3-quinoxalinedithiol have been prepared and used for determination of the acid ionization constants by potentiometric titration. The fully protonated form of the reagent was used in an alternative determination of the first ionization constant by a solubility method. The constants are pK(2), == 6.84 +/- 0.04 and pK(2) = 9.95 +/- 0.03. 相似文献
13.
CZE was employed to determine acid-base dissociation constants (pK(a)) of ionogenic groups of azahelicenes in methanol (MeOH). Azahelicenes are unique 3-D aromatic systems, which consist of ortho-fused benzene/pyridine units and exhibit helical chirality. The pK(a) values of pyridinium groups of the studied azahelicenes were determined from the dependence of their effective electrophoretic mobility on pH by a nonlinear regression analysis. The effective mobilities of azahelicenes were determined by CZE at pH range between 2.1 and 10.5. Thermodynamic pK(a) values of monobasic 1-aza[6]helicene and 2-aza[6]helicene in MeOH were determined to be 4.94 +/- 0.05 and 5.68 +/- 0.05, respectively, and pK(a) values of dibasic 1,14-diaza[5]helicene were found to be equal to 7.56 +/- 0.38 and 8.85 +/- 0.26. From these values, the aqueous pK(a) of these compounds was estimated. 相似文献
14.
Capillary electrophoresis was applied to investigate pKa values of 99mTc radiotracers used in nuclear medicine. Therefore, the protonation equilibria of the carboxyl groups of 99mTc-mercaptoacetylglycylglycylglycine (99mTc-MAG3) and 99mTc-ethylenecysteine dimer (99mTc-EC) were studied by pH-dependent determination of electrophoretical velocities. 99mTc-ethylenecysteine dimer diethyl ester (99mTc-ECD) was used as a non-protonable standard. The capillary electrophoresis system was equipped with a radioactivity detector. Measurements were performed using a pressure-driven capillary zone electrophoresis which allowed runs even in the low pH range. For the determination of pKa values, the electrophoretical velocities of the analytes were referred to the electrophoretical velocities of tetraphenyle arsonium chloride as a positively charged marker. Calculation of pKa values was accomplished by non-linear curve fitting of both structure-based equilibria equations and sigmoidal decay functions to the experimental data. 99mTc-MAG3 was shown to have a carboxyl group pKa value of 4.22. The value for the carboxyl groups of 99mTc-EC is 2.90 (determined by structure-based equilibria equations), which represents a common value for both carboxyl groups. By the use of sigmoidal functions, similar values were elucidated. As expected, 99mTc-ECD shows no protonation step. 相似文献
15.
The acidic dissociation constants of three vic-dioximes have been determined from spectrophotometric data. The results may be summarized as follows:
pK1 | pK2 | |
2,3-butanedionedioxime | 10.6 ± 0.1 | 11.9 ± 0.3 |
1,2-cyclohexanedionedioxime | 10.6 ± 0.2 | 12.4 ± 0.5 |
1,2-cycloheptanedionedioxime | 10.7 ± 0.2 | 12.3 ± 0.5 |