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Thermodynamic protonation constants of vardenafil by the nonlinear regression of multiwavelength pH-spectrophotometric titration data

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Meloun M., Ferenčíková Z., Niesnerová I., Pekárek T.
Open Chemistry
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2013
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vol. 11
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issue 2
271-279
EN
pH-spectrophotometric titration data were used to determine protonation constants of vardenafil at different ionic strengths I and temperatures of 25°C and 37°C. The use of two different multiwavelength and the multivariate treatment of spectral data, SPECFIT32 and SQUAD(84) nonlinear regression analyses and INDICES factor analysis is presented. The reliability of the protonation constants of the drug was proven with goodness-of-fit tests of the pH-spectra. The thermodynamic protonation constants log K Ti were estimated by a nonlinear regression of (log K, I) data using the Debye-Hückel equation, yielding log K 4T = 3.59(1) and 3.26(1), log K 3T = 5.64(1) and 5.81(1), log K 2T = 9.41(1) and 8.59(2), log K 1T = 10.92(2) and 10.05(1) at 25°C and 37°C, where the figure in brackets is the standard deviation in last significant digit. Concurrently, the experimental determination of four thermodynamic protonation constants was combined with the computational prediction of the MARVIN program based on knowledge of the chemical structures of the drug and was in good agreement with its experimental value. The factor analysis of spectra in the INDICES program predicts the correct number of light-absorbing components when the instrument error is known and when the signal-to-error ratio SER is higher than 10. [...]
2
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The thermodynamic dissociation constant of naphazoline by the regression analysis of potentiometric data

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Meloun M., Ferenčíková Z., Netolická L., Vrána A.
Open Chemistry
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2011
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vol. 9
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issue 1
66-74
EN
The mixed dissociation constant of naphazoline is determined at various ionic strengths I [mol dm−3] in the range of 0.01 to 0.26 and at temperatures of 25°C and 37°C using ESAB and HYPERQUAD regression analysis of the potentiometric titration data. A strategy of efficient experimentation is proposed in a protonation constant determination, followed by a computational strategy for the chemical model with a protonation constant determination. Two group parameters, L 0 and H T were ill-conditioned in the model and their determination is therefore uncertain. These group parameters, L 0 and H T, can significantly influence a systematic error in the estimated common parameter pKa and they always should be refined together with pK a. The thermodynamic dissociation constant pK aT was estimated by nonlinear regression of {pK a, I} data at 25°C and 37°C: for naphazoline pK alT = 10.41(1) and 10.13(2). Goodness-of-fit tests for various regression diagnostics enabled the reliability of the parameter estimates to be found. [...]
3
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Thermodynamic dissociation constants of risedronate using spectrophotometric and potentiometric pH-titration

100%
Meloun M., Ferenčíková Z., Málková H., Pekárek T.
Open Chemistry
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2012
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vol. 10
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issue 2
338-353
EN
Risedronate inhibits bone resorption in diseases like osteoporosis, Paget’s disease, tumor bone diseases or the malfunction of phosphocalcium metabolism. The acid-base properties of risedronate in an aqueous solution have been studied in a pH range from 2 to 12 and can be described in terms of four dissociation steps: pK a,2, pK a,4, pK a,5 (related to the dissociation of POH groups) and pK a,3 related to the dissociation of protonated amino group NH3+. The mixed dissociation constants were determined at different ionic strengths I = 0.02 to 0.20 mol dm−3 KCl and of 25°C and 37°C using pH-spectrophotometric and pH-potentiometric titration methods. Determination of group parameters L 0, H T might lead to false estimates of common parameters p K a;therefore, the computational strategy employed is important. A comparison between the two programs ESAB and HYPERQUAD demonstrated that the ESAB program provides a better fit of potentiometric titration curve. The thermodynamic dissociation constants pK aT were estimated by a nonlinear regression of (pK a, I) data and a Debye-Hückel equation at 25°C and 37°C, pK a,2T = 2.37(1) and 2.44(1), pK a,3T = 6.29(3) and 6.26(1), pK a,4T = 7.48(1) and 7.46(2) and pK a,5T = 9.31(7) and 8.70(3) at 25°C and 37°C using pH-spectroscopic data and pK a,2T = 2.48(3) and 2.43(1), pK a,3T= 6.12(2) and 6.10(2), pK a,4T = 7.25(2) and 7.23(1) and pK a,5T = 12.04(5) and 11.81(2) at 25°C and 37°C. The ascertained estimates of three dissociation constants pK a,3, pK a,4, pK a,5 are in agreement with the predicted values obtained using PALLAS [...]
4
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The thermodynamic dissociation constants of methotrexate by the nonlinear regression and factor analysis of multiwavelength spectrophotometric pH-titration data

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Meloun M., Ferenčíková Z., Vrána A.
Open Chemistry
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2010
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vol. 8
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issue 3
494-507
EN
The mixed dissociation constants of methotrexate - chemically (2S)-2-[(4-{[(2,4-diamino-7,8-dihydropteridin-6-yl)methyl] (methyl)amino}phenyl)formamido]pentanedioic acid (the cas number 59-05-2) at various ionic strengths I of range 0.01–0.4, and at temperatures of 25°C and 37°C, were determined with the use of two different multiwavelength and multivariate treatments of spectral data, SPECFIT32 and SQUAD(84) nonlinear regression analyses and INDICES factor analysis according to a general rule of first, determining the number of components, and then calculating the spectral responses and concentrations of the components. Concurrently, the experimental determination of the thermodynamic dissociation constants was in agreement with its computational prediction of the PALLAS programme based on knowledge of the chemical structures of the drug. The factor analysis in the INDICES programme predicts the correct number of light-absorbing components when the data quality is high and the instrumental error is known. Three thermodynamic dissociation constants were estimated by nonlinear regression of {pK a , I} data: for methotrexate pK a1T= 2.895(13), pK a2T= 4.410(14), pK a3T= 5.726(15) at 25°C and pK a1T= 3.089(15), pK a2T= 4.392(12), pK a3T= 5.585(11) at 37°C, where the figure in brackets is the standard deviation in last significant digits. The reliability of the dissociation constants of the drug were proven by conducting goodness-of-fit tests of the multiwavelength spectrophotometric pH-titration data. [...]
5
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Oligomers-model building in protonation equilibria of sitagliptin

100%
Meloun M., Ferenčíková Z., Niesnerová I., Pekárek T.
Open Chemistry
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2013
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vol. 11
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issue 11
1799-1807
EN
Mixed protonation constants of sitagliptin phosphate at various ionic strengths I (mol kg−1) in range 0.01 and 0.50 and at 298.15 K are determined using FBSTAC4 and HYPERQUAD nonlinear regression analyses of the potentiometric titration curve. At a low concentration c L = 1.1 mmol kg−1 the monomers L, LH, LH2, LH3 and LH4 dominate, while for a concentration range from c L = 13.7 to 24.7 mmol kg−1 dimers L2H2, L2H3, L2H4 and L2H are mainly present. The regression programme has almost no influence on the precision of the protonation constants. The accuracy of the protonation constants log βqr depends on the accuracy of the group parameters. As two group parameters c L,0, c H,T are ill-conditioned in a model, their determination is therefore uncertain: both can significantly cause a systematic error in the estimated common parameters log10βqr . Fitness tests using regression diagnostics have proven the reliability of the parameter estimates. [...]
6
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Reliability of protonation constants of vildagliptin dimers by the regression analysis of pH-titration data

100%
Meloun M., Ferenčíková Z., Niesnerová I., Pekárek T.
Open Chemistry
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2013
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vol. 11
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issue 7
1101-1111
EN
Protonation constants of protonated monomers and dimers of the vildagliptin are determined potentiometrically. For the low concentration c L = 3.3 mmol dm−3 the monomers L and LH dominate, while for a higher concentration c L = 6.3 mmol dm−3 the dimers L2H2, L2H3, L2H4 and L2H are mainly present. The algorithm used has little influence on the precision of the formation constants in comparison with the reproducibility of the titration. The mixed protonation constants of vildagliptin dimers Lq Hr at various temperatures are determined using FBSTAC4 and HYPERQUAD regression analysis of the potentiometric titration data. The accuracy of the protonation constants log10 βqr depends on the accuracy of the group parameters. As two group parameters L 0, H T are ill conditioned in a model, their determination is therefore uncertain; both can significantly cause a systematic error in the estimated common parameters log10 βqr . Using various regression diagnostics the goodness-of-fit proves the reliability of all parameter estimates. A rough estimation of thermodynamic enthalpies ΔH 0 (kJ mol−1) and entropies ΔS 0 (J K−1 mol−1) is determined from the temperature variation of protonation constants. The enthalpy shows the protonation process is exothermic, and the entropy indicates that it is spontaneous. [...]
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