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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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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 [...]
3
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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. [...]
4
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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. [...]
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