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Abstract

SRX Pharmacology
Volume 2010 (2010), Article ID 481497
doi:10.3814/2010/481497

Research Article

Recent Progress in the pKa Estimation of Druglike Molecules by the Nonlinear Regression of Multiwavelength Spectrophotometric pH-Titration Data

Milan Meloun,¹ Tomáš Syrový,¹ and Jahan Ghasemi²

¹	Department of Analytical Chemistry, University of Pardubice, Pardubice 532 10, Czech Republic
²	Chemistry Department, Faculty of Sciences, K. N. Toosi University of Technology (KNTU), 16167 Tehran, Iran

Received 2009-10-19
Revised 2009-12-03
Accepted 2009-12-21

Recent developments in the computational diagnostic tools for the pKa estimation of druglike molecules carried out by the nonlinear regression of multiwavelength spectrophotometric pH-titration data are demonstrated on the protonation equilibria of silybin. The factor analysis of spectra predict the correct number of components when the signal-to-error ratio SER is higher than 10. The mixed dissociation constants of the drug silybin at ionic strength I = 0.03 and a temperature of 25∘C were determined using two different programs, SPECFIT32 and SQUAD(84). A proposed experimental and computational strategy for the determination of the dissociation constants is presented. The dissociation constant pKa was estimated by nonlinear regression of the {pKa,I} data at 25∘C with SQUAD (and SPECFIT); that is, pKa1 = 6.898(0.022) and 6.897(0.002); pKa2 = 8.666(0.021) and 8.667(0.012); pKa3 = 9.611(0.010) and 9.611(0.004); pKa4 = 11.501(0.008) and 11.501(0.007). While great progress has been achieved in terms of the reliability of the protonation model estimation, among the most efficient diagnostics of the nonlinear regression of multiwavelength pH-spectra are the goodness-of-fit test, Cattel's scree plot of the factor analysis, spectra deconvolution, the signal-to-error SER ratio analysis, and other tools of efficient spectra analysis.