Multiple Dissociation Constants of the Intepirdine Hydrochloride Using Regression of Multiwavelength Spectrophotometric pH-Titration Data

Abstract

Spectrophotometric and potentiometric pH-titrations of the Neurotransmitter Intepirdine hydrochloride INN·HCl for three dissociation constants determination were compared. A nonlinear regression of the pH-spectra (REACTLAB, SQUAD84) and of the pH-titration curve (ESAB) determined three multiple dissociation constants. A sparingly soluble neutral molecule LH of INN·HCl was capable of protonation to form the still soluble three cations LH2+, LH32+ and LH43+ in pure water. Although the change of pH somewhat less affected changes in the chromophore, three consecutive thermodynamic dissociation constants were estimated pKTa1 = 5.64, pKTa2= 7.31, pKTa3=8.85 at 25 °C and pKTa1= 5.51, pKTa2 = 7.15, pKTa3= 8.77 at 37 °C. The graph of molar absorption coefficients of variously protonated species according to wavelength shows that the spectrum of species LH2+ and LH vary in colour, while protonation of chromophore LH2+ to LH32+ and LH43+ has less influence on chromophores in Intepirdine hydrochloride molecule. As the spectral response on the chromophore in the INN·HCl molecule is not large, it was necessary to test a reliability whether it is possible to estimate the dissociation constants even fromsuch small spectrum changes. Three multiple thermodynamic dissociation constants of 3×10−4M INN·HCl were determined by the regression analysis of potentiometric titration curves pKTa1=5.14, pKTa2=8.38, pKTa3=9.33 at25 °C and pKTa1=5.17, pKTa2=8.31, pKTa3=9.07 at 37 °C. The macro-dissociation constants of INN·HCl were estimated according to the chemical structure analyzed by two pKa predictors, the MARVIN and ACD/Percepta programs. The resulting protonation scheme of INN·HCl was suggested.