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 LH 2 + , LH 3 2+ and LH 4 3+ in pure water. Although the change of pH somewhat less affected changes in the chromophore, three consecutive thermodynamic dissociation constants were estimated p K T a1  = 5.64, p K T a2  = 7.31, p K T a3  = 8.85 at 25 °C and p K T a1  = 5.51, p K T a2  = 7.15, p K T a3  = 8.77 at 37 °C. The graph of molar absorption coefficients of variously protonated species according to wavelength shows that the spectrum of species LH 2 + and LH vary in colour, while protonation of chromophore LH 2 + to LH 3 2+ and LH 4 3+ 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 from such small spectrum changes. Three multiple thermodynamic dissociation constants of 3 × 10 −4  M INN·HCl were determined by the regression analysis of potentiometric titration curves p K T a1  = 5.14, p K T a2  = 8.38, p K T a3  = 9.33 at 25 °C and p K T a1  = 5.17, p K T a2  = 8.31, p K T a3  = 9.07 at 37 °C. The macro-dissociation constants of INN·HCl were estimated according to the chemical structure analyzed by two p K a predictors, the MARVIN and ACD/Percepta programs. The resulting protonation scheme of INN·HCl was suggested.