Solvent effect on Protonation Equilibria of L-Dopa and Catechol in CTAB-water Mixtures

Abstract

The impact of Cetyltrimethylammonium bromide (CTAB) on the protonation equilibria of L-Dopa and catechol has been studied in various concentrations (0.0-2.5% w/v) of CTAB solution maintaining an ionic strength of 0.16 mol L-1 at 303 K. The protonation constants have been calculated with the computer program MINIQUAD75 and the best fit models have been selected based on statistical parameters. Dopa has three dissociable protons and one amino group which associate with proton. It exists as LH4+ at low pH and gets deprotonated with the formation of LH3, LH-2 and LH2- successively with increase in pH. Catechol exists as LH2 at low pH and gets deprotonated with the formation of LH- and L-2 with increasing pH. The trend of log values of step-wise protonation constants with mole fraction of the medium has been explained based on electrostatic and non-electrostatic forces operating on the protonation equilibria. The effects of errors on the protonation constants have also been presented.