Solvent Effects on Tautomeric and Microscopic Protonation Constants of Glycine in Different Aqueous Solutions of Methanol and Ethanol

Abstract

The acid-base equilibria of glycine have been studied in different aqueous solutions of methanol and ethanol (0-45 % v/v) using a potentiometric method. In this study, the macro and micro protonation constants of the amino acid and its tautomeric constant have been determined at 25°C and constant ionic strength (0.1 mol·dm −3 NaClO 4 ). The protonation and the tautomeric constants of glycine in different binary mixtures were analyzed in terms of the Kamlet, Abboud and Taft (KAT) parameters. Single-parameter correlations of the constants versus α (hydrogen-bond donor acidity), β (hydrogen-bond acceptor basicity) and π * (dipolarity/polarizability) are poor in all solutions. Multi-parameter correlations show better results, but dual-parameter correlations represent significant improvements with regard to the single-and multi-parameter models. Linear correlation is observed when the experimental protonation constant values are plotted versus the calculated ones when the KAT parameters are considered. Finally, the results are discussed in terms of the effect of the solvent on protonation and tautomeric constants.

Solvent Effects on Protonation Constants of Betanin in Different Aqueous Solutions of Methanol at T = 25 ºC

“…The multiparametric equation, Eq. (8), has been proposed using the solvatochromic solvent parameters (α, β and π * ) which have been introduced in previous literatures [28][29][30][31][32]. logK = Aₒ + aα + bβ + pπ* (8) where Aₒ is the regression value and π* is the index of the solvent dipolarity/polarizability, which shows the ability of a solvent to stabilize a charge or a dipole by its own dielectric effects.…”