Solvent Effects on Protonation Constants of Tryptophan in Some Aqueous Aliphatic Alcohol Solutions

Abstract

The protonation constants of tryptophan (K1 and K2) were determined in binary mixtures of water with methanol, ethanol, and 1-propanol containing (0, 10, 20, 30, 40, 50, 60, 70, and 80) % (v/v) using a potentiometric method at 25 °C and constant ionic strength (0.1 mol·dm−3 sodium perchlorate). The autoprotolysis constant values (Kap) of the media were also determined in the same binary mixtures. The protonation constants of tryptophan and the autoprotolysis constant of the medium in different binary mixtures were analyzed in terms of 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. Multiparameter correlations show better results, but dual-parameter correlations represent significant improvements with regard to the single- and multiparameter models. Linear correlation is observed when the experimental log Kap, log K1, and log K2 values are plotted versus the calculated ones, while the KAT parameters are considered. Finally, the results are discussed in terms of the effect of the solvent on the protonation and autoprotolysis constants.