SHIFT IN PKA OFL-HISTIDINE AND L-GLUTAMIC ACID IN ORGANIC MEDIA: A POTENTIOMETRIC STUDY

Abstract

Protonation equilibria of L-histidine and in L-glutamic acid have been studied in various concentrations (0.0-50% v/v) in Dimethylformamide-water mixture by maintaining an ionic strength of 0.16 mol dm-3 at 310 K. The protonation constants have been calculated using the computer program MINIQUAD75 and the best-fit models are arrived at based on statistical grounds employing crystallographic R factor, χ2, skewness and kurtosis. The trend of log values of stepwise protonation constants with mole fraction of the medium have been explained based on electrostatic and non-electrostatic forces operating on the protonation equilibria. Distributions of species, protonation equilibria and effect of influential parameters on the protonation constants have also been discussed. The protonation constants from the best fit models show the formation of LH3 2+, LH2 +, LH and L- in the case of L-histidine and XH3 +, XH2, XH- and X2- in the case of Lglutamic acid. In both the cases, LH/XH is present to an extent of 90% in the pH range 4.0-11.0.The effect of experimental errors on the protonation constants has also been discussed.