Determination of Protonation Constants of L-Glutamic Acid and L-Methionine in 1,2-Propanediol-Water Mixtures

Muddapu Padma Latha, Vegi Maheswara Rao, Tirukkovalluri Siva Rao, Gollapalli Nageswara Rao*

School of Chemistry, Andhra University, Visakhapatnam-530 003, India.

The solute-solvent interactions of L-glutamic acid and L-methionine have been studied in 060% V/V 1,2-propanediolwater media using the pH-metric method. The protonation constants have been calculated with the computer program MINIQUAD75. Selection of the best fit chemical model of the acid-base equilibria is based on standard deviation in protonation constants and residual analysis using crystallographic R-factor and sum of squares of residuals in all massbalance equations. Linear variation of protonation constants with inverse of dielectric constant of the solvent mixture has been attributed to the dominance of the electrostatic forces. Distributions of species, protonation equilibria and effect of influential parameters on the protonation constants have also been presented.

Keywords: acid-base equilibria, L-glutamic acid, L-methionine, 1,2-propanediol