Effect of Solvent Composition on Protonation Constants of Some Glycine Dipeptides

Stoichiometric protonation constants (log(10) K (1) and log(10) K (2)) of some aliphatic dipeptides (Gly-Tyr, Gly-Phe, Gly-Val, Gly-Leu, Gly-Thr, Gly-Met and Gly-Pro) were determined potentiometrically in 20, 40, and 60 % (v/v) 1,4-dioxane-water and dimethyl sulfoxide-water mixtures at 25.0 (+/- 0.1) A degrees C with an ionic strength of 0.10 mol center dot L-1 sodium chloride. The protonation constants were calculated with the computer program PKAS and selection of the best fit chemical models is based on the statistical parameters. The effects of solvent composition on these protonation constants are discussed to determine the factors which control these processes. It has been observed that, while the correlation between log(10) K (1) and log(10) K (2) with the percentages of dimethyl sulfoxide in the dimethyl sulfoxide-water mixtures are not linear, these values linearly increase as the concentration of 1,4-dioxane increases in the solvent mixtures.