Solvent effects on protonation equilibria of some amino acids and peptides in different aqueous solutions of ethanol.

The protonation constants of glycine, L-alanine, glycyl-glycine, glycyl-glycyl-glycine and glycyl-L-alanine have been determined in 10-80% (v/v) ethanol-water mixtures at 25 °C and constant ionic strength (0.1 mol L-1 sodium perchlorate), by potentiometric technique and calculated using a suitable computer program, which employs a nonlinear least-squares method. The effect of solvent composition on the protonation constants in the mixed solvents were correlated with the Kamlet-Taft solvatochromic parameters (α, β and π*). It was shown that logK1 and logK2 values of glycine, L-alanine and peptides increasing with increase ethanol content up to 50% and then tend to decrease. Further, an overview of the effect of preferential solvation and solvent structure of electrolytes in ethanol-water mixtures on the values of the protonation constants in these media were also discussed.