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 degrees 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 (alpha, beta and pi*). It was shown that logK(1) and logK(2) 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.