Analysis of protonation equilibria of some alanyl dipeptides in water and aqueous ethanol mixtures.

The protonation constants are one of the most fundamental properties of biological molecules. The determination of the constants of the dipeptide is interesting and necessary for a full understanding of its activities in biological process. In this study, the protonation constants of some aliphatic alanine dipeptides (alanyl-alanine, alanyl-phenylalanine, alanyl-valine, alanyl-leucine, and alanyl-methionine) were studied in water and ethanol-water mixtures (20%ethanol-80%water;40%ethanol-60%water;60%ethanol-40%water, (v/v)) at 25 ± 0.1 °C under nitrogen atmosphere and ionic strength at 0.10 mol L by potentiometry. The constants of the systems were calculated using Best computer program. The effects of the different amino acids bound to the alanine on the acidity of the alanyl dipeptides were investigated. The constants were influenced by changes in solvent composition and their variations were discussed in terms of solvent and structural properties.