pKa values of peptides in aqueous and aqueous–organic media: Prediction of chromatographic and electrophoretic behaviour

In the present work, models describing the effect of the pH on the chromatographic and electrophoretic behaviour for polyprotic peptides were compared. The proposed models can be simultaneously used for determination of dissociation constants and selection of the optimum pH for the separation of peptides, in water and acetonitrile–water mixtures widely used in liquid chromatography and in capillary electrophoresis. The models use the pH value measured in the acetonitrile–water mixture instead of the pH value in water and take into account the effect of the activity coefficients. They permit the determination of the acidity constants in the aqueous and hydro–organic mobile phase from chromatographic retention and electrophoretic migration measurements, respectively. The values obtained by both proposed techniques agree with the potentiometric values previously determined. The suitability of the proposed models for predicting chromatographic and electrophoretic behaviour of compounds studied from a limited number of experimental data was also compared. The separation between solutes by both techniques in a complex mixture can be easily predicted, making simple and rapid pH selection to achieve optimum separation.