Net proton charge of beta- and kappa-casein in concentrated aqueous electrolyte solutions

Titration experiments have been carried out in order to measure the net proton charge of beta- and kappa-casein in NaCl solutions at 0.1 M and 1 M salt concentrations, at 4 degrees C, in the pH range between 5.5 and 10.5. Experimental data are compared with model values calculated through pK(a)'s of titrable groups neglecting the electrostatic perturbation term (deltapK(a)) in order to evaluate the magnitude of the error caused by this approximation and to delimit its effectiveness. At both ionic strengths, the agreement is good for kappa-casein in the pH range [5.5, 9.5], while errors of up to 2 charges are observed for beta-casein in the same range. These deviations are likely to be caused by strong electrostatic effects induced by the high density of negative charges of beta-casein 1-21 peptide. In order to account for these electrostatic effects, the net proton charge on this peptide is evaluated through a model based on the counterion condensation theory developed for the titration of polyelectrolytes with different types of ionizable groups.