The ionization state of amino acids is altered by a change in pH. At low pH, near the p K a pKa for the carboxylic acid, the − COOH −COOH proton is lost from the fully protonated form. As the pH approaches physiological levels, the zwitterionic form predominates. At high pH, near the p K a pKa for the amino group, one of the − NH + 3 −NH3+ protons is lost to form the fully deprotonated species.