Determine the net charge of the predominant form of Lysine a

Determine the net charge of the predominant form of Lysine at pH 10.0 (pK_a values are 2.16, 9,04 and 10.54).

Solution

The net charge on the lysine at pH 10.0 would be +1. The ionic state of the amino acids depends upon the pH of the solution in which they are dissolved. At low pH (pH ~ 1.0) the amino acid is in the cationic form and if placed in an electric field they will migrate toward the cathode. As the pH is increased the carboxylic acid group ionizes. When the pH is equal to the pKa the amino acid exists as a 50:50 mixture of the cationic and zwitter ionic forms. Adding more base results in continued ionization of the carboxylic acid group until the zwitter ionic form is the predominant form of the amino acid in solution. By the addition of more base, the pKa of the amino group is reached and at this point the amino acid exists as a 50:50 mixture of the zwitter ionic form and the anionic form.