Principles of Biochemistry Volume 1 - A. Lehninger 1985

Biomolecules
Water
The titration curve allows one to predict the electrical charge carried by a given amino acid

It also follows from The amino acid titration curves that There is a definite relationship between the pH of the solution and the net electrical charge of the amino acid. At pH 6.02, which corresponds to the inflection point of the titration curve where one titration stage transitions into the next, Alanine exists as a dipolar ion (zwitterion) that is fully ionized yet carries no net electrical charge (see Fig. 5-10). At this pH value, the alanine molecule is electrically neutral and does not migrate in an electric field. This characteristic pH value is termed the isoelectric point (pH_I or р_I). The isoelectric point is the arithmetic mean of two рК' values:

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hence, the isoelectric point of alanine is

At any pH above the isoelectric point, alanine possesses a net negative charge and migrates in an electric field toward the positive electrode (anode). At any pH below the isoelectric point, alanine carries a net positive charge, as seen in Fig. 5-10, and migrates toward the negative electrode (cathode). The further the pH of the alanine solution is from its isoelectric point, the greater the net electrical charge carried by the alanine molecules. For instance, at pH 1.0, all alanine molecules exist as ⁺NH₃—CHR—СООН ions with a net positive charge of 1.0. At pH 2.34, which represents a 1:1 mixture of ⁺NH₃—CHR—СООН and ⁺NH₃—CHR—COO^- ions, the average, or net, positive charge is 0.5. Similarly, the sign and magnitude of the net charge can be predicted for any other amino acid at any given pH.

As we shall soon see, this information is of great practical significance, as it implies that a mixture of Amino Acids can be separated by subjecting it to an electric field at a specific pH: under these conditions, different amino acids migrate in different directions and at varying relative speeds.