The isoelectric point is the pH at which a specified molecule's mean net electric charge is zero. Different protonation states can still carry nonzero electric charges; their contributions cancel in the average. For the two-step glycine speciation model, this gives . Additional ionizable amino acid side chains require their own chemical equilibrium calculations; averaging every pKa is not a general rule. [ref]
Organic Chemistry by John McMurry. §26.2, discussion following Figure 26.2: isoelectric points and the dissociations surrounding the neutral form
Below its isoelectric point, free glycine has positive mean electric charge and an electric field drives it toward the negative electrode; above its isoelectric point, the mean electric charge is negative and the direction reverses. At its isoelectric point the charge-driven contribution vanishes. Diffusion and bulk movement of the chemical solution can still move the sample. Gel electrophoresis applies this charge dependence to separation, although protein size, shape and interactions also affect migration. The pKa values of free glycine do not determine a protein's isoelectric point: its free N-terminus, C-terminus and ionizable amino acid side chains supply a different inventory. [ref]
Articles by others on the same topic
The isoelectric point (pI) is the pH at which a particular molecule, such as a protein or amino acid, carries no net electrical charge. At this point, the positive and negative charges of the molecule are balanced, and it becomes electrically neutral. In the context of proteins, the isoelectric point is significant because it influences the solubility and stability of the protein.