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Definition of the isoelectric point of a protein

What is the definition of the "isoelectric point" (pI) of a protein?

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The isoelectric point (pI) of a protein is the pH at which a zwitterionic molecule does not migrate in an electric field, because its NET CHARGE IS ZERO.

That's a definition that is easy to find but somewhat more difficult to understand. ...

Solution Summary

The solution contains an "official" definition of pI (isoelectric point), as well as an explanation of a more practical definition.