Definition of Isoelectric Point

The isoelectric point is a term that is defined as such a pH at which the molecule has no net charge present on it. This means that the molecule is electrically neutral. The charge on the molecule is being influenced by the acidity and basicity of the surrounding environment and due to the gain and loss of protons (H+), the molecule can become more positive or more negative respectively. It is represented by pI, IEP, or pH(I). The isoelectric point is capable of changing the solubility of molecules at a given pH.


Take an example of amino acids which have both acidic and basic functional groups and they impart an overall charge to the protein formed. If the pH is below pI, the protein has a net positive charge whereas if the pH is above the pI, the protein has a net negative charge.


The isoelectric point of an amino acid having one amino and one carbonyl group can be determined by the following formula:

Isoelectric Point

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