The Isoelectric point reference article from the English Wikipedia on 24-Apr-2004 (provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Isoelectric point

The isoelectric point (pI) is the pH at which a molecule carries no net electrical charge.

For an amino acid with only one amine and one carboxyl group, the pI can be calculated from the pKa's of this molecule.

For amino acids with more than two ionizable groups such as lysine for example, the same formula is used but this time, the two pKa's used are those of the two groups that lose and gain a charge from the neutral form of the amino acid.

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