Chemistry - 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.

$pI = {{\sum pK_a} \over 2}$

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.

Proteins can be separated according to their isoelectric point in a process known as isoelectric focusing.

At a pH below the pI, proteins carry a net positive charge. Above the pI they carry a net negative charge. This has implications for running electrophoretic gels (see Agarose gel electrophoresis). The pH of an electrophoretic gel is determined by the buffer used for that gel. If the pH of the buffer is above the pI of the protein being run, the protein will migrate to the positive pole (negative charge is attracted to a positive pole). If the pH of the buffer is below the pI of the protein being run, the protein will migrate to the negative pole of the gel (positive charge is attracted to the negative pole). If the protein is run with a buffer pH that is equal to the pI, it will not migrate at all.