Various acidic and basic functional groups on amino acid side chains and termini of proteins and peptides may be charged. The presence of a charge is dependent on the pKa values of the groups and the pH of the solution. The isoelectric point (pI) of a peptide is defined as the pH at which the peptide has a net charge of zero.
At pH values below the pI, peptides carry a net positive charge;
Above the pI, they carry a net negative charge
pI-values of peptides can be utilized as a basis for separation in solution. If a voltage is applied to a complex peptide mixture in a pH gradient, the peptide will migrate to the pH at which they are neutrally charged. The pI of a peptide maybe accurately calculated from its sequence, and provides information that complements mass spectrometric methods for protein and peptide identification.
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