Frequently Asked Questions
What is the isoelectric point?
The isoelectric point, or pI, is the pH at which a molecule carries no net electrical charge, so it does not move in an electric field.
How does a side chain change the pI?
An ionizable side chain shifts which pair of pK<sub>a</sub> values defines the neutral form. An acidic side chain averages with the α-carboxyl pK<sub>a</sub>, while a basic side chain averages with the α-amino pK<sub>a</sub>.
How do I find the pI of a simple amino acid like glycine?
For an amino acid with no ionizable side chain, average its two pK<sub>a</sub> values: pI = (pK<sub>a1</sub> + pK<sub>a2</sub>)/2. Glycine, with pK<sub>a1</sub> = 2.34 and pK<sub>a2</sub> = 9.60, has pI = (2.34 + 9.60)/2 = 5.97.
What is the net charge above and below the pI?
Below the pI, at a more acidic pH, the molecule carries a net positive charge; above the pI, at a more basic pH, it carries a net negative charge. Exactly at the pI the net charge is zero, which is why the molecule does not migrate in an electric field. For this reason the pI is sometimes described as the pH of zero net charge.
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