isoelectric point
Summary
The isoelectric point (pI) is the pH at which a molecule, such as an amino acid or protein, has no net charge. Below the pI it carries a net positive charge, and above it a net negative charge. For an amino acid with a nonionizable side chain, pI is the average of pKa1 and pKa2.
Detail
Meaning: At its pI, an amino acid is a zwitterion, with a positive ammonium group and a negative carboxylate, so the charges cancel. It does not migrate in an electric field. If pH < pI, the molecule is more protonated and net positive. If pH > pI, it is net negative. Acidic amino acids (Asp, Glu) have low pI values. Basic ones (Lys, Arg, His) have high pI values.
Equation: pI = (pKa1 + pKa2) / 2, where pKa1 is the α-carboxyl group and pKa2 is the α-amino group (both unitless). For an acidic amino acid, average the two lowest pKa values. For a basic one, average the two highest. In each case you average the two pKa values on either side of the neutral form.
On the MCAT: Passages give a titration curve or a table of pKa values and ask for the pI, or the charge at a stated pH. In isoelectric focusing, proteins move through a pH gradient until pH = pI, then stop. In native gel electrophoresis or ion-exchange chromatography, charge at the buffer pH sets the direction of movement or binding. Proteins are usually least soluble near their pI.
Common confusion: Students assume the pI is 7 or equals one pKa. Neither is true in general. Use the average of the two pKa values that flank the neutral species. Net charge zero also does not mean no charged groups are present.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Stryer)
- Organic Chemistry (Wade)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.