zwitterion
Summary
A zwitterion is a molecule carrying both a positive and a negative charge, so its net charge is zero. An amino acid near neutral pH is the standard case (−NH₃⁺ and −COO⁻). Testable point: the zwitterion predominates at the isoelectric point (pI), where the amino acid has no net charge.
Detail
Meaning: A zwitterion is a dipolar ion: one molecule with a positive and a negative formal charge and zero net charge. In water, an amino acid exists mainly in this form, not as neutral −NH₂ and −COOH. The α-carboxyl group (pKa about 2) loses its proton to become −COO⁻, and the α-amino group (pKa about 9) stays protonated as −NH₃⁺. In strong acid the net charge is +1, and in strong base it is −1.
Equation: pI = (pKa1 + pKa2)/2, where pKa1 and pKa2 are the two pKa values on either side of the net-neutral form (both unitless). For an amino acid with an ionizable side chain, use the two pKa values that flank the neutral species. Acidic side chains: average the two acidic pKa values. Basic side chains: average the two basic pKa values.
On the MCAT: Passages show a titration curve for glycine or another amino acid. You find the pI at the midpoint between the two buffering plateaus, or give the net charge at a stated pH. A group is mostly protonated when pH < its pKa and mostly deprotonated when pH > its pKa. In electrophoresis or isoelectric focusing, a molecule at its pI does not migrate in the electric field.
Common confusion: Zero net charge does not mean no charges. Also, the pI is not always 7. It depends on the side chain and is above 7 for basic amino acids and below 7 for acidic ones.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Stryer)
- Organic Chemistry (Klein)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.