Zwitterions and amino acid charge at different pH
A zwitterion is an amino acid that carries one positive and one negative charge, so its net charge is zero.
Remember
The amino group (NH2) becomes positively charged NH3+ and the carboxyl group (COOH) becomes negatively charged carboxylate (COO-), so the charges cancel when the side chain, the variable R group, is non-ionisable (it carries no charge of its own). The net charge of a free amino acid, one not joined into a peptide chain, changes with pH and can be positive, zero or negative.
Every amino acid has a central carbon atom bonded to an amino group (NH2), a carboxyl group (COOH), a hydrogen atom and a side chain, also called the R group. In water the amino group acts as a base: it accepts a proton, which is a hydrogen ion (H+), so NH2 becomes the positively charged NH3+. The carboxyl group acts as an acid: it donates its proton, so COOH becomes the negatively charged carboxylate, COO-.
When one positive and one negative charge sit on the same molecule, its net charge is zero. This is a zwitterion, from the German word for hybrid. A side chain that is non-ionisable adds no charge, so the net charge stays at zero. Zero does not mean uncharged, though: a zwitterion is an internal salt, with two charged groups. Near physiological pH, a free amino acid whose side chain is non-ionisable is mainly a zwitterion.
The charge state also depends on pH. In strongly acidic conditions the carboxylate gains a proton and becomes the neutral COOH, while the amino group stays as NH3+, so an amino acid whose side chain is non-ionisable is mainly a cation (a positive ion). At high pH the NH3+ group loses its proton, so an amino acid whose side chain is non-ionisable is mainly an anion (a negative ion).
This matters in electrophoresis, a technique that separates charged molecules by their movement between two electrodes. A cation moves towards the negative electrode and an anion towards the positive electrode, so the same amino acid can move either way depending on the pH of the solution. The isoelectric point is the pH at which the net charge is zero. There the molecule is mainly a zwitterion and shows little or no net movement.
Key facts
- Zwitterion
- one positive and one negative charge on the same molecule, with a net charge of zero when the side chain is non-ionisable.
- Amino group
- a base that accepts a proton and becomes positively charged (NH3+).
- Carboxyl group
- an acid that donates a proton and becomes negatively charged (COO-).
- Strongly acidic solution
- with a non-ionisable side chain, the carboxylate gains a proton and the amino group stays as NH3+, giving mainly a cation with a net positive charge.
- High pH
- with a non-ionisable side chain, the NH3+ group loses its proton, giving mainly an anion with a net negative charge.
- Isoelectric point
- the pH at which the net charge is zero, where little or no net movement occurs in an electric field.
- Electric field
- a cation moves towards the negative electrode and an anion towards the positive electrode.
Exam traps
- A zwitterion is not an amino acid with uncharged amino and carboxyl groups: that describes the un-ionised structure, with NH2 and COOH, but in water at physiological pH both groups are charged.
- In acid solution the group that gains a proton is the carboxylate, not the amino group, because the amino group already carries a proton as NH3+ and cannot accept an extra proton.
- Raising the pH does not add protons to an amino acid and make it positive: it removes protons, so the amino acid becomes more negative and, at high pH, moves towards the positive electrode.
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