Question
Explain what a zwitterion is, using glycine as an example. What is the isoelectric point (pI) of glycine? Predict the charge on glycine at pH 2, pH 6, and pH 10. In which direction would glycine migrate during electrophoresis at each pH?
(JEE Main 2022, similar pattern)
Solution — Step by Step
A zwitterion (from German, meaning “hybrid ion”) is a molecule that carries both a positive and negative charge simultaneously, with zero net charge.
Glycine in water exists predominantly as:
The amino group () accepts a proton to become , while the carboxyl group () loses a proton to become . The molecule has both charges but is overall neutral.
The isoelectric point (pI) is the pH at which the amino acid has zero net charge (exists as a pure zwitterion).
For glycine: (carboxyl group) and (amino group).
At pH 2 (strongly acidic, below ): Both groups are protonated: Net charge: +1. Migrates towards cathode (negative electrode).
At pH 6 (near pI): Zwitterion form: Net charge: 0. Does not migrate in electrophoresis.
At pH 10 (strongly basic, above ): Both groups deprotonated: Net charge: -1. Migrates towards anode (positive electrode).
Why This Works
The charge on an amino acid depends on the pH relative to its pKa values. Below , the carboxyl group is protonated (uncharged COOH), and the amino group is protonated (positive NH₃⁺) — net positive charge. Above , the amino group is deprotonated (uncharged NH₂), and the carboxyl is deprotonated (negative COO⁻) — net negative charge.
At the isoelectric point, the positive and negative charges balance exactly. This is why amino acids have minimum solubility and zero electrophoretic mobility at their pI. Proteins are purified using this property — isoelectric focusing separates proteins by moving them in a pH gradient until each reaches its pI and stops migrating.
Alternative Method
A quick visual rule: if pH < pI, the amino acid is positively charged (extra protons available in acidic solution). If pH > pI, it’s negatively charged (protons removed in basic solution). If pH = pI, net charge is zero.
NEET frequently asks: “At isoelectric point, amino acids…” — the answer involves zero net charge, minimum solubility, and no migration in electrophoresis. JEE may give you a dibasic amino acid (like aspartic acid with 3 pKa values) and ask you to calculate pI — use the average of the two pKa values that bracket the zwitterionic form.
Common Mistake
Students often draw amino acids in their un-ionised form () as the default. This form essentially does not exist in aqueous solution. In water, amino acids exist as zwitterions (). This explains their high melting points (250-300°C), water solubility, and solid crystalline nature — all properties of ionic compounds, not covalent organic molecules.