Amino Acids and Zwitterions Revision Notes | A Level AQA Chemistry

1. Why Amino Acids Are Chemically Unusual

Most organic molecules behave as either an acid or a base — amino acids do both. That dual personality is what makes them so chemically interesting, and it is exactly what AQA tests.

Picture the simplest amino acid, glycine. It has a carboxyl group ($-COOH$) at one end and an amino group ($-NH_2$) at the other, both attached to the same carbon — the alpha carbon. The R group on glycine is just a hydrogen atom, but in other amino acids it can be anything from a methyl group to a complex aromatic ring.

The key insight is this: the $-COOH$ group is acidic (it can donate a proton), and the $-NH_2$ group is basic (it can accept a proton). When these two groups are in the same molecule, an internal proton transfer becomes possible — and that is where the zwitterion comes from.

2. The Zwitterion: An Internal Proton Transfer

In aqueous solution near neutral pH, the carboxyl group donates a proton to the amino group within the same molecule. The result is a species that carries a positive charge on the nitrogen ($-NH_3^+$) and a negative charge on the oxygen ($-COO^-$), but has zero net charge overall. This is the zwitterion.

The transformation can be written as a simple equilibrium: $H_2N-CH(R)-COOH \rightleftharpoons ^+H_3N-CH(R)-COO^-$. The uncharged form on the left is sometimes called the "free acid" form, but at physiological and near-neutral pH, the zwitterion on the right dominates overwhelmingly in solution.

This matters for exam questions because AQA specifically asks you to draw the zwitterion structure. The charged groups must be shown correctly: $-NH_3^+$ (not $-NH_2$) and $-COO^-$ (not $-COOH$).

Worked example: Drawing the Zwitterion of Glycine

Draw the zwitterionic form of glycine ($H_2N-CH_2-COOH$) and explain the proton transfer that produces it.

1. $-NH_2$ (amino group, basic) and $-COOH$ (carboxyl group, acidic) — These are the two groups involved in the internal proton transfer. Both are attached to the alpha carbon.
2. $-COOH \rightarrow -COO^- + H^+$ and $-NH_2 + H^+ \rightarrow -NH_3^+$ — The carboxyl group loses a proton; the amino group gains it. This is an acid-base reaction within the same molecule.
3. $^+H_3N-CH_2-COO^-$ — The nitrogen now carries a positive charge and the oxygen carries a negative charge. Net charge = +1 + (−1) = 0.
4. Charge on $-NH_3^+$ = $+1$; charge on $-COO^-$ = $-1$; total = $0$ — A zwitterion always has zero net charge. If your drawn structure has a net charge, it is not a zwitterion.

Answer: The zwitterion of glycine is $^+H_3N-CH_2-COO^-$, with a positive charge on nitrogen and a negative charge on oxygen, and zero net charge overall.

3. Amphoteric Behaviour: Acids, Bases, and pH

Because amino acids carry both an acidic group and a basic group, they can respond to changes in pH in two directions. Add acid (lower the pH) and the $-COO^-$ group picks up a proton. Add alkali (raise the pH) and the $-NH_3^+$ group loses a proton. This is what chemists mean when they say amino acids are amphoteric.

Think of the zwitterion as the neutral starting point. Moving to lower pH pushes the equilibrium towards the fully protonated cation; moving to higher pH pushes it towards the fully deprotonated anion. AQA requires you to draw all three forms and to know which conditions produce each one.

Notice the pattern: as pH increases, protons are progressively removed. In acid, both groups are protonated. At the isoelectric point, the carboxyl group has lost its proton but the amino group retains one. In alkali, the amino group also loses its proton.

4. Drawing the Ionic Forms: Acid and Alkaline Conditions

Now that the principle is clear, the exam skill is to draw these structures accurately and quickly. The mechanism behind each transformation is straightforward proton transfer — the kind of acid-base chemistry you have already met with buffers and weak acids.

Worked example: Deriving the Ionic Form in Acid Solution

Starting from the zwitterion $^+H_3N-CH(R)-COO^-$, show what happens when excess hydrochloric acid is added, and draw the resulting ionic form.

1. The $-COO^-$ group can accept a proton (it is the conjugate base of $-COOH$). — In the zwitterion, the $-NH_3^+$ group is already fully protonated. The only group that can accept another proton is $-COO^-$.
2. $-COO^- + H^+ \rightarrow -COOH$ — The excess $H^+$ from HCl protonates the carboxylate group, converting it back to a carboxyl group.
3. $^+H_3N-CH(R)-COOH$ — The amino group remains as $-NH_3^+$ (it was already protonated). The carboxyl group is now $-COOH$. Net charge = +1.
4. Charge on $-NH_3^+$ = $+1$; charge on $-COOH$ = $0$; total = $+1$ — The cationic form carries a net positive charge of +1. This is the species that migrates towards the cathode in electrophoresis.

Answer: In acid solution, the amino acid exists as the cation $^+H_3N-CH(R)-COOH$, with a net charge of +1.

Worked example: Deriving the Ionic Form in Alkaline Solution

Starting from the zwitterion $^+H_3N-CH(R)-COO^-$, show what happens when excess sodium hydroxide is added, and draw the resulting ionic form.

1. The $-NH_3^+$ group can donate a proton (it is a weak acid). — In the zwitterion, the $-COO^-$ group is already deprotonated. The only group that can donate a proton to $OH^-$ is $-NH_3^+$.
2. $-NH_3^+ + OH^- \rightarrow -NH_2 + H_2O$ — The hydroxide ion acts as a base, accepting the proton from the ammonium group and forming water.
3. $H_2N-CH(R)-COO^-$ — The amino group is now $-NH_2$ (uncharged). The carboxylate group remains as $-COO^-$. Net charge = −1.
4. Charge on $-NH_2$ = $0$; charge on $-COO^-$ = $-1$; total = $-1$ — The anionic form carries a net negative charge of −1. This species migrates towards the anode in electrophoresis.

Answer: In alkaline solution, the amino acid exists as the anion $H_2N-CH(R)-COO^-$, with a net charge of −1.

5. Linking the Three Forms: A Complete Picture

Earlier, the zwitterion was introduced as the neutral-pH form. Now all three ionic forms can be connected into one coherent equilibrium scheme, which is exactly the kind of multi-step reasoning AQA rewards in longer questions.

The full equilibrium can be written as: $^+H_3N-CH(R)-COOH \rightleftharpoons ^+H_3N-CH(R)-COO^- \rightleftharpoons H_2N-CH(R)-COO^-$. Moving left (adding $H^+$, lowering pH) protonates the carboxylate. Moving right (adding $OH^-$, raising pH) deprotonates the ammonium group. The zwitterion sits in the middle.

6. Exam Technique and Mark-Winning Strategies

Questions on amino acids and zwitterions appear in the organic chemistry section of Paper 2. They typically ask you to draw structures, explain proton transfers, or compare the forms present at different pH values. The marks are almost always for precision of structure and accuracy of charge notation.

When a question says "draw the structure of the amino acid in acid solution", the examiner wants to see $^+H_3N-CH(R)-COOH$ with the charges clearly marked. A structure without charges, or with charges on the wrong atoms, will not score. Similarly, "explain why amino acids are amphoteric" requires you to state that they can act as both proton donors and proton acceptors — quoting the $-COOH$ and $-NH_2$ groups as the source of each behaviour.

For "compare" questions, always address both similarities and differences. For example: both the acid form and the zwitterion contain $-NH_3^+$, but they differ in that the acid form has $-COOH$ whereas the zwitterion has $-COO^-$.

Across this lesson, every ionic form of an amino acid has been derived from first principles using proton transfer logic — the same reasoning that underpins buffer chemistry and acid-base equilibria elsewhere in the course. The three forms (cation in acid, zwitterion at neutral pH, anion in alkali) are directly testable, and drawing them accurately with correct charge notation is the core exam skill for this topic.