Amino Acids and Zwitterions Revision Notes | A Level AQA Chemistry
A zwitterion forms when the –COOH group of an amino acid donates a proton to its –NH₂ group, giving zero net charge. Free AQA notes and 14 questions answered.
Updated 5 Sept 2026 · Revise Right Team
Amino Acids and Zwitterions is part of Amino acids, proteins and DNA (A-level only) in A Level AQA Chemistry.
5 min
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What this lesson covers
A zwitterion forms when the –COOH group of an amino acid donates a proton to its –NH₂ group, giving zero net charge. Free AQA notes and 14 questions answered.
Draw the structures of amino acids in their uncharged, zwitterionic, and ionic forms
Explain why amino acids behave as both acids and bases, using proton transfer reasoning
Describe the structure of a zwitterion and the conditions under which it forms
Draw the ionic forms of amino acids in acid solution and in alkaline solution
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Use this lesson to stay inside Amino acids, proteins and DNA (A-level only) while connecting the detail back to the wider Organic chemistry topic.
Zwitterion: A molecule that carries both a positive charge and a negative charge simultaneously, with an overall net charge of zero. Formed by amino acids at or near their isoelectric point.
Amphoteric: A substance that can act as both a Brønsted-Lowry acid (proton donor) and a Brønsted-Lowry base (proton acceptor) depending on the pH of the surrounding solution.
Isoelectric point: The pH at which an amino acid exists predominantly as a zwitterion, carrying equal positive and negative charges and therefore no net charge overall.
Alpha-amino acid: An organic compound containing both an amino group ($-NH_2$) and a carboxyl group ($-COOH$) attached to the same carbon atom, the alpha carbon, along with a variable side chain (R group).
Revision notes
Read the core explanations from Amino Acids and Zwitterions before testing yourself from memory.
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.
$-NH_2$ (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.
$-COOH \rightarrow -COO^- + H^+$ 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.
$^+H_3N-CH_2-COO^-$ — The nitrogen now carries a positive charge and the oxygen carries a negative charge. Net charge = +1 + (−1) = 0.
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.
Condition
Form present
Structure
Net charge
Acid solution (low pH)
Cationic form
$^+H_3N-CH(R)-COOH$
+1
Near isoelectric point (neutral pH)
Zwitterion
$^+H_3N-CH(R)-COO^-$
0
Alkaline solution (high pH)
Anionic form
$H_2N-CH(R)-COO^-$
−1
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.
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 $-COO^-$.
$-COO^- + H^+ \rightarrow -COOH$ — The excess $H^+$ from HCl protonates the carboxylate group, converting it back to a carboxyl group.
$^+H_3N-CH(R)-COOH$ — The amino group remains as $-NH_3^+$ (it was already protonated). The carboxyl group is now $-COOH$. Net charge = +1.
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.
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 $-NH_3^+$.
$-NH_3^+ + OH^- \rightarrow -NH_2 + H_2O$ — The hydroxide ion acts as a base, accepting the proton from the ammonium group and forming water.
$H_2N-CH(R)-COO^-$ — The amino group is now $-NH_2$ (uncharged). The carboxylate group remains as $-COO^-$. Net charge = −1.
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.
Sources and content standards
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-09-05.
A zwitterion is a dipolar ion formed when the –COOH group of an amino acid donates a proton to its –NH₂ group, giving –COO⁻ and –NH₃⁺ in the same molecule with zero net charge. Glycine's zwitterion is ⁺H₃N–CH₂–COO⁻, the main form near neutral pH.
What is the zwitterion of glycine?
The zwitterion of glycine is ⁺H₃N–CH₂–COO⁻. The carboxyl group has donated a proton to the amino group in the same molecule, so nitrogen carries a positive charge and oxygen a negative charge, giving zero net charge overall.
What form does an amino acid take in acid solution?
In acid solution, the –COO⁻ group of the zwitterion accepts a proton, giving the cation ⁺H₃N–CH(R)–COOH with a net charge of +1. The –NH₃⁺ group is already protonated, so only the carboxylate group changes.
What form does an amino acid take in alkaline solution?
In alkaline solution, OH⁻ removes a proton from the –NH₃⁺ group of the zwitterion, forming water and the anion H₂N–CH(R)–COO⁻, which has a net charge of −1. The –COO⁻ group is already deprotonated.
How does pH change the form of an amino acid?
As pH increases, protons are progressively removed. In acid both groups are protonated, giving ⁺H₃N–CH(R)–COOH; near neutral pH the zwitterion forms; in alkali the amino group also loses its proton, giving H₂N–CH(R)–COO⁻.
Why are amino acids amphoteric?
Amino acids are amphoteric because they contain both an acidic –COOH group, which can donate a proton, and a basic –NH₂ group, which can accept a proton. They exist as a cation at low pH, a zwitterion near neutral pH and an anion at high pH.
What is the general structure of an amino acid?
An alpha-amino acid has the general formula H₂N–CH(R)–COOH, with an amino group and a carboxyl group both bonded to the same carbon, the alpha carbon. R is the side chain that distinguishes one amino acid from another; in glycine, R is a hydrogen atom.
What is the net charge of a zwitterion?
A zwitterion always has zero net charge. The –NH₃⁺ group carries a charge of +1 and the –COO⁻ group carries −1, so the two charges cancel. A drawn structure with an overall charge is not a zwitterion.
Which way do amino acids move in electrophoresis?
In acid solution an amino acid exists as a cation with a net charge of +1, which migrates towards the cathode in electrophoresis. In alkaline solution it exists as an anion with a net charge of −1, which migrates towards the anode.
What form does an amino acid take at its isoelectric point?
Near the isoelectric point an amino acid exists as the zwitterion, with zero net charge. The carboxyl group has lost its proton to become –COO⁻, but the amino group retains one as –NH₃⁺.
What is the alpha carbon in an amino acid?
The alpha carbon is the carbon atom to which both the amino group and the carboxyl group of an alpha-amino acid are bonded. It also carries the R group, the side chain that distinguishes one amino acid from another; in glycine this is just a hydrogen atom.
How does the acid form of an amino acid differ from its zwitterion?
Both the acid form and the zwitterion contain –NH₃⁺, but they differ at the other end. The acid form has an uncharged –COOH group, giving a net charge of +1, whereas the zwitterion has –COO⁻, giving zero net charge.
Why do amino acids form zwitterions?
The –COOH group is acidic and can donate a proton, while the –NH₂ group is basic and can accept one. Because both groups are in the same molecule, an internal proton transfer happens in aqueous solution near neutral pH, producing the zwitterion.
What is the equation for an amino acid reacting with hydroxide ions?
The reaction is –NH₃⁺ + OH⁻ → –NH₂ + H₂O. The hydroxide ion acts as a base, accepting a proton from the ammonium group of the zwitterion and forming water, which leaves the anion H₂N–CH(R)–COO⁻ with a net charge of −1.