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Amino acids and zwitterions

Key idea: An amino acid contains acidic and basic groups. Its zwitterion carries opposite charges with no net charge; adding acid gives a positive ion and adding alkali a negative ion for glycine.

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  • GCE A-Level H2 Chemistry 2027
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An amino acid has a basic amino group and an acidic carboxylic acid group in the same molecule. In solid glycine and near-neutral aqueous glycine, the amino group is protonated and the carboxyl group is deprotonated. The same particle then carries both positive and negative charges. This lesson explains that ion, the zwitterion, and how an amino acid reacts with acids and with alkalis.

Check your understanding 1 Charges that add to zero

For H₃N⁺CH₂COO⁻, work out the overall charge and the number of charged groups. Enter integer counts.

Unit: elementary charges
Unit: groups
Show answer

There are two charged groups: -NH₃⁺ is +1 and –COO⁻ is −1. Their sum is zero. The particle is a zwitterion, not a molecule without charged groups.

Amino acids and zwitterions

The amino acids that make proteins are α-amino acids, RCH(NH₂)COOH: the amino group is attached to the carbon next to the carboxyl carbon. The simplest is glycine, aminoethanoic acid, H₂NCH₂COOH, where R is H.

Proton transfer converts the uncharged form to the zwitterion; in water, surrounding water molecules can help transfer the proton:

H₂NCH₂COOH → H₃N⁺CH₂COO⁻

The product is a zwitterion: an ion with a positive and a negative charge, and no overall charge. Solid amino acids exist as zwitterions, which is why they behave like ionic compounds: they have high melting points, because strong electrostatic attractions hold the lattice together, and glycine is soluble in water, where its charged groups interact with water, but poorly soluble in non-polar solvents. Water solubility varies with the side chain.

Check your understanding 2 Add hydroxide to a zwitterion

Predict what happens to the overall charge of glycine’s zwitterion when it reacts with one hydroxide ion.

Overall charge of the organic product
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The charge becomes −1. H₃N⁺CH₂COO⁻ + OH⁻ → H₂NCH₂COO⁻ + H₂O. The next section shows which group loses its proton.

Reactions with acids and alkalis

An amino acid is amphoteric: it reacts with both acids and bases.

  • With acid, the carboxylate group of the zwitterion accepts a proton: H₃N⁺CH₂COO⁻(aq) + H⁺(aq) → H₃N⁺CH₂COOH(aq)
  • With alkali, the ammonium group of the zwitterion gives up a proton: H₃N⁺CH₂COO⁻(aq) + OH⁻(aq) → H₂NCH₂COO⁻(aq) + H₂O(l)

So the form of an amino acid in solution depends on pH: a positive ion in strongly acidic solution, the zwitterion in near-neutral solution, and a negative ion in strongly alkaline solution.

Read in order of increasing pH: cation → zwitterion → anion.

  1. Strongly acidic

    H₃N⁺CH₂COOH

    Cation; net charge +1.

    Ammonium and carboxylic acid groups.

    Lose H⁺ from COOH to reach the zwitterion.

  2. Near neutral

    H₃N⁺CH₂COO⁻

    Zwitterion; net charge 0.

    Ammonium and carboxylate groups; both charges are present.

    Lose H⁺ from NH₃⁺ to reach the anion.

  3. Strongly alkaline

    H₂NCH₂COO⁻

    Anion; net charge −1.

    Amino and carboxylate groups.

    Adding acid reverses these proton losses.

Predominant aqueous forms of glycine as pH increases. Each proton loss lowers the net charge by one; the carbon skeleton stays the same.

Check your understanding 3

A learner says: “A zwitterion has no overall charge, so it cannot react with an acid or an alkali.” Explain the error, and say which group reacts with each reagent.

Show answer

No overall charge does not mean no charged groups. The zwitterion still has a -COO⁻ group, which accepts a proton from an acid, and an -NH₃⁺ group, which gives a proton to hydroxide. The products are the positive ion and the negative ion respectively.

Guided practice 1

Valine and hydroxide ions

About 5 min

Problem

Valine, in its zwitterion form (CH₃)₂CHCH(NH₃⁺)COO⁻, reacts with aqueous sodium hydroxide. Write the ionic equation.

Write the ionic equation

Hints

Hint 1

which proton

In the zwitterion, look for a proton on a charged group; the -COO⁻ group has already lost its acidic proton.
Hint 2

the other product

The proton and the hydroxide ion form water.
Show solution step by step

  1. Remove the proton

    Method

    -NH₃⁺ becomes -NH₂.

    Reason

    Hydroxide takes the proton; the carboxylate group is unchanged.

    Working

    Answer

    (CH₃)₂CHCH(NH₃⁺)COO⁻ → (CH₃)₂CHCH(NH₂)COO⁻

  2. Balance

    Method

    Add hydroxide and water.

    Reason

    The charge is −1 on both sides.

    Working

    Answer

    (CH₃)₂CHCH(NH₃⁺)COO⁻ + OH⁻ → (CH₃)₂CHCH(NH₂)COO⁻ + H₂O

Check your understanding 4 Check the valine product

For the reaction of valine’s zwitterion with hydroxide, choose the group that loses H⁺ and the organic ion formed.

Group that loses a proton
Organic ion formed
Show answer

The ammonium group loses H⁺, giving (CH₃)₂CHCH(NH₂)COO⁻. The carboxylate and alkyl side chain stay unchanged.

Check your understanding 5 Glycine at three pH values

Give the main form of glycine, and its overall charge, in strongly acidic solution, in near-neutral solution and in strongly alkaline solution.

Show answer

Strongly acidic: H₃N⁺CH₂COOH, charge +1. Near-neutral: H₃N⁺CH₂COO⁻, charge 0. Strongly alkaline: H₂NCH₂COO⁻, charge −1. Each step up in pH removes one proton.

Check your understanding 6 Alanine

Alanine is H₃N⁺CH(CH₃)COO⁻ as a zwitterion. Write its ionic equations with acid and with alkali. Does the methyl side chain react?

Show answer

Acid: H₃N⁺CH(CH₃)COO⁻ + H⁺ → H₃N⁺CH(CH₃)COOH

Alkali: H₃N⁺CH(CH₃)COO⁻ + OH⁻ → H₂NCH(CH₃)COO⁻ + H₂O

The methyl group has no acidic or basic site, so it does not react. Amino acids whose side chains contain another -COOH or -NH₂ group can gain or lose extra protons.

Check your understanding 7 From acid to alkali

A solution contains 5.00 mmol of glycine as H₃N⁺CH₂COOH and no excess acid. It is converted completely to H₂NCH₂COO⁻ by adding sodium hydroxide. Find the amount of hydroxide needed, in mmol.

Unit: mmol
Show answer

There are two proton losses per glycine cation, so n(OH⁻) = 2 × 5.00 = 10.0 mmol.

Check your understanding 8 Balance the whole change

Write the ionic equation for converting H₃N⁺CH₂COOH into H₂NCH₂COO⁻ with hydroxide. Show how both atoms and charge balance.

Show answer

H₃N⁺CH₂COOH + 2OH⁻ → H₂NCH₂COO⁻ + 2H₂O. Each removed proton makes one water molecule. The overall charge is + 1-2 = -1 on the left and −1 on the right. The two carbons stay in the organic product. Starting with the zwitterion would need only one hydroxide ion per particle; starting with the cation needs two.

Common mistakes

  • Writing H₂NCH₂COOH as the form of glycine in the solid or in neutral solution. It exists as the zwitterion.
  • Thinking a zwitterion is unreactive because its overall charge is zero.
  • Removing a proton from the -COO⁻ group of a zwitterion. It has already lost its proton.
  • Leaving spectator ions such as Na⁺ and Cl⁻ in an ionic equation.
  • Explaining the high melting point of an amino acid with hydrogen bonding alone, instead of attractions between ions.

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