Lesson 30 of 34

An amino acid can donate and accept protons

Its dominant charged form depends on pH; zero net charge does not mean no internal charges.

A-Level 9476 (2026-2027)

I can describe amino-acid acid-base properties.Syllabus
Syllabus 9476, 11.9(i). Describe amino-acid acid-base properties.

An amino acid can donate and accept protons

Its dominant charged form depends on pH; zero net charge does not mean no internal charges.

Aminoethanoic acid has an amino group and a carboxylic acid group. A proton can move from one to the other, giving the zwitterion H₃N⁺CH₂COO^-. It has a positive ammonium group and a negative carboxylate group. These groups explain why it reacts with both acids and bases, and why the solid behaves like an ionic substance.

Follow aminoethanoic acid as pH changes
Medium	Representative dominant form	Response of the zwitterion
Sufficiently acidic	H₃N⁺CH₂COOH; net +1.	COO^- accepts H⁺ to form COOH.
Intermediate pH near its isoelectric region	H₃N⁺CH₂COO^-; net 0.	Both internal charges are present.
Sufficiently alkaline	H₂NCH₂COO^-; net -1.	NH₃⁺ donates H⁺ to OH^-, giving NH₂ and H₂O.

Some amino acids have extra acidic or basic groups in their side chains, which add further steps. For an unfamiliar structure, find every acid and base site and use the pH or pK values given. Do not assume all amino acids have the same main form at one pH value.

Check your understanding

Alanine is CH₃CH(NH₂)COOH. What is its main form at pH 1, pH 6 and pH 13?

pH 1: CH₃CH(NH₂)COO^-; pH 6: CH3CH(NH3+)COO-; pH 13: CH3CH(NH3+)COOH
pH 1: CH3CH(NH3+)COOH; pH 6: CH3CH(NH3+)COO-; pH 13: CH₃CH(NH₂)COO^-
pH 1: CH3CH(NH3+)COOH; pH 6: CH₃CH(NH₂)COOH; pH 13: CH₃CH(NH₂)COO^-

Show the answer

pH 1: CH3CH(NH3+)COOH; pH 6: CH3CH(NH3+)COO-; pH 13: CH₃CH(NH₂)COO^-

In strong acid both groups are protonated (net +1). Near its isoelectric point it is the zwitterion (net 0). In strong alkali both groups have lost H⁺ (net -1). The zwitterion reacts with both H⁺ and OH^-, so it can act as a buffer.