Topic 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)

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 contains an amino group and a carboxylic acid group. Proton transfer can give the zwitterion H₃N⁺CH₂COO^-, with a positive ammonium group and a negative carboxylate group. These groups explain its amphoteric acid-base behaviour and the ionic character of the solid.

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.

Amino acids with additional ionisable side-chain groups can have further acid-base steps. For an unfamiliar structure, identify every acid and base site and use the stated pH or pK values. Do not assume that all amino acids have exactly the same dominant form at one numerical pH.