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.
| 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?
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.