Amino Acids

Specification Reference Organic chemistry, Amino acids, proteins and DNA 3.3.13.1

Quick Notes

Amino acids contain both an amine (–NH₂) and a carboxylic group (–COOH) functional group.

AQA A-Level Chemistry diagram showing the general alpha amino acid structure H2N–CHR–COOH

They have both acidic and basic properties.

Amino acids exist as zwitterions at their isoelectric point – the NH₂ group exists as NH₃⁺ and the carboxylic acid group exists as COO⁻.
In acidic solutions, amino acids gain a proton (amino acid becomes positively charged).
In alkaline solutions, amino acids lose a proton (amino acid becomes negatively charged).

Full Notes

General Structure of Amino Acids

Amino acids are molecules containing both an amine group (–NH₂) and a carboxylic acid group (–COOH).

Naturally occurring amino acids, known as alpha amino acids, share a common general structure but differ in their side chains (R groups). There are around 20 of these naturally occurring amino acids, and they are essential in biology as the building blocks of proteins.

(alpha) Amino acids have the general formula:
H₂N–CHR–COOH (where R is a side chain).

The amine (–NH₂) group acts as a base (H⁺ ion acceptor) and the carboxylic acid (–COOH) group acts as an acid (H⁺ ion donor).

AQA A-Level Chemistry diagram highlighting acidic carboxyl group and basic amine group in an amino acid

Zwitterions: Isoelectric Point

At a specific pH (isoelectric point), amino acids exist as zwitterions.

AQA A-Level Chemistry diagram showing formation of an amino acid zwitterion with NH3+ and COO− groups at the isoelectric point

The zwitterion contains both a positive (–NH₃⁺) and a negative (–COO⁻) charge but is overall neutral. They form at a specific pH when the amine (NH₂) group accepts a H⁺ ion and the carboxylic acid (COOH) group loses a H⁺ ion.

Amino Acids in Acidic and Alkaline Solutions

Amino acids can exist in different forms, depending on the pH of their conditions.

In acidic conditions (low pH):

AQA A-Level Chemistry diagram showing amino acid in acidic solution where the amine is protonated to NH3+

The amine group (–NH₂) is protonated to (–NH₃⁺).
The molecule carries a positive charge.

In alkaline conditions (high pH):

AQA A-Level Chemistry diagram showing amino acid in alkaline solution where the carboxyl group is deprotonated to COO−

The carboxyl group (–COOH) is deprotonated to (–COO⁻).
The molecule carries a negative charge.

Matt’s exam tip

Remember that the weak carboxylic acid group can lose a H⁺ ion at a specific pH, which might be below 7. The exact pH at which this happens depends on the strength of the acid — stronger acids lose H⁺ ions at lower pH values, forming a COO⁻ group. This explains why different amino acids exist as zwitterions at different pH levels. The same logic applies to the basic amine group gaining a H⁺ ion — the pH at which this occurs depends on the base strength. Different amino acids exist as zwitterions at slightly different pHs for this reason.

AQA A-Level Chemistry diagram comparing different amino acids showing different isoelectric points and zwitterion formation

Summary

Amino acids contain both –NH₂ and –COOH groups and therefore show both acidic and basic behaviour.
At the isoelectric point, amino acids exist as zwitterions with –NH₃⁺ and –COO⁻ groups.
In acidic conditions they are protonated overall (positive charge); in alkaline conditions they are deprotonated overall (negative charge).
Exact pH values depend on side chain (R group) effects on acid/base strength.