A-Level Chemistry (AQA) – Introduction to amino acids

30.1 - Introduction to Amino Acids

1The structure of amino acids
2Naming of amino acids
3Zwitterions

3 min

Amino acids contain amine and acid groups

Amino acids consist of both amine and acid functional groups:

An amino group (-NH₂).
A carboxyl group (-COOH). These groups are attached to the same carbon atom, the α-carbon, in α-amino acids.

The general structure of an α-amino acid is illustrated as:

Diagram showing the general structure of an alpha-amino acid with an amine group, a carboxyl group, and a side chain.

Here, R denotes a variable organic substituent i.e. an alkyl or aryl group, which is known as the side chain.

Naming amino acids

Amino acids are named using two methods:

Common names, for example, glycine and alanine.
Systematic IUPAC names, which are based on the carbon backbone.

The steps to name an amino acid systematically are:

Identify the longest carbon chain that includes the carboxyl carbon.
Start numbering the carbons from the carboxyl carbon, assigning it the number 1.
Specify the position and identity of the amine substituent.

Diagram showing the structure of 2-aminoethanoic acid with labelled carbon atoms.

For example, the systematic name of the amino acid glycine (shown above) is 2-aminoethanoic acid, indicating it has two carbons, with NH₂ attached to carbon 2.

Amino acids as zwitterions

Amino acids can form zwitterions - molecules that contain both positive and negative charges within the same structure.

Specifically, a zwitterion forms through:

Amine group protonation - The NH₂ group gains a proton to become positively charged NH₃⁺.
Carboxyl group deprotonation - The COOH group loses a proton to become negatively charged COO^-.

These opposite charges balance out, giving an overall neutral charge. This process involves an internal transfer of a proton (H⁺) from the carboxyl group to the amine group.

Whether the zwitterion forms depends on the pH:

Diagram showing the formation of amino acid zwitterions at different pH levels, including low pH, neutral pH, and high pH.

Lower pH (acidic) - The COOH group gets deprotonated.
Neutral pH - The zwitterion itself forms as both the carboxyl and amino groups ionise.
Higher pH (alkaline) - The NH₃⁺ group loses its proton.

The existence of these pH-dependent zwitterions demonstrates that amino acids are amphoteric. They can act as both acids (via the carboxyl group) and bases (via the amine group).

The exact pH where the zwitterion forms depends on the amino acid's R-group. Different R-groups alter the relative acidity and basicity of the functional groups.