Amino Acids and Polypeptides

Condensation Polymerisation Step-by-Step

When amino acids join together, they do so via condensation polymerisation. This is a reaction where monomers join together and eliminate a small molecule (water) for every bond formed.

To describe how amino acids form a polymer chain, follow these sequential steps:

1. First, an $-OH$ group is lost from the carboxyl group of one amino acid.
2. Then, an $-H$ atom is lost from the amine group of the adjacent amino acid.
3. These two parts combine to eliminate a molecule of water ($H_2O$).
4. Finally, a new covalent bond forms directly between the carbon atom of the carboxyl group and the nitrogen atom of the amine group.

This new $C-N$ bond is called a peptide link (or amide link), and it has the structure $-CONH-$. Because proteins contain this same amide linkage found in synthetic polymers like Nylon, they are often described as natural polyamides.

Representing the Polymerisation Reaction

You can represent the polymerisation of an amino acid using a chemical equation. For the polymerisation of glycine, the equation is:

$$n\ H_2NCH_2COOH \rightarrow (-HNCH_2CO-)_n + n\ H_2O$$

When drawing the repeat unit of this polymer, you must include trailing continuation bonds that extend through the brackets, with the subscript '$n$' placed outside the bottom right.

Polypeptides and Proteins

As the condensation polymerisation continues, the product grows in size and complexity:

• When two amino acids join, they form a dipeptide.
• As many amino acid monomers join, they form a polypeptide. A polypeptide is a long, continuous, and strictly unbranched chain.
• Finally, one or more polypeptide chains fold together into a specific 3D shape to form a functioning protein.

The specific order (sequence) of the different amino acids in the polypeptide chain is what determines the protein’s unique 3D structure and biological function.