R-groups and primary structureSpec B1.2.6, B1.2.7
In short
The R-groups of the 20 amino acids are chemically diverse: some are hydrophobic and others hydrophilic, and hydrophilic R-groups are polar or charged, acidic or basic. R-groups determine the properties of a polypeptide. The primary structure, the sequence of amino acids, determines how the chain folds, so proteins have precise, predictable and repeatable three-dimensional shapes.
Chemical diversity of R-groups
All amino acids share the same backbone, so the differences between them are entirely in their R-groups. R-groups vary in size, shape and chemistry. This variety is the basis for the immense diversity in protein form and function.
| Type | Property | Behaviour in a protein |
|---|---|---|
| Hydrophobic (non-polar) | No charge, no hydrogen bonding with water | Tend to be buried away from water or in membranes |
| Hydrophilic: polar | Uncharged but polar, can form hydrogen bonds | Tend to be on the surface, interacting with water |
| Hydrophilic: charged, acidic | Carboxyl group loses H⁺ and becomes negative | Can form ionic bonds |
| Hydrophilic: charged, basic | Amine group gains H⁺ and becomes positive | Can form ionic bonds |
R-groups determine the properties of the assembled polypeptide: how it folds, whether it is soluble, which molecules it binds and how it catalyses reactions.
Primary structure determines conformation
- Primary structure
- The sequence of amino acids in a polypeptide.
- Conformation
- The three-dimensional shape of a protein.
The sequence of amino acids and the precise position of each amino acid in the chain determine how the polypeptide folds, and so its three-dimensional shape. A polypeptide with a given sequence folds into the same shape every time. Proteins therefore have precise, predictable and repeatable structures, despite their complexity.
Changing even one amino acid can change the conformation. In sickle cell anaemia a single substitution in the beta chain of haemoglobin changes how the molecules interact, distorting red blood cells.
Linking question: what is the relationship between genome and proteome? The base sequence of a gene codes for the amino acid sequence, which determines conformation and so function.
Written and checked against the IB Biology HL specification · Updated October 2026