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Tertiary structure

The process of protein folding results in a compact structure in which secondary structure elements are packed against each other in a stable configuration, often called a `fold'. The tendency for the burial of hydrophobic side-chains in the interior, or core, of proteins has been observed in nearly all structures solved to date, and is believed to be the driving force of tertiary structure formation[Dill, 1990]. Other interactions help to stabilise the fold: hydrogen bonds, van der Waals forces, and oppositely charged amino acid side-chains forming salt-bridges. Many elegant structures have evolved, including curved, barrel-like $\beta$-sheets, parallel bundles of helices, and propellor-like structures. Folds are also referred to as `topologies' since they can be thought of as sets of connected secondary structure elements.