Definition
According to the most common definition, a turn is a structural motif where the Cα atoms of two residues separated by few (usually 1 to 5) peptide bonds are in close approach (< 7 Å), while the corresponding residues do not form a regular secondary structure element such as an alpha helix or beta sheet. Contrary to helices, the backbone dihedral angles are not (roughly) constant for all the residues in the turn.
Although the close approach of the two terminal Cα atoms is usually correlated with the forming of one or two hydrogen bonds between the corresponding residues, such hydrogen bond is not strictly required in the definition of the turn. That said, in most cases the H-bonding and Cα-distance definitions are equivalent.
Read more about this topic: Turn (biochemistry)
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