Turn (biochemistry)
A turn is an element of secondary structure in proteins.
According to the most common definition, a turn is defined by the close approach of two \mathrm{C^{\alpha}} atoms (< 7 A), when the corresponding residues are not involved in a regular secondary structure element such as an alpha helix or beta sheet.

# Types of turns
Turns are grouped by their hydrogen bonding and by their backbone dihedral angles.
At the level of hydrogen bonds, the nomenclature is similar to that of helices.
- A -turn (the most common form) is characterized by hydrogen bond(s) in which the donor and acceptor residues are separated by three residues (i \rightarrow i\pm 3 H-bonding).
- A \gamma-turn is characterized by hydrogen bond(s) in which the donor and acceptor residues are separated by two residues (i \rightarrow i\pm 2 H-bonding).
- An -turn is characterized by hydrogen bond(s) in which the donor and acceptor residues are separated by four residues (i \rightarrow i\pm 4 H-bonding).
- A \pi-turn is characterized by hydrogen bond(s) in which the donor and acceptor residues are separated by five residues (i \rightarrow i\pm 5 H-bonding).
Finally, an \omega-loop is a catch-all term for a longer loop with no internal hydrogen bonding.
Strictly speaking, a turn is defined by the close approach (\mathrm{C^{\alpha}} atoms and need not have a well-formed hydrogen bond.  Thus, it is more correct to define a -turn by the close approach of \mathrm{C^{\alpha}} atoms of residues separated by three peptide bonds, a \gamma-turn by the close approach of \mathrm{C^{\alpha}} atoms of residues separated by two peptide bonds, etc.  In most cases, the H-bonding and \mathrm{C^{\alpha}}-distance definitions are equivalent.
Within each hydrogen-bonding type, turns may be classified by their backbone dihedral angles.  A turn can be converted into its inverse turn (also called its mirror-image turn) by changing the sign on all of its dihedral angles.  (The inverse turn is not a true mirror image since the chirality of the \mathrm{C^{\alpha}} atoms is maintained.)  Thus, the \gamma-turn has two forms, a classical form with (, ) dihedral angles of roughly \left( 75^{\circ}, -65^{\circ} \right) and an inverse form with dihedral angles \left( -75^{\circ}, 65^{\circ} \right).  At least eight forms of the -turn have been identified, varying mainly in whether a cis isomer of a peptide bond is involved and on the dihedral angles of the central two residues.  The classical and inverse -turns are usually distinguished with a prime, e.g., type \mathrm{I} and type \mathrm{I}^{\prime} -turns.

# Hairpins vs. diverging turns
A hairpin is a special case of a turn, in which the direction of the protein backbone reverses and the flanking secondary structure elements interact.  For example, a -hairpin connects two hydrogen-bonded, antiparallel -strands.
However, turns can cause less drastic changes in direction and may connect regular secondary structure elements that do not interact with each other.  Such turns are called diverging turns.

# Role in protein folding
Two hypotheses have been proposed for the role of turns in protein folding.  In one view, turns play a critical role in folding by bringing together and fostering interactions between regular secondary structure elements.   This view is supported by mutagenesis studies indicating a critical role for particular residues in the turns of some proteins.  Also, nonnative isomers of X-Pro peptide bonds in turns can completely block the conformational folding of some proteins.  In the opposing view, turns play a passive role in folding.  This view is supported by the poor amino-acid conservation observed in most turns.  Also, non-native isomers of many X-Pro peptide bonds in turns have little or no effect on folding.