Torsion angle

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The angle between two bonds

Overview of the different torsion angles
Bond lengths, bond angles, and torsion angles

A torsion angle, also known as a dihedral angle, is formed by three consecutive bonds in a molecule and defined by the angle created between the two outer bonds. The backbone of a protein has three different torsion angles.

The ω-bond has a slightly double-bond character and is therefore almost always 180 degrees. The structure of a protein is mainly formed by the φ- and ψ-angles.
Every secondary structure element forces the backbone into a specific range of torsion angles, this can be visualized in a ramachandran plot. The figure shows the location of the φ, ψ and ω-angles
Side chains can also contain torsion angles, they are indicated as chi-angles.