The angle between two bonds
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 phi-angle (φ) - around the N-Cα bond
- The psi-angle (ψ) - around the Cα-C bond
- The omega-angle (ω) - around the peptide bond between C and N.
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.