Torsional angles are dihedral angles which is an angle between two intersecting planes on a third plane perpendicular to the line of intersection, it is defined by 4 points in space. It can vary +/- 180o from the co-planer
Torsional angles are one of the important parameters of the protein folding. In a protein chain there are three torsional angles : φ (phi), ψ (psi) and ω (omega). The side chain torsional angle are near 180°(trans), 60°(gauche+), and -60° (gauche-). Sometimes the stability of side-chain dihedral angles is affected by the values φ and ψ. The torsion angles φ (phi) and ψ (psi) maintains the flexibility of the polypeptide backbone to adopt a certain fold. Thus torsional angles play an important role in controlling the geometry of the protein structures on which the specific functions of proteins and biological activity depends. When a protein is denatured, secondary and tertiary structure is altered and it can no longer perform its function. So torsional angles have a great role in biological activities.