The curved line represents the torsion angle or dihedral angle between the intersecting red and blue planes. The green dots are points defining the planes.

A Torsion angle is a type of dihedral angle, the angle between two intersecting planes.

For proteins, the torsion or dihedral angles phi and psi largely determine the shape of the protein backbone. The term "torsion" emphasizes that these angles measure rotation or twisting of the backbone around the alpha carbon.

See dihedral angle for more.

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