Data type: char
Must appear in a looped list
Must match a value of '_atom_site_label'
The labels of the four atom sites which define the torsion angle specified by _geom_torsion. These must match codes specified as _atom_site_label in the atom list. The torsion angle definition should be that of Klyne and Prelog. The vector direction *_label_2 to *_label_3 is the viewing direction, and the torsion angle is the angle of twist required to superimpose the projection of the vector site2-site1 onto the projection of the vector site3-site4. Clockwise torsions are positive, anticlockwise torsions are negative. Ref: Klyne, W. & Prelog, V. (1960). Experientia, 16, 521-523.