Rotation and brotation activate a torsion. (To rotate models, see the commands turn and roll.) Rotation and brotation are equivalent; either makes the bond between the two specified atoms rotatable and opens the Adjust Torsions panel. Of course, atom1 and atom2 must be bonded. In addition, the bond cannot be terminal (not attached to any additional atoms on one or both ends) or part of a ring. The ability to activate a rotation does not imply that the bond is freely rotatable in the chemical sense; for example, it could be a double bond.
Activating a torsion results in movement of the smaller group of atoms during bond rotation while the larger group remains fixed. With the Adjust Torsions panel, the fixed and moveable portions can be switched, the torsion angle can be changed or reset to its original value, or the torsion can be deactivated so that the bond is no longer rotatable.
Two rotations cannot affect a common set of atoms unless one affected set is a complete subset of the other. That is, rotations must be properly nested.
Examples:
rotation #1:1@c8,c9- activate a rotation around the C8-C9 bond in residue 1 of model 1
brotation :1@c:2@n- activate a rotation around the bond between C in residue 1 and N in residue 2
See also: angle, the Adjust Torsions panel