text: Large molecules may have isomers that differ by the topology of their overall arrangement in space, even if there is no specific geometric constraint that separate them.  For example, long chains may be twisted to form topologically distinct knots, with interconversion prevented by bulky substituents or cycle closing (as in circular DNA and RNA plasmids). Some knots may come in mirror-image enantiomer pairs. Such forms are called topological isomers or topoisomers.
  Also, two or more such molecules may be bound together in a catenane by such topological linkages, even if there is no chemical bond between them.  If the molecules are large enough, the linking may occur in multiple topologically distinct ways, constituting different isomers. Cage compounds, such as helium enclosed in dodecahedrane (He@) and carbon peapods, are a similar type of topological isomerism involving molecules with large internal voids with restricted or no openings.