Although the anti conformation is lower in energy and occurs more frequently, there are situations that can enforce adoption of the syn conformation. For exam- ple, in some structures there is much better stacking of the bases in the syn con- formation, as seen in a form of DNA known as Z-DNA (see Section 2.12) and in some RNA loop structures. Chemical modifi cations of bases may also aff ect the relative energies of the anti and syn conformations