Additional Chirality Elements
A special case of chirality axis appears in molecules with a helical shape. Helices are chiral objects that look like screws and can therefore be right- or left-handed respectively. In nature, this type of chirality is frequently found, for example, in mussel or snail shells. A helically shaped molecule, that is of great interest for chemists and biologists, is deoxyribonucleic acid (DNA), which appears in two right-handed (A- and B-helix) and one left-handed (Z-helix) types of helices. The DNA backbone is winded around the chirality axis. A helix's rotational sense, or helicity, can be ascertained by following the chirality axis from the end of the helix. It does not matter from which end of the helix the molecule is viewed, because the result of ascertaining the helicity of a particular molecule is always the same.
Hexahelicene is a well-known helically-shaped molecule (view animation). Hexahelicene is chiral and its enantiomers do not easily interconvert. Therefore, it is optically active and the enantiomers can be separated from each other. One enantiomer forms a right-handed helix, while the other enantiomer forms a left-handed one. The helical shape of hexahelicene is forced by the steric interaction of the overlapping terminal aromatic rings.
Whether the molecule is viewed from one end of the chirality axis or the other makes no difference. The helicity of each particular hexahelicene molecule remains constant. Test it out by manipulating the interactive molecular models of hexahelicene above with your mouse.
- For the classification of absolute configuration of helices the (P,M nomenclature) is used. If the molecule has a right-handed helical shape, the absolute configuration is described by the descriptor (P) (plus). In a left-handed helical shape, the absolute configuration of a molecule is called (M) (minus).