Definition of a Helix in Chemistry

Helicity (screw sense) is a (geometric) structural property, by which the arrangement of atoms in molecules can be described as a combined rotation and translation process. Centro-chiral, planar-chiral molecules can be considered also as helical, having in mind their spatial environment (helicity around a point, a plane or an axis). In numerous cases, the understanding of chiral structures under the aspect of helicity is advantageous. 

Ingold and Prelog described and defined the helix as follows  

A molecular helix is defined by an axis, by its screw sense and its pitch. If the radius is constant, there is a cylindrical helix, if it changes constantly, a conical helix exists. A palindromic helix is characterized by a constant pitch. 

If a molecule contains several helical segments, which may be of opposite screw sense (amphiverse helix, see also Fig. 14), these helical subunits should be described using M or P (Fig. 11). 

In such cases, especially with large (biochemical) molecules, it is often useful to differentiate between the absolute helicity, which is the sum of all M- and P-helical domaines, and the net helicity, defined by the difference of the volumes of left-handed and right-handed helix domaines. The net helicity is of interest for chemical use with respect to the observed chiroptical properties. 

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