Science Poetry

Evenhanded Remarks

“The property known as chirality:
A helix’s handed spirality;
Two non-superposing
Mirrored molecules, chosen
To label by dext/sinist-rality.”  

The 23 September 2019 limerick addressed an interesting property observed in three-dimensional molecules, which is introduced in organic chemistry coursework.   

“The property known as chirality: /
A helix’s handed spirality…”
Organic Chemistry 1 is a challenging course for many reasons, one of which is the necessity of thinking about three-dimensional molecules and properties via largely two-dimensional communication: textbooks and chalkboard drawings.  One property that demands the ability to think three-dimensionally is chirality.  

It is obvious when someone puts shoes on the wrong foot or gloves on the wrong hand.  Feet and hands are chiral: they are non-superimposable mirror images.  Some molecules exist in “handed” forms, which means they react differently in “glove-like” chemical environments: some fit and some don’t.  Other molecules are achiral; they do not exhibit this quality.  (I’ve always liked the succinctness of “shoes are chiral; socks are achiral.”)   A DNA molecule, with its spiraling helix, is chiral, providing a pertinent rhyme.

“Two non-superposing /
Mirrored molecules, chosen /
To label by dext/sinist-rality.” 
Several precise vocabulary terms are introduced via topics of stereochemistry.  Molecules that are stereoisomers are molecules made up of the same atoms bonded in the same order but with different three-dimensional arrangements.  Stereoisomers that exist as pairs of the non-superimposable mirror images described above are called enantiomers.  Specific stereocenters (locations where chirality is evident) are distinguished as having “R” or “S” orientations.   Many such terms are used in organic coursework.    

Along with R/S notation, chiral molecules can also be described in terms of their optical rotation: the direction in which they rotate plane-polarized light, which is described with a positive or negative sign.  These terms are dextrorotatory (clockwise rotation) and levorotatory (counterclockwise rotation).  To fit the limerick’s rhythmic constraints, “dext/sinist-rality” was used as a shorthand for this last set of “handed” definitions.