A chiral molecule can occur as one of an otherwise identical pair which have the exact same composition, functional structure, but shapes which are mirror images of the other and so display a definite "handedness". These left–right handed pairs (enantiomers) even have identical physical and chemical properties unless they are interacting with some other chiral species or environment.
Such chiral molecules occur widely. Most commonly, the chirality originates at a tetrahedral, asymmetric carbon atom, though other chiral generating structures are also encountered. When four distinct substituent groups attach to the central carbon the 2nd, 3rd & 4th can arrange either clockwise or anti-clockwise as viewed from the 1st, resulting in a non-superimposable enantiomer pair — as seen in this picture of enantiomers of the amino acid alanine (CH3CH(NH2)COOH).
Many of the basic molecular building blocks of life are chiral species. It is an astonishing fact that in terrestrial life forms these occur with a unique handedness — for example, all naturally occurring amino acids are found only as the L-enantiomer. This homochirality is not understood, but is a key to the origins of life.
Because of homochirality, much of our basic biochemistry is chirally specific, meaning that organisms can have a differentiated response to other chiral molecules that are encountered in their environment.
Chiral Odour Molecules
Over 800 chiral natural product molecules whose enantiomers are relevant to the food and fragrance industries have been listed. Often the enantiomers of one of these chiral odour molecules are perceived to have very different smells. In fact the nose is an extremely sensitive tool for discriminating between such enantiomers. In contrast, current instrumental techniques are cumbersome, slow and relatively insensitive.
All this poses a challenge for instrumental breath analysis that attempts to analyse food odours — it is not sufficient to identify chemical composition and structure of an odour, the enantiomeric form needs to be established as well. This problem motivated our own development of the PECD technique for sensitive chiral analysis of gaseous samples.
Symmetry breaking associated with chiral phenomena is a theme that recurs across the sciences — from the intricacies of the electroweak interaction and nuclear decay to the environmentally influenced dimorphic chiral structures of microscopic plankton and the genetically controlled preferential coiling direction seen in the shells of snail populations.
The famous double helix of DNA presents a chiral – right-handed – structure; left-handed DNA is somewhat scarcer!
At the molecular level considerable capital, both intellectual and financial is invested in the study of chiral phenomena particularly because of their significance for the pharmaceutical industry.
Many pharmaceuticals are chiral and the response of an organism to these can be enantiomer-specific. Seven of the top ten selling drugs have a chiral active ingredient and increasingly such drugs are being developed in single enantiomer form, taking a rapidly expanding market share with a predicted value of $15–25 billion by 2008/9.
If a mixture of both enantiomers is administered – one perhaps active, the other at best benign, at worst harmful – one may anticipate selective degradation to smaller chiral units in the body and subsequent excretion. Experimental techniques that can identify and follow any such enantiomer-selective metabolism in vivo — perhaps chiral breath analysis — may come to offer better, more targetted drug design and delivery.