The great majority of compounds in nature are chiral. A substance is chiral when it and its mirror images are not identical (the mirror image of the molecule cannot be superimposed upon it). Chirality in natural products is a direct consequence of being produced by enzymes, which are formed from chiral amino-acids and are themselves chiral. The two mirror-image forms, called enantiomers, are different substances. Although most of their chemical properties are identical, they can have very different biological properties. Since our own taste receptors (on our tongues) and odour receptors (in our nasal passage) are themselves chiral, we can detect some chiral differences. For example the amino-acid l-asparagine, first isolated from asparagus juice, has a bitter taste, while its enantiomer, the unnatural d-asparagine tastes sweet (Figure 2.21). Our odour receptors can distinguish between (S)-(+)-carvone and {R)~ (-)-carvone (Figure 2.21), and we have a much lower threshold for detecting the first form. The different flavour of lemons and oranges is due to the enantiomeric forms of the monoterpene limonene (Figure 6.1).


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