Chirality
32.5 Properties of optical isomers
Enantiomers have - when present in a symmetric environment- identical chemical and
physical properties except for their ability to rotate plane-polarized light by equal amounts
but in opposite directions. A solution of equal parts of an optically-active isomer and its
enantiomer is known as a racemic solution and has a net rotation of plane-polarized light
of zero.
Enantiomers differ in how they interact with different optical isomers of other compounds.
In nature, most biological compounds (such as amino acids) occur as single enantiomers.
As a result, different enantiomers of a compound may have substantially different biological
effects. Different enantiomers of the same chiral drug can have very different pharmological
effects, mainly because the proteins they bind to are also chiral.
For example, spearmint leaves and caraway seeds respectively contain L-carvone and D-
carvone - enantiomers of carvone. These smell different to most people because our taste
receptors also contain chiral molecules which behave differently in the presence of different
enantiomers.
Figure 95 Limonene enantiomers have different smells.
D-form Amino acids tend to taste sweet, whereas L-forms are usually tasteless. This is
again due to our chiral taste molecules. The smells of oranges and lemons are examples of
the D and L enantiomers.
Penicillin’s activity is stereoselective. The antibiotic only works on peptide links of D-
alanine which occur in the cell walls of bacteria - but not in humans. The antibiotic can
kill only the bacteria, and not us, because we don’t have these D-amino acids.