Chirality in biologyThe electric and magnetic fields of polarized light oscillate in a geometric plane. An axis
normal to this plane gives the direction of energy propagation. Optically active isomers
rotate the plane that the fields oscillate in. The polarized light is actually rotated in a
racemic mixture as well, but it is rotated to the left by one of the two enantiomers, and to
the right by the other, which cancel out to zero net rotation.
32.6 Chirality in biology
Many biologically-active molecules are chiral, including the naturally-occurring amino acids
(the building blocks of proteins), and sugars. Interestingly, in biological systems most of
these compounds are of the same chirality: most amino acids are L and sugars are D. The
origin of this homochirality in biology is the subject of much debate.
Figure 96 Enantiomers of amino acids.
Chiral objects have different interactions with the two enantiomers of other chiral objects.
Enzymes, which are chiral, often distinguish between the two enantiomers of a chiral sub-
strate. Imagine an enzyme as having a glove-like cavity which binds a substrate. If this
glove is right handed, then one enantiomer will fit inside and be bound while the other
enantiomer will have a poor fit and is unlikely to bind.