Chemistry Reference
In-Depth Information
FIGURE 73
Optical isomers and amino acid racemization dating.
In mole-
cules in which an atom of carbon is linked to four identical atoms or chemical
groups, the four atoms or chemical groups are tetrahedrally distributed around
the central atom of carbon, forming a symmetric, triangular pyramid
(a)
. If the
linked atoms or chemical groups are different, however, the molecule is asym-
metricand the four atoms or chemical groups are arranged in two different spatial
distributions around the carbon
(b,c)
; the two molecules are said to be optical
isomers or stereoisomers. Each one of the two stereoisomers is known by a
slightly different name: one is known as the “
L
-isomer” and the other, as the “
D
-
isomer.” The amino acids
L
-isoleucine
(d)
and
D
-alloisoleucine
(e)
for example,
are optical isomers. The racemization of
L
-isoleucine into
D
-alloisoleucine is used
to estimate the age of dead remains by the “amino acid racemization dating”
method. The * next to carbon atoms indicates that the carbon is “asymmetric”.
acids - include in their composition at least one
chiral
or
asymmetric atom
of carbon and therefore exhibit stereoisomerism. For reasons that have as
yet not been explained, biological substances in living organisms occur only
as the
L
-isomers; thus all amino acids in living organisms occur exclusively
as
L
-isomers.
The
L
-isomer of any compound can transform into the
D
-isomer, and
vice versa. A mixture of equal amounts of the
L
- and
D
-isomers of a com-
pound is said to be a
racemic mixture
, and the process whereby an isomer,
