600
Chirality in Biology
4. Activity may require both enantiomers.
For honeybee queen mandibular phe-
romone (a mixture of compounds), both
enantiomers of
9-hydroxy-2-decenoic
acid must be present for full activity.
Another example is sulcatol.
3.4
Plant Growth Regulators
With
chiral
plant
growth
regulators,
one enantiomer of a chiral structure is
usually the most potent. Thus, for
α
-
phenoxypropionic acid, the (
R
) enantiomer
has
the
most
powerful
herbicidal
ac-
tivity. The naturally occurring form of
abscisic acid is (
S
), and the (
R
)f
o
rm
is signi±cantly
less
potent in
inhibit-
ing root growth of germinating barley;
there is little difference between these
two enantiomers in other physiological
effects. The growth regulator, paclobu-
trazol (
β
-[(4-chlorophenyl)-methyl]-
α
-(1,1-
dimethylethyl)-1H-1,2,4-triazole-1-etha-
nol), also has antifungal activity. Of the
four stereoisomers, the most effective
in growth regulation is the (2
S
,3
S
)-(
)
form (Fig. 13a), whereas the enantiomer
(2
R
,3
R
)-(
+
)i
smo
s
te
f
fe
c
t
ivea
safung
i
-
cide.
Jasmonic acid and its conjugates are
plant signaling substances synthesized
in
response
to
external
stimuli
(e.g.
wounding). The (3
R
,7
R
)-(
) enantiomer
is
particularly
common
(Fig. 13b).
In
barley leaves, treatment with jasmonic acid
yields upregulation of speci±c genes and
downregulation of housekeeping genes.
The (
) enantiomers of the methyl ester
and
amino
acid
conjugates
are
more
active than the (
+
) forms; conjugates with
an
L-amino acid also tend to be more
active. The responses of the jasmonate-
responsive genes to the various chiral
structures are complex.
Physiological effects can vary with dif-
ferent strains of the same plant. For the
synthetic auxin indole-3-succinic acid, a
10
8
M concentration of the (
R
) enan-
tiomer is most potent as a root growth
promoter for the SF (swingtime) strain of
Fuschia hybrida
, with the opposite enan-
tiomer being more potent for the VF
(variegated) strain at all concentrations.
3.5
Chiral Drugs
Reactions to pharmaceutical agents may
be influenced signi±cantly if chiral ele-
ments are present. Early in the twentieth
century, enantiomers and racemates of
hyoscyamine and epinephrine were dis-
tinguished by quantitative differences in
their physiological actions. However, there
was for many years little concern about
the possible impact of chirality on widely
used drugs. After World War II, attention
was focused on the role of chirality in
pharmacology and medicinal chemistry,
and there is now an enormous litera-
ture including conferences and symposia
on the topic of ‘‘chiral drugs.’’ For in-
stance, a query to PubMed on this topic
OH
Cl
N
N
N
3
2
COOH
O
(a)
(b)
3
7
Fig. 13
Chiral plant growth
regulators.
(
a
)
=
(
2
S
,3
S
)
stereoisomer of paclobutrazol;
(
b
)
=
(
3
R
,7
R
)-jasmonic acid.
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