Chemistry Reference
In-Depth Information
The human body can distinguish between enantiomers. For example,
consider the enantiomers of limonene. The R-(
+
) enantiomer has the odor
of orange and the S-(
−
) enantiomer the odor of lemon. The designation
(
) indicates that plane polarized light is rotated in the clockwise direction
or that the compound is dextrorotatory and (
+
) indicates counterclockwise
rotation, also known as levorotatory. R and S indicate the positioning of the
substituents on the chiral carbon. Some R compounds are dextrorotatory and
other R compounds are levorotatory. R-(
−
−
) carvone has a spearmint odor
and S-(
+
) carvone has a caraway odor. R-(
−
) amphetamine has a musty
+
odor and S-(
) a fecal odor. There are examples of other compounds where
enantiomers do not have different odors, but there are several examples that
prove that enantiomers can have different odors [30].
R-(+) limonene
S-(
−
) limonene
O
O
S-(+) carvone
R-(
−
) carvone
H
H
Ph
Ph
NH
2
H
2
N
CH
3
CH
3
R-(
−
) amphetamine
S-(+) amphetamine
It is also true that enantiomers can have different bioactivity, bioavail-
ability, and pharmacokinetics. There are many examples. One is albuterol,
used for over 40 years as a racemic mixture to treat asthma. It is the R(
)
enantiomer which is the effective bronchodilator. There are some side effects
with the use of racemic albuterol. The S(
−
) enantiomer does not exhibit the
beneficial selective binding. At best, it can be considered inert and there is
even some indication that it can act as an agonist with effects contrary to
the R(
+
) enantiomer is metabolized up to 10 times
more slowly and therefore persists in the circulation up to 12 hours [31].
Even if the S(
−
) enantiomer. The S(
+
) enantiomer were inert, a patient taking the racemic mix-
ture needs twice the amount versus taking the effective R(
+
−
) enantiomer.
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