Chemistry Reference
In-Depth Information
up to 2
x
stereoisomers. The amino acid, threonine, has two chiral centers and
can have up to 2
2
or four stereoisomers.
CO
2
H
CO
2
H
CO
2
H
CO
2
H
H
H
2
N
H
NH
2
H
2
N
NH
2
H
H
H
H
OH
HO
H
HO
H
OH
CH
3
CH
3
CH
3
CH
3
2R,3R
2S,3S
2R,3S
2S,3R
Threonine stereoisomers
The relationship between the 2R,3R and the 2S,3S stereoisomers is that
they are nonsuperimposable mirror images of each other; they are enan-
tiomers. Similarly, the 2R,3S and the 2S,3R stereoisomers are enantiomers of
each other. The relationship between the 2R,3R and the 2R,3S stereoisomers
is that they are diastereomers. Diastereomers are stereoisomers that are
not mirror images. Diastereomers generally have different, albeit similar,
physical properties. Of the four stereoisomers of threonine, only the 2S,3R
stereoisomer occurs naturally and is an essential human nutrient [29].
Consider 2,3-diaminobutane. There are two chiral centers and therefore
we could have up to four stereoisomers. The four possibilities are arbitrarily
labeled a-d. Stereoisomers a and b are nonsuperimposable mirror images and
are enantiomers. Stereoisomers c and d are also mirror images but they are
superimposable. Therefore they are the same compound. There are only three
stereoisomers, a-c.
CH
3
CH
3
CH
3
CH
3
H
2
N
H
2
N
H
H
H
H
NH
2
NH
2
H
NH
2
H
2
N
H
H
2
N
H
H
NH
2
CH
3
CH
3
CH
3
CH
3
a
b
c
d
2,3-diaminobutane
If we look closely at stereoisomer c, we see that there is a plane of sym-
metry within the molecule. Stereoisomer c is achiral yet has chirality centers.
Compounds like this are called meso compounds. A meso compound is opti-
cally inactive. The relationship between a and c (or between b and c) is that
they are diastereomers.
CH
3
H
NH
2
Plane of symmetry
H
NH
2
CH
3
c
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