Chirality in Biology
583
Diastereoisomers differ in their general
chemical properties.
The major biological concern is with the
chirality of carbon compounds. The sim-
plest organic chiral compound is formed
by a carbon atom linked to four differ-
ent noncarbon atoms as in CHBrClF.
The spatial arrangements in such a com-
pound are termed
confgurations
,an
di
f
the precise arrangement is known, it is
described as an absolute con±guration (ar-
rangements resulting from rotation about
single bonds are excluded). Absolute con-
±gurations are speci±ed unambiguously
by the Cahn–Ingold–Prelog system. The
four different atoms or groups of atoms
attached to the carbon atom are ±rst ar-
ranged in a carefully described priority
sequence. The ‘‘sequence rules’’ are based
on parameters such as atomic number and
atomic mass. To be unambiguous, they are
complex but are readily available in texts
of organic chemistry or stereochemistry.
For many groups of biological interest, the
following sequence will be adequate; a
>
b
indicates that a has a higher priority than b:
Br
>
Cl
>
SH
>
F
>
OCH
3
>
OH
>
NHCOCH
3
>
NH
2
>
COOR
>
COOH
>
CHO
>
CH
2
OH
>
C
6
H
5
>
C
2
H
5
>
CH
3
>
3
H
>
2
H
>
1
H
A model of the molecule is then viewed
with the atom or group of lowest priority
pointing away from the observer. If the
remaining atoms or groups of atoms,
when examined in order of decreasing
priority, show a clockwise or right-handed
arrangement, the absolute con±guration is
denoted as
R
(rectus). A counterclockwise,
left-handed path yields the
S
(sinister)
absolute con±guration. For CHBrClF, the
priority is Br
>
Cl
>
F
>
H, and the two
possible enantiomeric models are shown
in Fig. 1(a, b). The (
R
) enantiomer (Fig. 1a)
F
Br
Cl
(
R
)
(
R
)(
S
)
H
O
(
S
)
F
Cl
Br
H
O
(a)
(b)
C
Cl
Br
F
H
C
Br
Cl
F
H
F
C
Br
H
Cl
F
C
Cl
H
Br
Fig. 1
Enantiomers of CHBrClF. In this and subsequent ±gures a
‘‘dashed’’ line indicates a bond below the paper plane and a ‘‘wedge’’
shape indicates a bond above the paper plane. Structures (a) and
(b) show the molecular models viewed to determine (
R
)or(
S
)
con±guration; O
=
eye of observer. Note that the sequence,
Br
Cl
F, describes a right-handed path in (a) and a left-handed
path in (b). The bottom drawings determine con±guration using
projection drawings with the group of lowest priority, H, at the
bottom. For each enantiomer, there is a three-dimensional
representation and a conventional projection formula.
previous page 1257 Encyclopedia of Molecular Cell Biology and Molecular Medicine read online next page 1259 Encyclopedia of Molecular Cell Biology and Molecular Medicine read online Home Toggle text on/off