Chirality in Biology
585
Fig. 3
Confgurational
standards.
(
a
)
=
D-glyceraldehyde [also (
R
),
see Fig. 2] and
(
b
)
=
D-serine.
CHO
C
H
OH
CH
2
OH
(a)
COOH
C
H
NH
2
CH
2
OH
(b)
Fig. 4
Amino acid confgurations.
(
a
)
=
L-confguration o± the 19 chiral,
protein amino acids: R indicates the side
chains;
(
b
)
=
L-serine;
(
c
)
=
L-cysteine;
(
d
)
=
L-serine; also (
S
)-serine;
(
e
)
=
L-cysteine, also (
R
)-cysteine.
COOH
C
H
2
N
H
R
COOH
C
H
2
N
H
CH
2
OH
COOH
C
H
2
N
H
CH
2
SH
(a)
(c)
(b)
(d)
NH
2
C
HOOC
CH
2
OH
H
(e)
NH
2
C
HOOC
CH
2
SH
H
(
R
)
(
S
)
(Fig. 4a). As indicated, L-serine also has
the (
S
)c
o
n
f
g
u
r
a
t
i
o
n(
F
i
g
.4
b
,d
)
.B
e
-
cause o± the vagaries o± the sequence
rule, not all
L-amino acids are (
S
); ±or
instance, L-cysteine has (
R
)confgura
t
ion
(Fig. 4c, e).
Carbohydrates
have
multiple
carbon
atoms carrying H and OH groups. D/L
assignments are based on the arrange-
ment in a projection ±ormula o± the H
and OH groups at the highest num-
bered chiral carbon; the numbering starts
with the most oxidized group, CHO, C-1
(Fig. 5). Projection ±ormulae ±or carbo-
hydrates are not in±ormative since the
linear carbon chain would normally have
a zigzag arrangement; they were devised
many decades be±ore computer drawing
programs ±or chemical structures were
available. More realistic representations
use either vertical or horizontal zigzag
carbon chains (Fig. 5d, e). Since carbo-
hydrates usually adopt cyclic structures
in aqueous solutions, alternate structures
are necessary (e.g. Haworth or Mills struc-
tures).
2.3
Axial Chirality
The chirality type so ±ar described is
termed
central
chirality
.
Another
type,
axial chirality, occurs in planar struc-
tures and in compounds o± the allene
type, abC
=
C
=
Cab. Some natural products
are allenes; one example is mycomycin:
HC
C
C
C
CH
=
C
=
CH
CH
=
CH
CH
=
CH
CH
2
COOH. This compound
is a toxic antibiotic. Moreover, a sex at-
tractant ±or male Dried Bean beetles is
CH
3
(CH
2
)
6
CH
2
CH
=
C
=
CH
CH
=
CH
COOCH
3
.
Grasshopper
ketone,
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