584
Chirality in Biology
is levorotatory and, conversely, the (
S
)
enantiomer (Fig. 1b) is dextrorotatory.
Three-dimensional molecular arrange-
ments
are
often
represented
as
two-
dimensional
‘‘projection’’
drawings
by
carefully
following
conventions
estab-
lished by Emil Fischer. In a projection
formula, a vertical line indicates a bond
pointing below the plane of the paper,
while a horizontal line indicates a bond
pointing above the plane of the paper;
see CHBrClF enantiomers (Fig. 1). If the
group of lowest priority is at the bottom
of a projection formula, the con±guration
can be determined from the handedness
of the remaining three groups (Fig. 1).
For chirality speci±cation, any projec-
tion formula can be transformed so that
the group of lowest priority is at the
bottom; hence the handedness can be
ascertained as just described. This trans-
formation must involve two interchanges
of any two groups; such a process re-
tains the original con±guration (a single
interchange of any two groups inverts
the original con±guration). For exam-
ple, (
+
)-glyceraldehyde is conventionally
written with the most oxidized group lo-
cated at the top (Fig. 2a). The priority
sequence is OH
>
CHO
>
CH
2
OH
>
H.
If the groups H and CH
2
OH are inter-
changed so that the lowest priority H is
at the bottom, a structure with inverted
con±guration is obtained (Fig. 2b). A sec-
ond interchange of CHO and OH restores
the original con±guration (Fig. 2c). ‘‘Read-
ing’’ the handedness of the sequence,
OH
>
CHO
>
CH
2
OH, gives the con±g-
uration of (
+
)-glyceraldehyde as (
R
).
2.2
Chirality Descriptors: the D/L Notation
Certain
groups
of
metabolites
having
structures with identical handedness –
α
-
amino acids,
α
-hydroxy acids, and carbo-
hydrates – are often described by the D/L
notation. A projection formula is drawn
with the most oxidized group (usually
CHO or COOH) placed at the top. The
next carbon atom in the vertical chain has
invariant groups, H and NH
2
for amino
acids, and H and OH for hydroxy acids.
Two con±gurational ‘‘standards’’, glycer-
aldehyde and serine, are used (Fig. 3). If in
such formulae, the NH
2
or OH group is
to the right, the con±guration is assigned
as D; if to the left, the assignment is L.
If necessary, the modi±cations D
g
and D
s
are used to emphasize a relationship with
D-glyceraldehyde or D-serine. D/L descrip-
tors refer only to con±gurations and not
to optical rotation. The classical 20 protein
amino acids have the same L-con±guration
C
CHO
OH
H
CH
2
OH
(a)
C
CHO
OH
HOCH
2
H
(b)
C
OH
CHO
HOCH
2
H
(c)
Fig. 2
Transformation of projection
formulae. The structure drawn is that of
(
+
)-glyceraldehyde. One interchange of
two groups,
(
a
)
(
b
)
, inverts the
conFguration, and a second,
(
b
)
(
c
)
,
restores the original conFguration. As
shownin(c),(
+
)-glyceraldehyde has the
(
R
)conFgurat
ion
.
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