582
Chirality in Biology
1
Chirality – An Introduction
The earliest people must have recognized
that one human hand, although very
similar, cannot be superposed on the other
hand and that the reflected image (e.g.
on a water surface or in a mirror) of
one hand was superposable on the other.
In modern terminology, the two human
hands are in a mirror-image relationship.
The ‘‘handedness’’ of biological structures,
molecules, objects, and so on, generates
much interest. For understanding, it is
necessary to de±ne right and left. Although
children are taught to distinguish hands,
feet, eyes, ears, and so on, as right or left, a
precise de±nition of right or left in terms of
direction is dif±cult if not impossible. The
normal de±nition is a statistical one – the
right hand is the stronger, or that which
is used preferentially by the majority.
Most of the world’s population favors
the right hand, and populations with the
majority preferentially using the left hand
are not known.
A comprehensive terminology describ-
ing handedness is based on the words
chiral (adjective) and chirality (noun) in-
troduced by Lord Kelvin in 1894 as follows:
‘‘I call any geometric ±gure, or group of
points
chiral
and say that it has
chirality
,if
its image in a plane mirror, ideally realized,
cannot be brought to coincide with itself.’’
Beginning in about 1958, they began to be
very widely used. Although based on the
Greek word ‘‘
χειρ
’’ for hand, the de±nition
of chirality requires only the observation of
a mirror image without requiring knowl-
edge of right or left. The two human hands
are chiral regardless of their names.
While chirality usually involves three-
dimensional structures, it is possible in
two dimensions. Thus, the mirror image
of the letter F cannot be superposed on
the original by translation or rotation in
the plane; F is chiral in two dimensions.
The letter E is achiral; mirror image and
original are superposable.
Chirality, a very pervasive phenomenon
in biology, operates at two levels. The ±rst
is that of easily observed structures such
as the hands. The second concerns atoms
and molecules. Since the macrobiological
structures depend on enzyme reactions for
their formation, the chirality of metabo-
lites, both large and small, determines the
chirality of the larger structures. Hence,
a brief review of general stereochemical
principles will be given before discussing
the biological implications. In this article,
all of the common amino acids will in
some cases be described by the conven-
tional three-letter abbreviations.
2
Molecular Chirality in Biology
2.1
Chirality Descriptors: the
R
/
S
Notation
Isomers are compounds having the same
molecular formula but differing in the
nature or sequence of bonding of their
atoms, or
in the
spatial
arrangement
of their atoms. Those isomers differ-
ing in their spatial atomic arrangements
are
called
stereoisomers
.
Stereoisomers
showing a mirror-image relationship are
termed
enantiomers
, whereas stereoiso-
mers not in a mirror-image relationship
are
diastereoisomers
. Enantiomers show
the
same
general
chemical
properties
but differ in their interaction with po-
larized light. One enantiomer will pro-
duce a right-handed dextro (
+
)ro
ta
t
ion
,
the other a left-handed levo (
)r
o
t
a
-
tion. Enantiomers frequently exhibit dif-
ferent properties in biological systems.
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