614
Chirality in Biology
CC
genotype results in ‘‘fluctuating asym-
metry,’’ that is, a 1 : 1 distribution. The
DC
genotype yields 25% left-handedness.
The original hypotheses have been mod-
iFed and subject to criticism and the
topic cannot be summarized simply. It
is noteworthy that in a 1991 Symposium,
‘‘Biological Asymmetry and Handedness,’’
a paper by McManus (The Inheritance
of Left-handedness) was extensively dis-
cussed by the participants. The recorded
comments comprise about 7000 words.
The genetics of left/right asymmetry in
bodily organs are under study, but appar-
ently none of the identiFed genes have
been implicated in behavior.
8
Origin of Chirality on Earth
Many attempts to explain the emergence
of life on planet Earth and the estab-
lishment of homochirality for the major
macromolecules, nucleic acids, proteins,
and carbohydrates have led to little agree-
ment. One difFculty is that experimental
chemical approaches to prebiotic condi-
tions produce racemic compounds if a
chiral center is involved. Two general pos-
sibilities are (1) that the Frst signiFcant
molecules for evolution of life were al-
ready formed with one chiral sense in place
or (2) that initially produced racemic mix-
tures were subjected to chiral enrichment
by a selection process.
Actual mechanisms under consideration
are legion-interactions with polarized
β
-
radiation from nuclear decay, interactions
with circularly polarized light, selective ad-
sorption, or polymerization on crystals or
clays, stochastic sorting of conglomerate
crystals,
enantioselective
magnetochiral
photochemistry, stirring, and chiroselec-
tive ampliFcation. A recent development is
the suggestion that life may have arisen in
high-temperature environments with pre-
biotic synthesis in an environment of iron
sulFde and hot magmatic exhalations.
Another general suggestion is that ex-
traterrestrial organic compounds may have
been transported to Earth and used for evo-
lutionary processes. Certain amino acids
are deFnitely present in meteorites, most
famously, the Murchison. There has been
some evidence concerning possible enan-
tiomeric excess in such amino acids, partic-
ularly those that do not occur in proteins.
There are, however, conflicting reports,
and the possible contamination of mete-
orite material by terrestrial material is real.
To overcome such difFculties, instrumen-
tal approaches to detecting chirality in, for
example, amino acids, are being prepared
for space missions to Mars and comets.
Another major area of discussion relates
to the possibility that earthly homochirality
reflects the fundamental structure of mat-
ter itself, in particular, the nonconserva-
tion of parity with the weak force as carried
by W
+
and Z
o
particles. The weak force is
the only one that is chiral. The universe
has an intrinsic ‘‘left-handedness’’ – it is
made of matter and not antimatter, and
the parity-violating differences between
enantiomers might determine biomolec-
ular chirality. The handedness of particles
implies that L and D molecules are actu-
ally diastereoisomers; the true enantiomer
of, say, L-alanine would be D-alanine made
from antimatter. Some authors believe that
the possible selectivity in such processes
is too small and yet others view the initial
choice as a random event.
±o
rl
i
f
ea
sw
eknowi
t
,a
l
l D-proteins
and
nucleic
acids
with
L-ribose
and
L-deoxyribose
would (presumably) have
been equally acceptable – this point of
view is bolstered by the work previously
described with all-D HIV protease. Perhaps
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