Homochirality is a uniformity of chirality,
or handedness. Objects are chiral when they
cannot be superposed on their mirror images.
For example, the left and right hands of a
human are approximately mirror images of each
other but are not their own mirror images,
so they are chiral. In biology, 19 of the
20 natural amino acids are homochiral, being
L-chiral (left-handed), while sugars are D-chiral
(right-handed). Homochirality can also refer
to enantiomerically pure substances in which
all the constituents are the same enantiomer
(a right-handed or left-handed version of
an atom or molecule), but some sources discourage
this use of the term.
It is unclear whether homochirality has a
purpose, however, it appears to be a form
of information storage. One suggestion is
that it reduces entropy barriers in the formation
of large organized molecules. It has been
experimentally verified that amino acids form
large aggregates in larger abundance from
enantiopure substrates than from racemic ones.It
is not clear whether homochirality emerged
before or after life, and many mechanisms
for its origin have been proposed. Some of
these models propose three distinct steps:
mirror-symmetry breaking creates a minute
enantiomeric imbalance, chiral amplification
builds on this imbalance, and chiral transmission
is the transfer of chirality from one set
of molecules to another.
== In biology ==
Amino acids are the building blocks of peptides
and enzymes while sugar-peptide chains are
the backbone of RNA and DNA. In biological
organisms, amino acids appear almost exclusively
in the left-handed form (L-amino acids) and
sugars in the right-handed form (R-sugars).
Since the enzymes catalyze reactions, they
enforce homochirality on a great variety of
other chemicals, including hormones, toxins,
fragrances and food flavors. Glycine is achiral,
as are some other non-proteinogenic amino
acids are either achiral (such as dimethylglycine)
or of the D enantiomeric form.
Biological organisms easily discriminate between
molecules with different chiralities. This
can affect physiological reactions such as
smell and taste. Carvone, a terpenoid found
in essential oils, smells like mint in its
L-form and caraway in its R-form. Limonene
tastes like lemons when right-handed and oranges
when left-handed.Homochirality also affects
the response to drugs. Thalidomide, in its
left-handed form, cures morning sickness;
in its right-handed form, it causes birth
defects. Unfortunately, even if a pure left-handed
version is administered, some of it can convert
to the right-handed form in the patient. Many
drugs are available as both a racemic mixture
(equal amounts of both chiralities) and an
enantiopure drug (only one chirality). Depending
on the manufacturing process, enantiopure
forms can be more expensive to produce than
stereochemical mixtures.Chiral preferences
can also be found at a macroscopic level.
Snail shells can be right-turning or left-turning
helices, but one form or the other is strongly
preferred in a given species. In the edible
snail Helix pomatia, only one out of 20,000
is left-helical. The coiling of plants can
have a preferred chirality and even the chewing
motion of cows has a 10% excess in one direction.
== Origins ==
=== 
Symmetry breaking ===
Known mechanisms for the production of non-racemic
mixtures from racemic starting materials include:
asymmetric physical laws, such as the electroweak
interaction; asymmetric environments, such
as those caused by circularly polarized light,
quartz crystals, or the Earth's rotation;
and statistical fluctuations during racemic
synthesis. Once established, chirality would
be selected for. A small enantiomeric excess
can be amplified into a large one by asymmetric
autocatalysis, such as in the Soai reaction.
In asymmetric autocatalysis, the catalyst
is a chiral molecule, which means that a chiral
molecule is catalysing its own production.
An initial enantiomeric excess, such as can
be produced by polarized light, then allows
the more abundant enantiomer to outcompete
the other.One supposition is that the discovery
of an enantiomeric imbalance in molecules
in the Murchison meteorite supports an extraterrestrial
origin of homochirality: there is evidence
for the existence of circularly polarized
light originating from Mie scattering on aligned
interstellar dust particles which may trigger
the formation of an enantiomeric excess within
chiral material in space. Interstellar and
near-stellar magnetic fields can align dust
particles in this fashion. Another speculation
(the Vester-Ulbricht hypothesis) suggests
that fundamental chirality of physical processes
such as that of the beta decay (see Parity
violation) leads to slightly different half-lives
of biologically relevant molecules. Homochirality
may also result from spontaneous absolute
asymmetric synthesis.It is also possible that
homochirality is simply a result of the natural
autoamplification process of life—that either
the formation of life as preferring one chirality
or the other was a chance rare event which
happened to occur with the chiralities we
observe, or that all chiralities of life emerged
rapidly but due to catastrophic events and
strong competition, the other unobserved chiral
preferences were wiped out by the preponderance
and metabolic, enantiomeric enrichment from
the 'winning' chirality choices. The emergence
of chirality consensus as a natural autoamplification
process has been associated with the 2nd law
of thermodynamics.
=== Amplification ===
==== 
Theory ====
In 1953, Charles Frank proposed a model to
demonstrate that homochirality is a consequence
of autocatalysis. In his model the L and D
enantiomers of a chiral molecule are autocatalytically
produced from an achiral molecule A
A + L → 2L, A + D → 2Dwhile suppressing
each other through a reaction that he called
mutual antagonism
L + D → ∅.In this model the racemic state
is unstable in the sense that the slightest
enantiomeric excess will be amplified to a
completely homochiral state. This can be shown
by computing the reaction rates from the law
of mass action:
d
[
L
]
d
t
=
k
a
[
L
]
−
k
d
[
L
]
[
D
]
d
[
D
]
d
t
=
k
a
[
D
]
−
k
d
[
L
]
[
D
]
,
{\displaystyle {\begin{aligned}{\frac {d[{\ce
{L}}]}{dt}}&=k_{a}[{\ce {L}}]-k_{d}{\ce {[L][D]}}\\{\frac
{d[{\ce {D}}]}{dt}}&=k_{a}[{\ce {D}}]-k_{d}{\ce
{[L][D]}},\end{aligned}}}
where
k
a
{\displaystyle k_{a}}
is the rate constant for the autocatalytic
reactions,
k
d
{\displaystyle k_{d}}
is the rate constant for mutual antagonism
reaction, and the concentration of A is kept
constant for simplicity. By defining the enantiomeric
excess
Φ
{\displaystyle \Phi }
as
Φ
=
[
D
]
−
[
L
]
[
D
]
+
[
L
]
,
{\displaystyle \Phi ={\frac {[{\ce {D}}]-[{\ce
{L}}]}{[{\ce {D}}]+[{\ce {L}}]}},}
we can compute the rate of change of enatiomeric
excess using chain rule from the rate of change
of the concentrations of enantiomeres L and
D.
d
Φ
d
t
=
(
2
k
d
[
L
]
[
D
]
[
D
]
+
[
L
]
)
Φ
.
{\displaystyle {\frac {d\Phi }{dt}}=\left({\frac
{2k_{d}{\ce {[L][D]}}}{[{\ce {D}}]+[{\ce {L}}]}}\right)\Phi
.}
Linear stability analysis of this equation
shows that the racemic state
Φ
=
0
{\displaystyle \Phi =0}
is unstable. Starting from almost everywhere
in the concentration space, the system evolves
to a homochiral state.
It is generally understood that autocatalysis
alone does not yield to homochirality, and
the presence of the mutually antagonistic
relationship between the two enantiomers is
necessary for the instability of the racemic
mixture. However, recent studies show that
homochirality could be achieved from autocatalysis
in the absence of the mutually antagonistic
relationship, but the underlying mechanism
for symmetry-breaking is different.
==== Experiments ====
There are several laboratory experiments that
demonstrate how a small amount of one enantiomer
at the start of a reaction can lead to a large
excess of a single enantiomer as the product.
For example, the Soai reaction is autocatalytic.
If the reaction is started with some of one
of the product enantiomers already present,
the product acts as an enantioselective catalyst
for production of more of that same enantiomer.
The initial presence of just 0.2 equivalent
one enantiomer can lead to up to 93% enantiomeric
excess of the product.
Another study concerns the proline catalyzed
aminoxylation of propionaldehyde by nitrosobenzene.
In this system, a small enantiomeric excess
of catalyst leads to a large enantiomeric
excess of product.
Serine octamer clusters are also contenders.
These clusters of 8 serine molecules appear
in mass spectrometry with an unusual homochiral
preference, however there is no evidence that
such clusters exist under non-ionizing conditions
and amino acid phase behavior is far more
prebiotically relevant. The recent observation
that partial sublimation of a 10% enantioenriched
sample of leucine results in up to 82% enrichment
in the sublimate shows that enantioenrichment
of amino acids could occur in space. Partial
sublimation processes can take place on the
surface of meteors where large variations
in temperature exist. This finding may have
consequences for the development of the Mars
Organic Detector scheduled for launch in 2013
which aims to recover trace amounts of amino
acids from the Mars surface exactly by a sublimation
technique.
A high asymmetric amplification of the enantiomeric
excess of sugars are also present in the amino
acid catalyzed asymmetric formation of carbohydratesOne
classic study involves an experiment that
takes place in the laboratory. When sodium
chlorate is allowed to crystallize from water
and the collected crystals examined in a polarimeter,
each crystal turns out to be chiral and either
the L form or the D form. In an ordinary experiment
the amount of L crystals collected equals
the amount of D crystals (corrected for statistical
effects). However, when the sodium chlorate
solution is stirred during the crystallization
process the crystals are either exclusively
L or exclusively D. In 32 consecutive crystallization
experiments 14 experiments deliver D-crystals
and 18 others L-crystals. The explanation
for this symmetry breaking is unclear but
is related to autocatalysis taking place in
the nucleation process.
In a related experiment, a crystal suspension
of a racemic amino acid derivative continuously
stirred, results in a 100% crystal phase of
one of the enantiomers because the enantiomeric
pair is able to equilibrate in solution (compare
with dynamic kinetic resolution).
=== Transmission ===
Many strategies in asymmetric synthesis are
built on chiral transmission. Especially important
is the so-called organocatalysis of organic
reactions by proline for example in Mannich
reactions.
== Optical resolution in racemic amino acids
==
There exists no theory elucidating correlations
among L-amino acids. If one takes, for example,
alanine, which has a small methyl group, and
phenylalanine, which has a larger benzyl group,
a simple question is in what aspect, L-alanine
resembles L-phenylalanine more than D-phenylalanine,
and what kind of mechanism causes the selection
of all L-amino acids. Because it might be
possible that alanine was L and phenylalanine
was D.
It was reported in 2004 that excess racemic
D,L-asparagine (Asn), which spontaneously
forms crystals of either isomer during recrystallization,
induces asymmetric resolution of a co-existing
racemic amino acid such as arginine (Arg),
aspartic acid (Asp), glutamine (Gln), histidine
(His), leucine (Leu), methionine (Met), phenylalanine
(Phe), serine (Ser), valine (Val), tyrosine
(Tyr), and tryptophan (Trp). The enantiomeric
excess ee = 100 ×(L-D)/(L+D) of these amino
acids was correlated almost linearly with
that of the inducer, i.e., Asn. When recrystallizations
from a mixture of 12 D,L-amino acids (Ala,
Asp, Arg, Glu, Gln, His, Leu, Met, Ser, Val,
Phe, and Tyr) and excess D,L-Asn were made,
all amino acids with the same configuration
with Asn were preferentially co-crystallized.
It was incidental whether the enrichment took
place in L- or D-Asn, however, once the selection
was made, the co-existing amino acid with
the same configuration at the α-carbon was
preferentially involved because of thermodynamic
stability in the crystal formation. The maximal
ee was reported to be 100%. Based on these
results, it is proposed that a mixture of
racemic amino acids causes spontaneous and
effective optical resolution, even if asymmetric
synthesis of a single amino acid does not
occur without an aid of an optically active
molecule.
This is the first study elucidating reasonably
the formation of chirality from racemic amino
acids with experimental evidences.
== History of term ==
This term was introduced by Kelvin in 1904,
the year that he published his Baltimore Lecture
of 1884. Kelvin used the term homochirality
as a relationship between two molecules, i.e.
two molecule are homochiral if they have the
same chirality. Recently, however, homochiral
has been used in the same sense as enantiomerically
pure. This is permitted in some journals (but
not encouraged), its meaning changing into
the preference of a process or system for
a single optical isomer in a pair of isomers
in these journals.
== See also ==
Chiral life concept - of artificially synthesizing
chiral-mirror version of life
CIP system
Stereochemistry
Pfeiffer Effect
Unsolved problems in chemistry
