Homochirality

Homochirality describes a geometric property of some materials that are composed of chiral units. Chiral bodies are objects which are non-superposable on their mirror images. For example, left and right hands are chiral. A substance is said to be homochiral if all the constituent units have the same chiral form (enantiomer).

In biology, homochirality is a common property of amino acids and sugars; almost all biologically produced chiral amino acids are L-chiral, while sugars are D-chiral. While the origin of this phenomenon is not clearly understood, many different mechanism for the emergence of homochirality are proposed. One of the difficulties in determining the correct mechanism for the origin of homochirality stems from the ambiguity in its time line, and it is not clear when homochirality occurred in comparison to the origin of life.

It is unclear if 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.

Homochirality is said to evolve in three distinct steps: mirror-symmetry breaking creates a minute enantiomeric imbalance and is key to homochirality, chiral amplification is a process of enantiomeric enrichment and chiral transmission allows the transfer of chirality of one set of molecules to another.

It is also entirely 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.

Mirror-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. 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.

Chiral amplification

Phase portrait of Frank's model: starting from almost everywhere in L-D plane (except L = D line), the system approaches to one of the homochiral states

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 → 2D

while 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:

${\begin{aligned}{\frac {d[L]}{dt}}&=k_{a}[L]-k_{d}[L][D]\\{\frac {d[D]}{dt}}&=k_{a}[D]-k_{d}[L][D],\end{aligned}}$

where $k_{a}$ is the rate constant for the autocatalytic reactions, $k_{d}$ is the rate constant for mutual antagonism reaction, and the concentration of A is kept constant for simplicity. By defining the enantiomeric excess $\Phi$ as

$\Phi ={\frac {[D]-[L]}{[D]+[L]}},$

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.

${\frac {d\Phi }{dt}}=\left({\frac {2k_{d}[L][D]}{[D]+[L]}}\right)\Phi .$

Linear stability analysis of this equation shows that the racemic state $\Phi =0$ 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

Laboratory experiments exist demonstrating how in certain autocatalytic reaction systems the presence of a small amount of reaction product with enantiomeric excess at the start of the reaction can result in a much larger enantiomeric excess at the end of the reaction. In the Soai reaction, pyrimidine-5-carbaldehyde (Scheme 1) is alkylated by diisopropylzinc to the corresponding pyrimidyl alcohol. Because the initial reaction product is also an effective catalyst the reaction is autocatalytic. The presence of just 0.2 equivalent of the alcohol S-enantiomer at the start of the reaction is sufficient to amplify the enantiomeric excess to 93%.

Scheme 1. Soai autocatalysis

Another study concerns the proline catalyzed aminoxylation of propionaldehyde by nitrosobenzene (scheme 2). In this system too the presence of enantioenriched catalyst drives the reaction towards one of the two possible optical isomers.

Scheme 2. Blackmond autocatalysis

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 carbohydrates

One 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)

Chiral 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=100x(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

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

• Chirality (biology)
• Chiral life concept - of artificially synthesizing chiral-mirror version of life
• CIP system
• Stereochemistry
• Pfeiffer Effect
• Unsolved problems in chemistry

External links

• Noise-induced symmetry breaking far from equilibrium and the emergence of biological homochirality, 2017 Link
• On the Genesis of Homochirality A. Maureen Rouhi Chemical & Engineering News June 17, 2004 Link
• Observations Support Homochirality Theory Photonics TechnologyWorld November 1998 Link
• Scienceweek digest 1998 Link
• How left-handed amino acids got ahead: a demonstration of the evolution of biological homochirality in the lab Press release Imperial College London 2004 Link
• Origins of Homochirality conference in Nordita Stockholm, February 2008, talks available online [1]
• The Handedness of the Universe by Roger A Hegstrom and Dilip K Kondepudi http://quantummechanics.ucsd.edu/ph87/ScientificAmerican/Sciam/Hegstrom_The_Handedness_of_the_universe.pdf