Symmetry breaking Theories for the origin of homochirality in the molecules of life can be classified as deterministic or based on chance depending on their proposed mechanism. If there is a relationship between cause and effectthat is, a specific chiral field or influence causing the mirror symmetry breakingthe theory is classified as deterministic; otherwise it is classified as a theory based on chance (in the sense of randomness) mechanisms.
Deterministic theories Deterministic theories can be divided into two subgroups: if the initial chiral influence took place in a specific space or time location (averaging zero over large enough areas of observation or periods of time), the theory is classified as local deterministic; if the chiral influence is permanent at the time the chiral selection occurred, then it is classified as universal deterministic. The classification groups for local determinist theories and theories based on chance mechanisms can overlap. Even if an external chiral influence produced the initial chiral imbalance in a deterministic way, the outcome sign could be random since the external chiral influence has its enantiomeric counterpart elsewhere. In deterministic theories, the enantiomeric imbalance is created due to an external chiral field or influence, and the ultimate sign imprinted in biomolecules will be due to it. Deterministic mechanisms for the production of non-racemic mixtures from racemic starting materials include: asymmetric physical laws, such as the
electroweak interaction (via cosmic rays) or asymmetric environments, such as those caused by
circularly polarized light (CPL),
quartz crystals, or the Earth's rotation, β-Radiolysis or the magnetochiral effect. Shortwave circularly polarized light, for example, can induce enantiomeric bias because chiral molecules will preferentially absorb either the right-handed or left-handed CPL.
Parity Violation Parity, a physical property of symmetry, is conserved for strong, electromagnetic, and gravitational interactions. However, experimental work involving weak forces, such as the interactions between subatomic particles, found that parity is not conserved in these interactions (see
Parity Violation). This theoretically would result in slight energy differences between mirror-image enantiomers of a single compound, which could cause a small stereochemical bias for the lower-energy enantiomer. 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. The amino acids in asteroids
Bennu and
Ryugu show no chiral bias.The
Murchinson meteorite contained only racemic amino acids.
Amplification Most mechanisms of symmetry breaking focus on amplification of an initial stochastic
enantiomeric excess. The most likely path for this amplification step is by
asymmetric autocatalysis. An autocatalytic chemical reaction is that in which the reaction product is itself a reactive, in other words, a chemical reaction is autocatalytic if the reaction product is itself the catalyst of the reaction. In asymmetric autocatalysis, the catalyst is a chiral molecule, which means that a chiral molecule is catalyzing 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.
Theory 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 (L=0 or D=0). 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 :\begin{align} A + L \xrightarrow{k_a} 2L,\\ A + D \xrightarrow{k_a} 2D, \end{align} while suppressing each other through a reaction that he called
mutual antagonism \begin{align} L + D \xrightarrow{k_d} \varnothing.\\ \end{align} 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{align} \frac{d[\ce L]}{dt} &= k_a [\ce A] [\ce L] - k_d \ce{[L] [D]}\\ \frac{d[\ce D]}{dt} &= k_a [\ce A] [\ce D] - k_d \ce{[L] [D]}, \end{align} 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. The analytical solutions for are found to be [L]/[D] = [L]_0/[D]_0\,e^{{kd([L]_0-[D]_0)(e^{k_a t}-1)}} . The ratio [L]/[D] increases at a more than exponential rate if ([L]_0-[D]_0) is positive (and vice versa). Every starting conditions different to [L]_0 = [D]_0 lead to one of the asymptotes [L] = 0 or [D] = 0. Thus the equality of [L]_0 and [D]_0 and so of [L] and [D] represents a condition of unstable equilibrium, this result depending on the presence of the term representing mutual antagonism. By defining the enantiomeric excess ee as :ee = \frac{[\ce D] - [\ce L]}{[\ce D] + [\ce L]}, the rate of change of enantiomeric excess can be calculated using
chain rule from the rate of change of the concentrations of enantiomers L and D. : \frac{d(ee)}{dt} = \left(\frac{2k_d \ce{[L] [D]}}{[\ce D] + [\ce L]}\right)ee. Linear stability analysis of this equation shows that the racemic state ee = 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 homochirality, and the presence of the mutually antagonistic relationship between the two enantiomers is necessary for the instability of the racemic mixture. Homochirality could be achieved from autocatalysis in the absence of the mutually antagonistic relationship, but the underlying mechanism for symmetry-breaking is different.
Experiments A small amount of one enantiomer at the start of a reaction can lead to a large enantioenrichment in 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 can catalyze the production of more of that same enantiomer. For the
proline catalyzed
aminoxylation of
propionaldehyde by
nitrosobenzene, a slight enantiomeric excess of catalyst leads to a large enantiomeric excess of product.
Serine octamer clusters containing 8 serine molecules appear in mass spectrometry. No evidence supports such clusters existing under non-ionizing conditions, however. Fractional
sublimation of a 10% enantioenriched sample of
leucine results in up to 82% enrichment in the sublimate. Partial sublimation processes could in principle 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 Sodium chlorate, which crystallizes in a chiral habit, can deposit almost single enantiomers of crystals when evaporating solutions are stirred. This effect, illustrates the role of seed crystals in
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 Once a significant enantiomeric enrichment has been produced in a single biomolecule or biological class of molecules in a system, the transference of chirality through the entire system is possible. This last step is known as the chiral transmission or propagation step. Independently achieving homochirality in every biomolecule (e.g., creating significant enantiomeric enrichment or complete homochirality for all 19 chiral amino acids separately) would be statistically improbable for compounds with different physical and chemical properties and has not yet been experimentally demonstrated. Stereoselective pressure from one biomolecule or biological class to others would eliminate the need to supply all prebiotically-relevant biological precursors in their enantiopure form. Some proposed models for the transmission of chiral asymmetry are polymerization, epimerization or copolymerization. Experimental work has demonstrated that enantiomerically enriched amino acids could assert chiral pressure on sugars and RNA precursors, and vice versa. For example, laboratory experiments demonstrated enantioenrichment of the 3-carbon sugar D-glyceraldehyde from a racemic solution via the interaction of L-proline-valine dipeptide.
The hypothesized central dogma of biological homochirality Source: Common criticisms of previously proposed mechanisms of symmetry breaking, amplification, or transmission include that they only induce an enantiomeric excess in one class of biological compounds, that the induced enantiomeric excess is not high enough or cannot persist for long enough for full homochirality to be achieved, or that the mechanism is not plausible under prebiotic conditions on the early Earth. In the early 2020s, a framework for achieving homochirality across all major biological molecule classes was proposed. Over three decades later, the abiotic synthesis of RAO was achieved from the reaction of cyanamide and the simple 2- and 3-carbon compounds glycoaldehyde and glyceraldehyde, a demonstration of prebiotically feasible
cyanosulfidic chemistry.
Symmetry-breaking and chiral amplification of ribose aminooxazoline (RAO) The chirality of the ribose sugar in RAO is conserved in its transformation into ribonucleotides, so obtaining RAO in its enantiopure form could lead to the formation of homochiral RNA. Experimental work shows that enantiopure RAO could be achieved via the interaction of a racemic mixture of RAO and a
spin-polarized magnetic surface, such as the surface of the prebiotically abundant mineral
magnetite, due to the
chirality-induced spin selectivity (CISS) effect and the unique conglomerate crystallization properties of RAO (i.e. its tendency to crystallize as enantiopure crystals). This process explains the observation that an enantiopure layer of chiral molecules can selectively filter for electrons of a particular spin. Some early theories of biological homochirality in the 1950s suggested that homochiral biopolymers of both enantiomeric forms (e.g., both D-RNA and L-RNA) could have been present in the prebiotic Earth, and that polymers of the canonical enantiomeric forms were selected for due to improved chemical function. As an extension of this idea, it was also proposed that lifeforms containing both enantiomeric forms of molecular machinery could have arisen independently, and that evolution by
natural selection eventually led to the complete dominance of one enantiomeric lifeform. This purely biotic theory is difficult to assess experimentally, and is not widely accepted or discussed in the scientific community. ==Optical resolution in racemic amino acids==