Through the Glass Darkly
Mechanism to Explain Origin of Homochirality Questioned
My teenage daughters spend a lot of time in front of the mirror. For my girls, life without a mirror is unimaginable.
But they aren’t the only ones who feel that way. Origin-of-life researchers are obsessed with mirror images, too. But not because they have extreme grooming habits. Instead these scientists are concerned with explaining why life’s molecules only reflect one mirror image—a condition called homochirality—a defining and necessary feature of life. In a sense, life without a mirror is unimaginable.
Some molecules are mirror images of each other. Molecular mirror images result when four different chemical constituents bind to a central carbon atom. (The central carbon atom is called the chiral carbon.) These chemical groups are oriented in space in one of two possible arrangements that turn out to be reflections of each other. As mirror images, these compounds cannot be overlaid on one another so that all the chemical groups coincide in space. Because they can’t be superposed, molecular mirror images (called enantiomers) are distinct chemical entities.
Some of the compounds that play key roles as life’s building blocks, such as amino acids and the sugars deoxyribose and ribose are chiral compounds.
It turns out that the amino acids that comprise proteins and the sugars that are part of the constituents of DNA and RNA have uniform chirality, a condition biochemists call homochirality. In other words, all the amino acids in proteins have the same chirality. And all the sugars in DNA and RNA have identical chirality as well.
Homochirality is a strict requirement for life. Chirality dictates the three-dimensional positioning of chemical groups in space. And the spatial location of the chemical moieties (equal parts) plays an essential role in the interactions that stabilize the three-dimensional structure of proteins. (A protein’s structure determines its function.) As a case in point, for some proteins the incorporation of even one amino acid of the opposite mirror image into its backbone will disrupt the protein’s structure, and hence, function. Additionally, as Hugh Ross and I point out in Origins of Life, laboratory experiments demonstrate that the “wrong” enantiomeric form of a nucleotide inhibits the formation of DNA and RNA assembly.
Homochirality and the Origin of Life
In order to adequately explain the spontaneous emergence of life, origin-of-life researchers have to account for the origin of homochirality. This is no easy feat. Up until now, these scientists have been looking through a dark glass. While numerous proposed explanations for the genesis of homochirality have been advanced, none seem compelling and most are riddled with problems. (For a detailed discussion of some of the difficulties researchers encounter in their attempts to explain the origin of homochirality, see Origins of Life.
Part of this challenge stems from the fact that chemical reactions that generate chiral compounds from achiral starting materials produce a 50:50 mixture of both mirror images. (This type of mixture is called racemic.) In other words, chemical processes, as a rule of thumb, do not yield homochiral products—unless a chiral excess already exists at the outset for one of the reactants or the reaction catalyst.
In the midst of the difficulties, a new discovery has been touted by some origin-of-life researchers as the breakthrough needed to explain the origin of homochirality. A recent chemical analysis of a meteorite recovered in Antarctica suggests that materials delivered to early Earth from extraterrestrial sources may provide an answer. These ingredients may have seeded chemical processes yielding a homochiral surplus in prebiotic molecules which, in turn, spawned the homochiral conditions needed for life.
Homochiral Compounds and the Murchison Meteorite
In 1969, a meteorite fell in Murchison, Australia. Chemical analysis of this meteorite fueled excitement within the origin-of-life research community. Scientists look to the Murchison, and meteorites like it, as a proxy for the chemistry operating on the early Earth, since it’s a remnant from the time that the solar system formed. Chemists found organic compounds, like amino acids, in Murchison that are reminiscent of those formed in the famed Miller-Urey experiment.
It turns out that some of the amino acids in the Murchison meteorite display a slight chiral excess, suggesting that either a chemical or physical mechanism exists that is capable of generating chiral enrichment. Origin-of-life researchers view these results with ambiguity, however, since the Murchison meteorite suffers from terrestrial contamination, although the nitrogen isotopic fingerprint of the chirally-enriched amino acids suggests they are endogenous to the meteorite.
The GRA 95229 Meteorite
Researchers from Arizona State University and Brown University recently recognized that GRA 95229, a meteorite recovered in the Graves Nunataks ice fields of Antarctica, offers an opportunity to resolve some of the uncertainty surrounding the studies of the Murchison amino acids. Scientists regard this meteorite as pristine, experiencing little, if any, contamination from the terrestrial environment after it fell to Earth. The meteorite is a carbonaceous chondrite meteorite, the same class as the Murchison.
The research team recovered a number of organic compounds from GRA 95229, including amino acids. These amino acids appear to be native to the meteorite, since biologically relevant amino acids were found to be racemic and their deuterium fingerprint indicated an extraterrestrial origin.
The Genesis of Homochirality?
The team from ASU and BU also noted that one of the amino acids found in the meteorite, (isoleucine, a biologically-significant amino acid) displays a 14% enantiomeric excess. This result indicates that some mechanism must exist that favors the production of one mirror image over the other and may portend the advent of homochirality. The scientists proposed that the aldehyde precursor to the isoleucine must have already possessed a chiral excess that was transferred to the amino acid during isoleucine’s production. They note that aldehydes have been detected in interstellar space. Accordingly, circularly polarized radiation impinging on these aldehydes could selectively destroy one enantiomer, creating a chiral excess.
Once a chiral excess exists within amino acids, it could conceivably create a chiral excess in sugars like ribose and deoxyribose. Previous work demonstrates that reactions which generate sugars from simple organic starting materials, such as formaldehyde (likely to be present on early Earth), can be catalyzed by amino acids. When these amino acids possess a chiral excess, the amino acid catalyst creates a chiral surplus in the sugars produced by the reaction.
Though seemingly plausible, this proposal raises significant concerns upon closer inspection. For example, no astronomical source of circularly polarized UV radiation has ever been observed. Additionally, laboratory experiments have demonstrated that circularly polarized UV radiation destroys both enantiomers too rapidly for a chiral excess to be established. These two problems make it unlikely that the source of chiral excess in isoleucine comes from an aldehyde precursor.
Yet, a chiral excess of isoleucine exists in GRA 95229, indicating that some mechanism must produce it. But still it is questionable if this relatively low level of chiral excess in isoleucine can explain the origin of homochirality. A 14% surplus of one enantiomer is a far cry from the 100% required for living systems.
It is also questionable if the transfer of the chiral excess from amino acids to sugars can serve as a sufficient explanation for homochirality. It’s not clear if this reaction, successfully conducted in a laboratory under exacting conditions, would operate on early Earth. Even if it did, the extent of chiral excess generated in sugars is limited to about 10%. Again, a long way from the 100% required for life. Lab work shows that only a 10% chiral excess is generated in sugars if the amino acid catalyst possesses a 100% surplus of one enantiomer. Once the level of enantiomeric excess falls to about 15% for the amino acid catalyst (the level observed for isoleucine in GRA 95229), the chiral excess generated in the sugars plummets to around 1%.
The bottom line: The detection of a slight enantiomeric excess of isoluecine in the GRA 95229 meteorite appears to be an unequivocal finding that requires an explanation. Though interesting, the discovery of the slight chiral excess of isoleucine in GRA 95229 provides little, if any, reason to believe that the scientific community is on their way to explaining the origin of homochirality.
It looks like origin-of-life researchers need to spend a little more time in front of the mirror.