How Life Shatters Chemistry’s Mirror
Amino acids, sugars and other chiral molecules come in two varieties that are mirror images of each other.
All of us are left-handed. At least, the bits that make up our proteins are. This is surprising since Nature is predominantly ambidextrous when it comes to assembling these molecules from scratch. Some scientists argue that left-handedness was handed down to us from space, but others say it just happened by chance in some little nook on the primordial Earth.
Handedness, or as it’s technically called, "chirality," occurs in certain molecules that come in two varieties that are mirror images of each other, like right- and left-handed gloves.
In living creatures, amino acids – the building blocks of proteins – are almost exclusively left-handed. Whereas, the sugars that make up the helical backbone of DNA and RNA are all right-handed. (Scientists tend to focus on the left-handed amino acids because they are easier to analyze than the right-handed sugars.)
Life’s exclusive use of one hand over the other is referred to as homochirality. Its origin is a mystery, since when chemists brew up amino acids and sugars in the lab the results are an equal mixture of both hands (unless special chemical templates are used).
Finding out where homochirality comes from could tell us which chemical reactions sparked the creation of life on our planet. It might also tell us what we should expect from life on other planets. Will they be left-handed just like us?
The only thing that distinguishes the two hands of a chiral molecule is the shape, but that can lead to very different chemical behaviors. For example, the right-handed version of the molecule carvone smells to us humans like caraway seeds, but the left-handed one has a peppermint odor.
The sugars that make up the helical backbone of DNA and RNA are all right-handed.
The distinction is not always so benign, however. In the case of the drug thalidomide, the left-handed form helps relieve morning sickness, while the right-handed form induces birth defects.
Indeed, drug companies spend more than $100 billion per year to ensure that drugs contain pure chiral molecules, says Robert Hazen of the Carnegie Institution of Washington.
"Chirality is an essential, diagnostic characteristic of cellular life. But how did this selectivity emerge in the seemingly random prebiotic world?" Hazen wonders.
Louis Pasteur was the first to recognize the handedness of biomolecules more than 150 years ago. Since then, many different theories have been put forth to try to explain its emergence.
Some scientists have suspected that homochirality was a by-product of life. In this case, the first organisms would have been a mixture of different handed molecules, but at some point being purely left-handed afforded some evolutionary advantage.
However, most scientists now believe that homochirality is a precondition for life. They say that the first complex molecular structures (e.g. the sugar-based helices of DNA and the amino acid sheets found in proteins) could only have emerged from a chemical soup in which one hand dominated.
"If you pulled one hundred gloves from a drawer with only left-handed or only right-handed gloves and arranged them in sequence, you would get a well-defined structure with the homochiral ones," says chemist Ronald Breslow from Columbia University.
An excess of left-handed amino acids has been found in a few meteorites, including the Murchison meteorite, which landed in Australia in 1969.
But you’d get a terrible mess, he says, if you tried to arrange the same sequence from a drawer with both left- and right-handed gloves randomly mixed together.
This implies that the origin of homochirality came before the origin of life. Uncovering how Nature purified the handedness of life’s raw ingredients should bring us closer to understanding how those ingredients came together to form life.
Some researchers contend that living things were destined to be left-handed. The deck was stacked from the beginning.
The reason is that particle physics is left-handed in its own way. The electrons emitted during certain radioactive decays all spin in a left-handed fashion. The force that controls these decays, the so-called weak force, is thought to have an influence on the formation of biomolecules.
However, the effect is extremely tiny: the predicted excess of left-handed amino acids due to the weak force is less than one out of a million billion.
"The energies involved in any global-scale or universal-scale chiral influence are very small," Hazen says.
Hazen and others suspect that much greater influences work on the local scale to tip the hand of chemistry in one direction or another.
Although chemical reactions tend to make left and right handed molecules in equal amounts, there are autocatalysis could have occurred in various small pools or puddles on the primordial Earth. Some spots would have ended up predominantly left-handed, while others would have been more right-handed.
The right-handed version of carvone smells like caraway seeds, but the left-handed one has a peppermint odor.
Credit: National Museum of American History
A similar idea is that left and right handed molecules were segregated into separate "micro-pools" by mineral surfaces. Hazen and his colleagues have shown that calcite – a common crystal found in a wide range of rock types – is chiral selective. In other words, some calcite surfaces adsorb predominantly left-handed amino acids, while others favor right-handed ones.
"In the process, we find the molecules are selected, concentrated and organized on the surfaces in ways that might lead to additional molecular assembly," Hazen says.Although Hazen’s team has yet to show it, amino acids may be able to bond together on crystal faces as a first step towards protein formation. If this is the case, then life may have emerged from a single left-handed crystal face.
Assuming this localized chemistry is the whole story, then possible life on Mars or other nearby worlds will just as likely be right-handed as left.
However, homochirality may not be a complete crap shoot. It could have been fixed in space and dropped down by meteorites.
The evidence for this is in a small number of meteorites found to have an excess of left-handed amino acids over right-handed ones.
"We’re still in the dark how it happened," says Sandra Pizzarello from Arizona State University, who has analyzed many of the meteorite samples.
This is an artist’s concept of asteroids delivering amino acids to Earth. The jagged white line at the bottom of the image is the actual data from the analysis of the Murchison meteorite. The two largest peaks are the amounts of right-handed and left-handed versions of the amino acid isovaline. Note that the highest of these two peaks is the amount of left-handed isovaline, revealing an excess of the left-handed variety in the meteorite.
Credit: NASA/Mary Pat Hrybyk-Keith
The most popular notion is that amino acids formed on interstellar dust grains that were subsequently exposed to circularly polarized light, which destroyed more of the right-handed than left-handed amino acids. The problem is finding a source of circularly polarized light (candidates include neutron stars and molecular clouds).
Whatever the cause of the left-handed excess, the fact that the meteorites match the chiral preference of life on our planet cannot be a mere coincidence, according to many researchers.
"Various other theories [for the origin of homochirality] are certainly less attractive once we know that chirality is being delivered to Earth by some meteorites," Breslow says.
One concern, however, is that the amino acids in the meteorites are not the ones used by living things. To address this, Breslow and his colleagues have recently put together a plausible storyline for how the uncommon amino acids delivered by the meteorites would mix with the raw ingredients on Earth to form the common amino acids that led to life.
Through these meteorite seeds, the left-handedness in space "spread" to Earth’s primordial soup.
"We filled in the blanks that made it clear how the meteorite materials could transfer their chirality to normal amino acids," Breslow says.
The left-handed excess in the meteorites is small – around 10 percent – but there are ways to amplify this through evaporation and crystallization to create a homochiral environment.
"We assume that the potential for life to arise is widespread, but only where homochirality is present – near the meteorite sites – would it be successful," Breslow says. "Of course, the early Earth was heavily bombarded, so such sites could be widespread."
If life got started around a meteorite impact, that could tell us what were the early conditions and the likely ingredients for life’s big debut on our planet.
And since the whole solar system would have been "seeded" by the same meteorites, any life on other planets or moons should also be left-handed.
But around distant stars, the chiral mechanisms may have been reversed (e.g. the polarized light may have been in a different direction), so any aliens in these systems will likely be right-handed.
It will be a long, long time before we can test this out.