Why is life left-handed? The answer is in the stars

Why is life left-handed? The answer is in the stars
A needle in a haystack? Search for the first ever biological molecule. Credit: Hubble Heritage/Flickr, CC BY-SA

While most humans are right-handed, our proteins are made up of lefty molecules. In the same way your left and right hands mirror one another, molecules can assemble in two reflected structures. Life prefers the left-handed version, which is puzzling since both mirrored types form equally in the laboratory. But a new study suggests that this may be because the star-forming cloud that created the first-ever biological molecule, before our sun was even born, made it left-handed.

In 2004, NASA's Stardust spacecraft swept through the nebulous halo surrounding a comet. What it found was the simplest of life's building blocks: the amino acid glycine. Comets are frozen remnants from the earliest days in our . Their material is therefore not made in planets, but likely originates in the natal gas cloud that formed our sun.

A research team recently recreated the freezing conditions inside such a star-forming cloud. In apparatus sealed completely from the already crisp air in the laboratory, the temperature can be brought down to -263 degrees Celsius, just ten degrees above absolute zero where even molecules stop vibrating. They believed that on the surface of dust grains suspended in this chilly gas, may have undergone a change that made it left-handed.

At the core of the glycine molecule is a carbon atom with four bonds. If two of these bonds attach to hydrogen atoms, then the molecule is symmetric and neither right nor left handed. However, swap a hydrogen for a heavier atom and this symmetry is broken. The molecule can then form two mirrored versions, giving it handedness or "chirality" as it is called in chemistry.

Why is life left-handed? The answer is in the stars
Protein molecules: just a bunch of lefties. Credit: Perhelion/wikimedia

The experiments suggest that a glycine hydrogen atom could be displaced by an atom of deuterium, which is a heavier version of hydrogen that contains an extra neutron in its nucleus, doubling its weight. It is abundant inside star-forming clouds, which is why they create many deuterium-enriched compounds, including heavy water. Once a deuterium atom has replaced a hydrogen, it is very hard to dislodge. This means that the fraction of chiral glycine steadily increases, until the main species of glycine inside the cloud shows left or right handedness.

Chiral glycine is very similar to original glycine, but with an important extra property. Laboratory experiments have shown that chiral glycine is a catalyst for other chiral molecules. That is, it promotes the production of other species with the same handedness as itself.

The result is that if glycine became a left-handed molecule, then future would also be predominantly left-handed. When life developed on Earth, it would therefore build from a pool of left-handed molecules, giving it the bias we observe today.

Pinning down glycine in space

This discovery potentially resolves another issue. While glycine is expected to be abundant inside star-forming clouds, it has never actually been observed. Individual molecules absorb different wavelengths of the starlight passing through them. Which wavelengths are absorbed depends on the atoms and their arrangement, providing a fingerprint for the presence of a particular molecule. Glycine's fingerprint has never been seen. However, these searches have been looking for the symmetric version of glycine, not its left-handed twin. If most of the glycine was left-handed, it would absorb different wavelengths and be missed.

It is an exciting idea, but many questions still remain. In the new experiment, the scientists could tell that deuterium had replaced hydrogen to form chiral glycine, but the quantities were too small to see which mirrored version had formed.

It could be that the dust grain structure favours left or right handedness. Alternatively, both types could form but one might be more easily destroyed. The answer to this would tell us if life beyond our own solar system is expected to share our left-handed bias.


Explore further

Fingerprint of dissolved glycine in the Terahertz range explained

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Jul 21, 2015
Alternatively, both types could form but one MIGHT be more easily destroyed

Herein lies the whole problem with this idea. So far all observed results of naturally forming isomers show absolutely no preference for one side or the other. Hence this whole idea flies in the face of real, true, observed and repeatable experiments and outcomes.
There just is no known way for purely non-intelligent, dead materials to self-assemble in either a fully exclusive left-handed or right-handed chain of isomers, all by themselves, without outside interference - exactly as we've seen was done in this little experiment.

Jul 21, 2015
Alternatively, both types could form but one MIGHT be more easily destroyed

Herein lies the whole problem with this idea. So far all observed results of naturally forming isomers show absolutely no preference for one side or the other. Hence this whole idea flies in the face of real, true, observed and repeatable experiments and outcomes.
There just is no known way for purely non-intelligent, dead materials to self-assemble in either a fully exclusive left-handed or right-handed chain of isomers, all by themselves, without outside interference - exactly as we've seen was done in this little experiment.

gravity and spin...

Jul 21, 2015
This comment has been removed by a moderator.

Jul 22, 2015
Besides the new mechanism to predict the observed chirality differences in the cosmic neighborhood - where L-amino acids and D-sugars have slight excesses, more likely for the reason docile describe, assuming lipid/grain interfaces in early hot and wet asteroids stand in for micelles - there isn't much news here. The title is certainly reader bait.

I don't think the mechanisms, like curved membrane (or micelle) action, that confer slight excesses, were deciding. First, cross-chiral replicases works better than a homochiral RNA world. And the tRNA/rRNA mechanism has still a two-tier chiral filter frozen in, despite that modern cells enforce homochirality by metabolism. Likely then the RNA/RNA cells were racemic, and it was first then RNA/protein cells evolved coding that homochirality was necessitated.

Possibly cell membrane selection was deciding at the time, though it is hard to envision exactly what use the difference had.

Jul 22, 2015
@Fred: "no preference".

Anyone has just to open an astrobiology book to see that you don't know anything abut the subject. We see natural differences (see my previous comment [PC]), we know many mechanisms that makes them, and we know many mechanisms that amplifies them into homochirality (an example in PC)

"no known way".

Same here, many pathways are known and actively researched on, see the Astrobiology 2015 conference which has a whole session dedicated to testing. Astrobiology is normal science now, theories (2 main ones) _and_ testing them.

The 'intelligence' of molecules - biochemistry complexity - is sufficient, we know how the evolved information in the cellular machinery can build complex, multicellular populations. Hence, since emergence involved simpler systems, we have _always_ known in modern biology that natural mechanisms were sufficient.

If you want to troll science efficiently, 1) know the subject and 2) don't propose magic alternatives. It is useless.

Jul 22, 2015
By the way, I have a 'proof' of how life emerged out of Hadean geochemical systems. But the margin is too small to contain it.

No, really, there are good pathways now, not only in the sense that there are ongoing, successful tests (i.e. accumulating non-rejections by data), but in the sense that all previous major, speculative/possible/out-of-ignorance suggested roadblocks have fallen. Chirality is not a problem but - it turned out - a boon (shorter and more efficient replicases); strand shortening doesn't happen in the proposed reactors (alkaline vents); pyruvate is a reachable _and_ key molecule (produced from CO2 in Hadean alkaline vents); gluconeogenesis/glycolysis is a non-enzymatic first pathway (occurs spontaneously in Hadean alkaline vents from pyruvate); chemiosmosis can be bootstrapped after lipid membranes evolved (but only in Hadean alkaline vents); et cetera.

From that the evidence ('proof') is simple (but long). I leave it to the reader. Tip: use evolution.

Jul 22, 2015
The amino acids involved in the synthesis of proteins in all living organisms are left handed. However in laboratory conditions with the synthesis of amino acids we always get the equal amounts of left and right handed amino acid of the same kind. On the other hand sugars used by living organisms are always right handed. It is obvious that this imaginary proto cloud can rotate simultaneously in both opposite directions. More interesting is why this fictioal proto claud is rotating according to the mythical theory of the big bang after has expanded at a speed close to the speed of light from the center to the periphery? God knows how to leave evidence for His presence. It is enough to have an honest approach the facts to see His works.

Jul 22, 2015
This comment has been removed by a moderator.

Jul 22, 2015
This comment has been removed by a moderator.

Jul 22, 2015
Alternatively, both types could form but one MIGHT be more easily destroyed

Herein lies the whole problem with this idea. So far all observed results of naturally forming isomers show absolutely no preference for one side or the other. Hence this whole idea flies in the face of real, true, observed and repeatable experiments and outcomes.
There just is no known way for purely non-intelligent, dead materials to self-assemble in either a fully exclusive left-handed or right-handed chain of isomers, all by themselves, without outside interference - exactly as we've seen was done in this little experiment.

gravity and spin...

gravity being the constant and spin, the variable...

Jul 23, 2015
Not sure why this H2O2 mirror symmetry origin of life story from last year is running again? http://phys.org/n...rth.html
also what happened to Dreiling and Gay 'weak force' model?
http://phys.org/n...ife.html

http://www.extrem...g-puzzle

Jul 25, 2015
Contrary to this article's proposal that Deuterium is the cause of molecular chirality, Earth life actually prefers to use ordinary Hydrogen instead of Deuterium, and has even been observed to "select out" deuterium when possible in cellular chemistry.

It makes sense, because a lone Proton is half the mass of Deuterium, therefore a much more efficient carrier for ion pumps in the body.

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