Researchers find a clue to how life turned left

Researchers find a clue to how life turned left
This is an artist's concept of excess left-hand aspartic acid created in asteroids and delivered to Earth via meteorite impacts. The line at the bottom is a chromatogram showing that left-hand aspartic acid (tall peak in the center, with diagram of left-hand aspartic acid molecule on top) was four times more abundant in the meteorite sample than right-hand aspartic acid (smaller peak to the left, with right-handed aspartic acid molecule on top). Credit: NASA/Hrybyk-Keith, Mary P.

Researchers analyzing meteorite fragments that fell on a frozen lake in Canada have developed an explanation for the origin of life's handedness – why living things only use molecules with specific orientations. The work also gave the strongest evidence to date that liquid water inside an asteroid leads to a strong preference of left-handed over right-handed forms of some common protein amino acids in meteorites. The result makes the search for extraterrestrial life more challenging.

"Our analysis of the in meteorite fragments from Tagish Lake gave us one possible explanation for why all known life uses only left-handed versions of amino acids to build proteins," said Dr. Daniel Glavin of NASA's Goddard Space Flight Center in Greenbelt, Md. Glavin is lead author of a paper on this research to be published in the journal Meteoritics and Planetary Science.

In January, 2000, a large meteoroid exploded in the atmosphere over northern British Columbia, Canada, and rained fragments across the frozen surface of Tagish Lake. Because many people witnessed the fireball, pieces were collected within days and kept preserved in their frozen state. This ensured that there was very little contamination from terrestrial life. "The Tagish Lake meteorite continues to reveal more secrets about the early Solar System the more we investigate it," said Dr. Christopher Herd of the University of Alberta, Edmonton, Canada, a co-author on the paper who provided samples of the Tagish Lake meteorite for the team to analyze. "This latest study gives us a glimpse into the role that water percolating through asteroids must have played in making the left-handed amino acids that are so characteristic of all life on Earth."

Proteins are the workhorse molecules of life, used in everything from structures like hair to enzymes, the catalysts that speed up or regulate chemical reactions. Just as the 26 letters of the alphabet are arranged in limitless combinations to make words, life uses 20 different amino acids in a huge variety of arrangements to build millions of different proteins. Amino acid molecules can be built in two ways that are mirror images of each other, like your hands. Although life based on right-handed amino acids would presumably work fine, they can't be mixed. "Synthetic proteins created using a mix of left- and right-handed amino acids just don't work," says Dr. Jason Dworkin of NASA Goddard, co-author of the study and head of the Goddard Astrobiology Analytical Laboratory, where the analysis was performed.

Since life can't function with a mix of left- and right-handed amino acids, researchers want to know how life – at least, life on Earth -- got set up with the left-handed ones. "The handedness observed in biological molecules – left-handed amino acids and right-handed sugars – is a property important for molecular recognition processes and is thought to be a prerequisite for life," said Dworkin. All ordinary methods of synthetically creating amino acids result in equal mixtures of left- and right-handed amino acids. Therefore, how the nearly exclusive production of one hand of such molecules arose from what were presumably equal mixtures of left and right molecules in a prebiotic world has been an area of intensive research. The team ground up samples of the Tagish Lake meteorites, mixed them into a hot-water solution, then separated and identified the molecules in them using a liquid chromatograph mass spectrometer. "We discovered that the samples had about four times as many left-handed versions of aspartic acid as the opposite hand," says Glavin. Aspartic acid is an amino acid used in every enzyme in the human body. It is also used to make the sugar substitute Aspartame. "Interestingly, the same meteorite sample showed only a slight left-hand excess (no more than eight percent) for alanine, another amino acid used by life."

"At first, this made no sense, because if these amino acids came from contamination by terrestrial life, both amino acids should have large left-handed excesses, because both are common in biology," says Glavin. "However, a large left-hand excess in one and not the other tells us that they were not created by life but instead were made inside the Tagish Lake asteroid." The team confirmed that the amino acids were probably created in space using isotope analysis.

Isotopes are versions of an element with different masses; for example, carbon 13 is a heavier, and less common, variety of carbon. Since the chemistry of life prefers lighter isotopes, amino acids enriched in the heavier carbon 13 were likely created in space.

"We found that the aspartic acid and alanine in our Tagish Lake samples were highly enriched in carbon 13, indicating they were probably created by non-biological processes in the parent asteroid," said Dr. Jamie Elsila of NASA Goddard, a co-author on the paper who performed the isotopic analysis. This is the first time that carbon isotope measurements have been reported for these amino acids in Tagish Lake. The carbon 13 enrichment, combined with the large left-hand excess in aspartic acid but not in alanine, provides very strong evidence that some left-handed proteinogenic amino acids -- ones used by life to make proteins -- can be produced in excess in asteroids, according to the team.

Some have argued that left-handed amino acid excesses in meteorites were formed by exposure to polarized radiation in the solar nebula – the cloud of gas and dust from which asteroids, and eventually the Solar System, were formed. However, in this case, the left-hand aspartic acid excesses are so large that they cannot be explained by polarized radiation alone. The team believes that another process is required.

Additionally, the large left-hand excess in aspartic acid but not in alanine gave the team a critical clue as to how these amino acids could have been made inside the asteroid, and therefore how a large left-hand excess could arise before life originated on Earth.

"One thing that jumped out at me was that alanine and aspartic acid can crystallize differently when you have mixtures of both left-handed and right-handed molecules," said Dr. Aaron Burton, a NASA Postdoctoral Program Fellow at NASA Goddard and a co-author on the study. "This led us to find several studies where researchers have exploited the crystallization behavior of molecules like aspartic acid to get left-handed or right-handed excesses. Because alanine forms different kinds of crystals, these same processes would produce equal amounts of left- and right-handed alanine. We need to do some more experiments, but this explanation has the potential to explain what we see in the Tagish Lake meteorite and other meteorites."

The team believes a small initial left-hand excess could get amplified by crystallization and dissolution from a saturated solution with liquid water. Some amino acids, like aspartic acid, have a shape that lets them fit together in a pure crystal – one comprised of just left-handed or right-handed molecules. For these amino acids, a small initial left- or right-hand excess could become greatly amplified at the expense of the opposite-handed crystals, similar to the way a large snowball gathers more snow and gets bigger more rapidly when rolled downhill than a small one. Other amino acids, like alanine, have a shape that prefers to join together with their mirror image to make a crystal, so these crystals are comprised of equal numbers of left- and right-handed molecules. As these "hybrid" crystals grow, any small initial excess would tend to be washed out for these amino acids. A requirement for both of these processes is a way to convert left-handed to right-handed molecules, and vice-versa, while they are dissolved in the solution.

This process only amplifies a small excess that already exists. Perhaps a tiny initial left-hand excess was created by conditions in the solar nebula. For example, polarized ultraviolet light or other types of radiation from nearby stars might favor the creation of left-handed amino acids or the destruction of right-handed ones, according to the team. This initial left-hand excess could then get amplified in asteroids by processes like crystallization. Impacts from asteroids and meteorites could deliver this material to Earth, and left-handed amino acids might have been incorporated into emerging life due to their greater abundance, according to the team. Also, similar enrichments of left-handed amino acids by crystallization could have occurred on Earth in ancient sediments that had water flowing through them, such as the bottoms of rivers, lakes, or seas, according to the team.

The result complicates the search for extraterrestrial life – like microbial hypothesized to dwell beneath the surface of Mars, for example. "Since it appears a non-biological process can create a left-hand excess in some kinds of amino acids, we can't use such an excess alone as proof of biological activity," says Glavin.

Explore further

More asteroids could have made life's ingredients

Citation: Researchers find a clue to how life turned left (2012, July 25) retrieved 22 August 2019 from
This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.

Feedback to editors

User comments

Jul 25, 2012
The 'bottom line' is that non-terrestrial amino acids were evidently synthesized in a chiral environment.
The implication is that the prevailing conditions (temperature, pressure, elemental abundance, density, etc.) were associated with a charged fluid moving through a strong magnetic field.

Jul 25, 2012
Nice graphic. All it needs is a big string of 101010100101010001010011101 streaking out of space into the planet and it will be the most internet sciency science graphic ever produced by the hand of man.

Jul 25, 2012
There are many known pathways to enantiomer excess, like the one Glavin's et al has tested. But while it isn't necessary to know how and when this excess arose, it is interesting and contribute knowledge.

There was always a slight left handed amino acid excess in meteorites originating in our system (and no other have been found yet), but they were always to small to tie to our own LH AAs. Now they are so large that they mess with our ability to identify fossils elsewhere. Who ordered that? =D

An article giving more context to the wide extension of the original LH AA finds Glavin et al have done, and how they have tested the excess pathway theory, is here: http://www.nasa.g...ids.html .

Jul 25, 2012
Creationists shouldn't comment on science, it is hilarious.

As here for example, when a symmetry broken chiral environment contains _less_ information than a symmetrical one, in the same way that noise necessarily contains more information on signal amplitudes than an ordered wave do. Information is relative a physical system, it doesn't mean anything by itself. It wouldn't even be 'sciency'.

The whole point was that this came out of a natural process that happens way before the process of chemical to biological evolution continued on a terrestrial.

@ tadchem:

That sounds like an EU idea.

More problematic for you, the research contradicts what you say by the found mechanisms.

The initial enantiomer excess had to happen in the gas nebula, which is why they suggest well known chiral phenomena as synchrotron radiation from rotational jets. It is there and it will suffice.

The enhanced excess happens spontaneously by a water based alteration process without any chiral input.

Jul 25, 2012
i thought they were going to talk about the post-McCarthy liberalization of America and the pervasive media bias for leftist politics. Boy, was I wrong!

Jul 25, 2012
Not that this discovery has necessarily anything to do with life whatsoever, but it does support an argument I made just a few hours ago, which is that chirality is a creative bias, and NOT an evolutionary or selective bias.

It's quite a different thing to go from amino acids in an asteroid to an actual life form.

Spare us the infinite monkey nonsense.

As here for example, when a symmetry broken chiral environment contains _less_ information than a symmetrical one, in the same way that noise necessarily contains more information on signal amplitudes than an ordered wave do. Information is relative a physical system, it doesn't mean anything by itself.

Not true, necessarily.

Bias is itself a form of information, and your analogy is a poor one anyway.

If you were creating an ecosystem you would want maximum modular compatibility of materials and compounds across all life forms in order to maintain maximum recyclability. continues.

Jul 25, 2012
Examples of creative modular bias:

Screws, nuts, bolts, etc, all turn right with only some very, very rare exceptions, in order to maintain maximum reusability and modularity. This is not an accident, but is by human intelligent design for both convenience and practicality.

If there was not an intelligently designed, modular chirality bias in these fasteners and other tools and machines, then buying replacement parts from a hardware store or auto parts dealer would be at least twice as complicated, and as such would produce wrong parts purchase errors far more often, likely as much as half the time...

Chirality bias is a creation bias; intelligently designed, we may certainly add, particularly in the case of life.

Jul 25, 2012
I think they could be incorrect that the excess left handed molecules caused by polarised radiation, (perhaps from a super nova explosion) is not enough to account for the handedness in life. The reasoning is the same as why life prefers left handed sugar, that is, there was enough of an excess of handed molecules so that life (meaning early prokaryote cells) began with handedness and as "all life is one", it is only natural that all life has pretty much the same handedness, in sugars and proteins.

Jul 26, 2012
With America being destroyed by Conservatism, who can blame it?
I thought America's president was considered a "liberal"?

Jul 26, 2012
America's president is considered Liberal. But Americans are a pretty stupid bunch and immediately elected a Republican congress that refuses to act upon virtually any of the president's plans. Those Republicans. then insist that he be removed from office because his unimplemented plans aren't working.

I was going to say something much the same.

Also, the Republicans deceived the public on the nature of their platform, as always. The Tea Party claimed to be about helping average people, but really they were mostly about helping the top 1% evade taxes. The Republican presidential candidates were exposed as anti-social extremists and tyrants.

Now perhaps some candidates were caught off guard by some difficult questions that nobody can quite answer fully at this point, but some quite remarkably anti-social answers were given, and as the good book says, "Out of the abundance of the heart the mouth speaks."

Jul 26, 2012
Question: From what I gather from the article some amino acids like to form crystals with one type of handedness while others may forms crystals more easily if there's an equal mix. So far, so good.

But somehow the article seems to suggest that for the ones that form crystals from just one type the (e.g.) left handed crystal is more likely than the right handed crystal (doesn't say so in the article directly but the there are allusions to why one type of chirality is preferred in living things).

That strikes me as odd. Sure, if you start of with a left handed molecule you're more likley to get a left handed crystal - but there's no selection mechanism for thatinitial amino acid's handedness, is there? So it's still left up to chance whether life started left handed or right handed

(Also: individual crystals in the sample should be randomly left and right handed - so an average analysis should still not give you an excess).

What am I missing here?

Jul 26, 2012
IMO the homochirality of life doesn't require any ad-hoced explanation based on panspermia event - it just follows from the fact, the chiral molecules are adsorbing at the highly curved surfaces (micelles) with different speed at the moment, when they're hydrophobic or hydrophilic at the same moment. It's classical "lotus leaf" effect combined with stereoisomer geometry, which explains too, why hydrophilic molecules (sugars) are attracted to the opposite side of cells, like the somehow more hydrophobic molecules (aminoacids and proteins). The "polarized nebulae light" theory doesn't explain this duality. It means, the separation of optically active molecules at the curved surfaces of micelles would occur anytime later, even during lab experiments.

Jul 26, 2012
@ Lurker2358:

"the infinite monkey".

Why do you discuss religion on a science site? What does your religious expression even mean?

But as long as we are on the subject of nature, is is creationists that prefer infinite creation chains, as the insist on the necessity of creative agents. Natural processes have no problem with either physics causality nor eternity - the standard cosmology is a process resulting in an eternal universe.

Hence creationism isn't about facts of nature but about created fairy tales.

Jul 26, 2012

"your analogy is a poor one anyway".

It isn't just an analogy, I was alluding to Kolmogorov complexity which is measurable. [ http://en.wikiped...mplexity ]

KC is maximized for noise, since you can't compress a string describing its random amplitudes as much as a regular string.

Similarly a symmetrical string (describing chemical racemic solutions) can be compressed by a factor of 2 by mirroring it, while an asymmetrical string (describing chiral solutions) can't.

Isn't it hilarious how creationist woo includes throwing in "information", while they haven't informed themselves on what it is? =D

Jul 26, 2012
@ elektron: Read the article (and perhaps my previous comments) on the proposed stages again. Their staging of mechanisms is enough, even if they don't know the exact mechanism for the first stage. It is known from lab work that crystal growth constraint can amplify small chiral excess (on the order of 10^-5 IIRC), and that synchrotron radiation can result in such small excess due to stereochemistry.

@ antalias, VD:

I don't see the bias [sic] of the article. It suggest a lefthand excess for our system, but they also discuss that "vice-versa" excess would work.

The left-hand excess of meteorites is known before albeit not as convincingly strong as here. And it happens to correlate with our amino acid handedness bias. Note that nucleotides and most hydrocarbons have the reverse bias, and that the extended aa alphabet (metylation and what not) can often be righthanded because such compounds can have a biological effect. (Remember the drug sideeffects of early racemic compounds.

Jul 26, 2012
We could illustrate the above explanation with following analogy: the molecules are behaving like an insect with four legs arranged into vortices of tetrahedron. One of its leg is oiled and as such hydrophobic and the another ones are hydrophilic. To give a chirality to such a molecule, we can imagine, the whole body of insect is hydrophilic (sugar molecule) or hydrophobic (aminoacids molecules). The hydrophilic molecules would climb more easily around positively curved hydrophilic surface represented with sphere (biological cell formed with lipidic bilayer), where the higher number of hydrophilic parts of its body would get into contact with the surface of sphere. Whereas the hydrophobic molecules would prefer the internal negatively curved surface of the sphere from the same reason. For hydrophilic molecules it would be advantageous to have oiled leg oriented outside of sphere, for second ones toward center of sphere. Their separation would increase with decreasing radius of the cell

Jul 26, 2012
@ Satene:

Agreed, chirality can turn up later. But it is now very suggestive with the correlation between primordial and extant biochemistry because of the large exogenously delivered excess. As they not, it can also mean chirality was early indigenously as well.

Can you explain in more detail how chirality appears in your model? I am curious if one needs an already chiral membrane, because phylogenetic studies of membrane metabolism turns up the same as similar studies on carbon fixation metabolism, an early robust dual pathway mechanism. In the case of membranes it means they likely were non-stereospecific up to the non-diversified DNA LUCA. ["Ancestral lipid biosynthesis and early membrane evolution", Pereto et al, TRENDS in Biochem Sci, 2004]

[Edit: And so you just did. Seems non-stereospecific membranes would work. But I don't see the chiral separation; I'll have to think about that.]

Jul 26, 2012
Even better would be if you have refs, for the stereochemistry challenged.

Jul 26, 2012
The left-hand excess of meteorites is known before

Yes, but there's not one aspartic acid crystal in these (and past) asteroid samples but many. Getting an 80:20 result (as reported in the article) seems an incredible outlier. The more crystals you analyze the more you should get an overall 50/50 result.

So I think we're dealing with one of several scenarios here:
1) Left and right handedness is not equally likely for some molecules due to some special circumstances of production in space - which we haven't figured out yet (on Earth you get 50/50 if you make them artificially)

2) The stuff on asteroids is not the result of 'artificial' creation but the remainders of a selective mechanism (e.g. life...which seems a bit far fetched)

3) Contamination of the samples during reentry or handling.

I find scenario 1) the most intriguing and something that should be looked at (because it may yield new strategies for synthesizing molecules).

Jul 26, 2012
it is now very suggestive with the correlation between primordial and extant biochemistry because of the large exogenously delivered excess
IMO this correlation doesn't imply causation, it just points to the similar physical origin of homochirality. For example it's well known, some L-aminoacids are more prone to acid hydrolysis. We could assume, they're adhering selectively to the surface of water clusters, which contain only limited number of water molecules (20, 100 or 280). The density of water increase toward center of cluster because of strong intramolecular forces, which could mimic the hydrophobic gradient of micelles for polar molecules. Blair et al. (1981) suggested that repeated wet-dry cycles on the prebiotic Earth might have led to partial polymerization of amino acids followed by partial hydrolysis of the polypeptide products and thus similarly enhanced any initially small enantiomeric excesses.

Jul 26, 2012
There's also a density difference between L- and D-alanine that could account for the discrepancy.

Jul 27, 2012
The reason why life uses a specific enantiomer (handedness) is pretty simple. The enzymes that make and use these small molecules have complex 3D shapes and so by their very nature, if one enantiomer "fits" into the enzyme, the other one won't fit. In this way enzymes are like a hand. Because a hand is not symmetric, it will only fit either a right or left handed glove, but not both. Given this fact, it's easy to see why life eventually chose one handedness for chiral molecules. The question then is why did they choose left-handed (for amino acids) rather than right handed. This work is exploring the hypothesis that there was an initial excess of left-handed amino acids in the primordial material, so the earliest organisms that utilized left-handed amino acids had a slight advantage. This work is exploring a possible physical mechanism that could give rise to an excess of left-handed amino acids. Pretty simple really...

Jul 29, 2012
@ antalias:

The article describes it well. It is a two stage process, the known slight excess (induced by chemistry of polarized UV light, say) gets amplified in the next stage by repeated crystal growth & dissolution in a finite volume with a finite source. This is covered in the text book in astrobiology of the course I once attended.

The new thing is that these lab mechanisms are know seen in nature, and that the excess now makes the correlation to our own AAs alluring.

@ Satene:

Too bad you can't explain chirality from membranes, because the UCA doesn't seem to have had a stereo-chemic membrane. I therefore have to assume such a mechanism can't predict original chirality.

As for similar physical origin of homochirality, this result doesn't point to that. Precisely because correlation doesn't imply causation. This is a known pathway, it has just been tested as observed in nature.

Jul 29, 2012

The reason why a strong chiral excess is alluring is because you have to dilute the organic contribution from impactors until it didn't act as the original chiral seed for chiral takeover anywhere locally in the evolving biochemistry. This is now a tall order.

Jul 29, 2012

The reason why a strong chiral excess is alluring is because you have to dilute the organic contribution from impactors until it didn't act as the original chiral seed for chiral takeover anywhere locally in the evolving biochemistry. This is now a tall order.

@ Vendicar Decarian:

Exactly, that is what I describe. So I don't see how antalias can see a universal bias, if that is what he claims.

Aug 01, 2012
Well, the article says pretty plainly
The work also gave the strongest evidence to date that liquid water inside an asteroid leads to a strong preference of left-handed over right-handed forms of some common protein amino acids in meteorites

So there must be some 'universal' bias mechanism at work, right? Not just a local mechanism that would dicatate that one crystal is all-left or all-right (because then OVERALL we'd see no bias whatsoever - just local bias which should add up to a 50/50 distribution if taking all findings together.

Problem is: I'm having trouble figuring out where that bias is coming from, since artificially produced amino acids don't show that bias (and indeed one of the two compounds studied inthe asteroid which shows a bias in life on Earth didn't show any bias in the asteroid - so we're probably not dealing with stuff formed from life processes).

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more