© NASA/Hrybyk-Keith, Mary P.
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.
New clues have been unveiled that shed light on why living things use only molecules with specific orientations.
Research analyzing meteorite fragments that fell on a frozen lake in Canada is providing strong evidence that liquid water inside an asteroid leads to a preference of left-handed over right-handed forms of common protein amino acids in meteorites.
"Our analysis of the amino acids 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," Dr. Daniel Glavin of NASA's Goddard Space Flight Center and lead author of a paper being published in the journal Meteoritics and Planetary Science
, said in a press release.
A meteorite landed on Earth back in January 2000, and because many people witnessed the event, pieces were able to be collected and preserved in their frozen state for research.
"The Tagish Lake meteorite continues to reveal more secrets about the early Solar System the more we investigate it," Dr. Christopher Herd, a co-author on the paper, said. "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 used in everything from structures like hair to enzymes, and are the catalysts that speed up or regulate chemical reactions. Life uses just 20 different amino acids in a 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, and although life based on right-handed amino acids would presumably work fine, they cannot be mixed.
"Synthetic proteins created using a mix of left- and right-handed amino acids just don't work," Dr. Jason Dworkin of NASA Goddard, co-author of the study, said.
Because life is unable to function with a mix of left- and right-handed amino acids, scientists are trying to know how life 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," Dworkin said.
The team used the meteorite samples and 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 that is used in every enzyme in the human body, and is also used to make 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," he added.
He said that at first, the findings did not make sense because if both amino acids come from contamination by terrestrial life, both should have large left-handed excesses.
"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," Glavin said.
The team was able to confirm that amino acids were most likely created in space using isotope analysis.
Isotopes are elements with different masses, and since the chemistry of life prefers lighter isotopes, amino acids that have a heavier isotope like 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," Dr. Jamie Elsila of NASA Goddard, a co-author on the paper who performed the isotopic analysis, said.
The research marks the first time carbon isotope measurements have been reported for these amino acids in Tagish Lake. The carbon 13 enrichment, combined with the left-hand excess in aspartic acid, provides strong evidence that some left-handed proteinogenic amino acids can be produced in excess in asteroids.
Some say that large left-handed amino acid access in meteorites formed by exposure to polarized radiation in the solar nebula, however these are so large they cannot be explained by this alone. The team believes that another process would be required.
The large left-hand excess found in aspartic acid and not alanine gave the team a critical clue as to how these amino acids could have been made inside the asteroid.
"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," Dr. Aaron Burton, a NASA Postdoctoral Program Fellow at NASA Goddard and a co-author on the study, said. "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."
He said that because alanine forms different types of crystals, these 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," Burton added.
The team believes a small left-hand excess could get amplified by crystallization and dissolution from a saturated solution with liquid water. Some amino acids have a shape that allows them to come together in a pure crystal form. These amino acids have a small initial left- and right-hand excess that could become greatly amplified at the expense of the opposite-handed crystals.
Other amino acids have a shape that prefers to join together with their mirror image to make a crystal, which are comprised of equal numbers of left- and right-handed molecules. As these crystals grow, any small initial excess would be washed out for these amino acids.
Both of these processes can convert to left-handed to right-handed molecules, and vice-versa while they dissolve in the solution.
The process only amplifies a small excess that already exists. The team said that polarized ultraviolet light or other types of radiation from nearby stars may fair the creation of left-handed amino acids or the destruction of right-handed ones. This left-hand excess could then get amplified in asteroids by processes like crystallization.
Impacts by asteroids and meteorites could help deliver this material to Earth, and left-handed amino acids may have been incorporated in emerging life due to their greater abundance. Similar enrichments of left-handed amino acids by crystallizations could have taken place on Earth in ancient sediments that had water flowing through them, the team said.
"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," Glavin said.