[meteorite-list] NASA and University Researchers Find a Clue to How Life Turned Left (Tagish Lake Meteorite)

[Ron Baalke]

http://www.nasa.gov/topics/solarsystem/features/life-turned-left.html

NASA and University Researchers Find a Clue to How Life Turned Left
Bill Steigerwald (William.A.Steigerwald at nasa.gov)
NASA's Goddard Space Flight Center, Greenbelt, Md.
July 25, 2012
 
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 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," 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 life 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.

The research was funded by the NASA Astrobiology Institute, the Goddard 
Center for Astrobiology, the NASA Cosmochemistry Program, and the Natural 
Sciences and Engineering Research Council of Canada.