Buat yang mau mengikuti penelitian dan kajian ilmuwan...
-
Web address:
http://www.sciencedaily.com/releases/2012/07/
120725120706.htm
How Life Turned Left: Meteorite Fragments Help Explain Why Living Things Only
Use Molecules With Specific Orientations
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.)
ScienceDaily (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 composed 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 composed 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.
Share this story on Facebook, Twitter, and Google:
Other social bookmarking and sharing tools:
Share on blogger Share on digg Share on fark Share on linkedin Share on myspace
Share on newsvine Share on reddit Share on stumbleupon | 48
Story Source:
The above story is reprinted from materials provided by NASA/Goddard Space
Flight Center.
Note: Materials may be edited for content and length. For further
information, please contact the source cited above.
Journal Reference:
Glavin DP, Elsila JE, Burton AS, Dworkin JP, Hilts RW and Herd CDK. Unusual
non-terrestrial L-proteinogenic amino acid excesses in the Tagish Lake
meteorite. Meteoritics & Planetary Science, 2012 (in press)
Need to cite this story in your essay, paper, or report? Use one of the
following formats:
APA
MLA
NASA/Goddard Space Flight Center (2012, July 25). How life turned left:
Meteorite fragments help explain why living things only use molecules with
specific orientations. ScienceDaily. Retrieved July 26, 2012, from
http://www.sciencedaily.com /releases/2012/07/120725120706.htm
Note: If no author is given, the source is cited instead.
Disclaimer: Views expressed in this article do not necessarily reflect those of
ScienceDaily or its staff.
------------------------------------
Post message: [email protected]
Subscribe : [email protected]
Unsubscribe : [email protected]
List owner : [email protected]
Homepage : http://proletar.8m.com/Yahoo! Groups Links
<*> To visit your group on the web, go to:
http://groups.yahoo.com/group/proletar/
<*> Your email settings:
Individual Email | Traditional
<*> To change settings online go to:
http://groups.yahoo.com/group/proletar/join
(Yahoo! ID required)
<*> To change settings via email:
[email protected]
[email protected]
<*> To unsubscribe from this group, send an email to:
[email protected]
<*> Your use of Yahoo! Groups is subject to:
http://docs.yahoo.com/info/terms/
