[meteorite-list] Murchison's Amino Acids: Tainted Evidence?

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:18:26 2004
Message-ID: <200302121705.JAA26132_at_zagami.jpl.nasa.gov>

http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=375&mode=thread&order=0&thold=0

Murchison's Amino Acids: Tainted Evidence?
By Anne M. Rosenthal
Astrobiology Magazine
February 12, 2003

Summary: Few meteorite discoveries can rival the one
that fell in 1969, just 60 miles north of Melbourne,
Australia. Called the Murchison meteorite, the rock's
interior showed signs of the protein building blocks of
life. But whether those amino acids arose after the rock
fell is one question that the experts still debate three
decades later.


Could meteorite bombardment in Earth's early
history have delivered the chemical building
blocks essential for life? One way to answer
that question is to study the chemistry of
meteorites from observed falls, says University
of Oklahoma geochemist Michael H. Engel.
Quickly collected, such meteorites are exposed
only minimally to weathering processes and
contamination by earthly materials.

Over the last two centuries, stones have been
collected from about 35 observed falls of
carbonaceous chondrites. These meteorites
contain materials coalesced from dense
molecular clouds during or prior to the formation
of the solar system. They are of special interest
because, like life on Earth, they contain organic
or carbon-based
compounds.

But chemical studies of these meteorites have
often been challenged as unreliable by scientists
claiming that contamination has occurred through
exposure, storage, or handling. Over time, says
Jeff Bada, of the Scripps Institute of Oceanography, even
carefully stored meteorites gradually become contaminated.

If organic compounds such as amino acids from Earth's
biosphere have penetrated meteorite samples, they would
no longer be representative of early solar system chemistry,
nor could they provide evidence of an extraterrestrial
source for the components of Earth's first life. But
figuring out whether or not a meteorite has been
contaminated has proven to be a thorny problem.

The Murchison meteorite, which fell about 100 kilometers
(about 60 miles) north of Melbourne, Australia, in 1969,
is one of the world's most closely studied carbonaceous
chondrites.

According to Engel, several lines of evidence indicate
that the interior portions of well-preserved fragments
from Murchison are pristine. Engel points to the array of
amino acids Murchison contains and to isotope studies to
bolster his position. Other scientists are equally
convinced that the evidence proves the opposite: that
Murchison is now thoroughly contaminated by terrestrial
organic material.

Indeed, the results of various experiments performed on
Murchison are a bit of a head-scratcher - and a good
window into how science works when data are ambiguous.

Tallying Amino Acids

Over the past three decades, Murchison studies,
particularly those of John Cronin and Sandra Pizzarello of
Arizona State University (ASU), showed that the meteorite
contains a fascinating assemblage of amino acids. More than
fifty of the amino acids found in Murchison are not present
on Earth. Murchison also contains many of the protein amino
acids incorporated into Earth's living systems - but it
doesn't include all of them.

The portfolio of amino acids in Murchison provides evidence
that it has not been contaminated, says Engel. He and Stephen
Macko of the University of Virginia, Charlottesville, maintain
that if amino acids from Earth had found their way into the
interior of Murchison samples, scientists, including Engel and
Macko, would find the complete spectrum of amino acids present
on Earth, not just some of them.

Bada and others counter that the missing amino acids were
probably there, but that Engel and Macko simply didn't detect
them, perhaps because their equipment wasn't sufficiently
sensitive.

Mirror Image

There are also other differences between the
amino acids in Murchison and those typically
found on Earth, differences that pertain to the
issue of contamination. One of these differences
has to do with the chirality, or handedness, of
the molecules.

To understand the concept of chirality, Engel
suggests, bring your hands together with palms
facing each other, and note that your thumbs
and fingers line up. But if instead you place the
palm of one hand atop the back of the other,
your hands no longer coincide. That's because
your hands are mirror images of each other.
When a molecule comes in two mirror-image
forms, it is termed chiral. The majority of amino
acids are chiral molecules.

A curious aspect of Earth's life forms is that they
contain (with few exceptions) only left-handed amino acids.
In contrast, when scientists synthesize amino acids from
nonchiral precursors, the result is always a "racemic"
mixture - equal numbers of right- and left-handed forms.
Scientists have been unable to perform any experiment
that, when starting with conditions believed to emulate
those of early Earth, results in a near-total dominance of
left-handed amino acids, says George Cody, a
geochemist at the Carnegie Institute of Washington.

Scientists are especially interested in amino acids that
are the building blocks for proteins, which serve as both
structural components and enzymes in terrestrial life
forms.

Engel and colleagues published chemical analyses of some
of the protein amino acids from Murchison in Nature in
1982 and 1990. They found that Murchison had neither
the complete dominance of left-handed amino acids found on
Earth nor the racemic mixture expected, on the basis of
laboratory experiments, for materials formed in
space.

Instead, their results showed a moderate to
strong predominance of left-handed forms.
Engel and colleagues interpreted their results
as showing that the extraterrestrial material in
Murchison was not racemic, but had a different
mixture of the left- and right-handed forms
than materials found on Earth.

They did not see their findings as evidence of
contamination, even though their interpretation
was at odds with the first analysis of
Murchison amino acids, published in 1971 by
Keith Kvenvolden (then of NASA Ames Research Center -
Kvenvolden is now at the US Geological Survey) and
his NASA colleagues.

The results of Kvenvolden, et al., had shown a left-handed
excess, but only a small one. Therefore, Kvenvolden and
colleagues had interpreted their results as indicative
that the amino acids in Murchison were racemic, as
anticipated - if they were extraterrestrial. They
concluded that the most likely explanation was that after
Murchison arrived bearing a racemic mixture, it was
contaminated by amino acids from Earth. Kvenvolden points
outs that it's possible that impurities in the chemicals
used to perform the analysis could have distorted the results.

Because Engel and Macko had found a much greater left-handed
excess than Kvenvolden, et. al., they had interpreted it as
evidence that the extraterrestrial material in Murchison was
not racemic. This interpretation was rebuffed by Bada,
Kvenvolden, and others, kind of who attributed the excess to
contamination, as Kvenvolden had for his own data.

Circumventing Contamination

To skirt the issue of contamination, Cronin and Pizzarello
decided to study four of the Murchison amino acids that are
not found on Earth.

What Cronin and Pizzarello found was a small predominance of
left-handed forms. Their conclusion: At least for some amino
acids, there must have been an excess of left-handed forms
indigenous to the meteorite. Earthly materials penetrating
Murchison could not have influenced the percentage of
left-handed forms in these non-terrestrial amino acids.

Cronin and Pizzarello published their results in Science in
1997. While these results would appear to support the findings
of Engel and Macko, in the same study the ASU scientists also
looked at two Murchison amino acids that are present in
terrestrial proteins and found them to be racemic.

How did Cronin and Pizzarello explain this confusing result?
The non-protein amino acids must have formed by a different
process than the protein amino acids, they wrote in their report.
What those processes might have been, however, have not yet
been clarified.

Weighing the Evidence

Meanwhile, Engel and Macko were conducting
another set of experiments to show that the
Murchison interior was pristine. This work
looked at stable-isotopic signatures.

Isotopes of an element are variants that have
the same number of protons but different
numbers of neutrons, and thus different atomic
weights. Because life on Earth preferentially
uses lighter isotopes of carbon and nitrogen,
scientists can use isotopic signatures to
distinguish terrestrial organic materials from
extraterrestrial.

Engel and Macko first separated amino acids
extracted from Murchison into right-handed
and left-handed forms, using a technique called gas
chromatography. Then, with an instrument called a mass
spectrometer, they determined carbon and nitrogen
stable-isotope ratios separately for the left-handed and
right-handed forms of the molecules.

The investigators found that the stable isotope ratios
were identical for the left-handed and right-handed forms.
This, says Engel, indicates, that they had to have come
from the same source - that is, not from Earth. If, he
argues, a portion of the left-handed forms were from
terrestrial organics, these forms would have exhibited a
different isotopic signature than the right-handed forms.
They would have contained more light carbon and nitrogen.

Kvenvolden and Bada aren't convinced. The new stable-isotope
evidenc e notwithstanding, says Kvenvolden, a left-handed
excess like that found in previous research by Engel and
Macko, "is inconsistent with the observations of Cronin,
Pizzarello and myself for protein amino acids in the
meteorite." Kvenvolden firmly believes Engel and Macko were
seeing contamination.

Cronin and Pizzarello suggest that Engel and Macko's most
recent findings could be caused by coelution: extra, unwanted,
compounds exiting from the gas chromatograph column at the
same time as the left-handed amino acids. This would skew the
data.

Engel disputes this explanation, pointing out that work was
done separately for the C and N isotopes. It would be a highly
unlikely coincidence, he says, for coelution to mask a
contamination signature for both C and N stable isotope ratios.
Thus, Engel concludes, there are portions of Murchison that
remain pristine, uncontaminated.

What's Next?

Even if it can be demonstrated conclusively that Murchison
contains amino acids with a slight left-handed excess, and
that this excess is not due to contamination or experimental
artifacts, would that explain the world of left-handed amino
acids in which we live? Not necessarily.

On the one hand, says Cody, "the only evidence of the
prebiotic world is carbonaceous meteorites, and remarkably,
they appear to have a slight [left-handed] enhancement." On the
other hand, he points out, that this may not tell us anything
about how the almost complete dominance of left-handed forms in
terrestrial life got its start.

According to Bada, it doesn't much matter whether the amino
acids that rained down on Earth in meteorites before life
began had a slight left-handed excess. Once they arrived and
mixed with the environment, Bada says, commonplace chemical
reactions would have erased the left-handed signature.

As for contamination, only laboratory analysis designed to
eliminate the possibility of coelution, preferably with
carefully handled samples from fresh meteorite falls, is
likely to settle the question to everyone's satisfaction. As
Cody notes, "It's really difficult to be 100 percent definitive
in this because there's still so may unknowns. Contamination
will always be an issue."
Received on Wed 12 Feb 2003 12:05:13 PM PST