Murchison's Amino Acids: Tainted Evidence?
|Carbonaceous chondrites, such as this CO3 specimen, are considered the most primitive meteorites, with solar major element composition (Fe, Si, Mg, Ca, Al).
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.
|When a molecule comes in two mirror-image forms, it is termed chiral. The majority of amino acids are chiral molecules (shown above).
Credit: Bernhard Rupp
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.
|Keith Kvenvolden (shown above) published the first analysis of Murchison amino acids in 1971.
Image Credit: Thomson ISI
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.
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
|The Murchison Meteorite crashed on September 28, 1969, near Murchison, Australia. The meteorite contains minerals, water, and complex organic molecules such as amino acids.
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 evidence 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.
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."
Related Web Pages
Carbonaceous Clues to the Early Solar System
Identifying Molecular Fingerprints: Gathering Clues to the Origin of Life
Well Preserved Meteorite Yields Clues to Carbon Evolution in Space
Amino Acid Asymmetry in the Murchison Meteorite
Chirality and the Origin of Life