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Murchison Meteorite Still Contentious
An anonymous reader writes "The well-known 1969 meteorite that fell 60 miles north of Melbourne, Australia, remains remarkably contentious today. The 100 kilogram carbon rock : a) contains pre-biotic proteins and 12% water; b) harbors 50 amino acids not found on Earth; c) favors the tell-tale signature of biochemistry based on a dominant left-handed chirality, compared to random or racemic mixtures found in test-tube syntheses. While terrestrial contamination (even interior to the meteor) may discount this so-called 'Murchison meteor', its light isotopes of carbon and nitrogen suggest the left-handed amino acids not found elsewhere on Earth have the same ratios as the right-handed ones. This would not be the case if, say, bacteria was just making the left-handed ones after impact. Seems quite a controversy from down-under."
Rocks are porous. It's unlikely scientists are going to find a pigeon in the center of a meteorite, but anything dust particle or smaller could make it in.
Try not. Do or do not, there is no try.
-- Dr. Spock, stardate 2822-3.
From the article: 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.
In many cases, the levorotary forms are lower energy structures and would be favored during synthesis. The fact that many L based systems are almost exclusively so is dependant upon the larger structures that are based upon amino acids and other small molecules. Often a D form of a molecule will not be able to integrate into a L structure.
This is not to say that D forms cannot have biological activity however as there are many instances I can think of where racemic mixtures of molecules can have biological activity. For instance, the 2 chiral forms of carvone have completely different smells due to receptors in the olfactory epithelium being activated by each of the racemic forms.
Some instances of similarity of molecular structure but different chirality have also resulted in catastophies. One only has to think of MPTP poisoning the neurons of the substantia nigra or potentially thalidomide.
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The earth is only roughly 4.55 billion years old.
But that would just mean the asteroid in your example would only have to hit...say...2 billion years ago.
The first Apollo mission found the moon to be sterile, but later Apollo missions found strep bacteria from previous missions.
That's a little misleading. Apollo 12 found microbes inside the camera of Surveyor 3 (which landed three years before). Its not like the strep bacteria are colonizing the moon -- I'm pretty sure that the lunar environment is still sterile.
"The dinosaurs died because they didn't have a space program." - Niven
c) favors the tell-tale signature of biochemistry based on a dominant left-handed chirality, compared to random or racemic mixtures found in test-tube syntheses.
If a molecule has a carbon with 4 different groups bonded to it then there are two different ways of making the same thing but with different physical layouts eg:
W
|
X -C- Y
|
Z
Or:
X
|
W -C- Y
|
Z
Basicaly these have a "non superimposable mirror image" (no matter how much you rotate them and you can never have all the x,y,x and z's lined up)
Generaly the left handed and right handed molcules have very quite different behaviours, for instance some drugs use only one of the versions, whilst the other version is a poision.
A racemic mixture is a mixture of 50-50 of the left handed and right handed molecules, and generaly chemical processes will produce a racemic mixture.
so..... this is probably immoral, if not illegal, but their javascript source for it is in the cleverly named file javascript/showimages.php...
actually took a little while to find it, thanks to the somewhat deceiving name.
All amino acids produced by living things on earth are left-handed. Ones that occur in lab synthesis have random chirality. I don't fully understand the process behind this, but the handedness of the amino acids assures they were generated biotically, and the fact that many of the amino acids are new suggests that they're extraterrestrial in origin. If true, this would be a much less contentious evidence of the existence of extraterrestrial life than the martian fossils you mentioned.
Recent research suggests that there is an excess of L-amino acids (the specific enantiomer used in life-proteins) found in space, suggesting that the chiral specific process involving circular polarized light (mentioned in the article) could have lead to the amino acids that were found on the Murchison (and other meteorites).
...
From the article:
Recently it has been discovered that an excess of L-amino acids is present in the Murchison and Murray meteorites indicating that a preference for L-amino acids existed in solar system material before there was life on Earth. This supports an idea, first proposed by Rubenstein et al. (1983, Nature 306, 118), for an extraterrestrial origin for homochirality.
In this model the action of circular polarized light on interstellar chiral molecules introduced a left handed excess into molecules in the material from which the solar system formed.
If our own solar system formed in such a region of high circular polarization, it could have led to the excess of L-amino acids which we see in meteorites and to the homochirality of biological molecules. It is possible that without such a process operating it would not be possible for life to start. This may have implications for the frequency of occurrence of life in the universe.
Or sugar - right-handed - and sucralose - left-handed. Sucralose is equally sweet, but non-metabolizable, hence its use in diet food.
-T
Assides from the non sequetor, which applies regardless, did you read the article? If you had read it and the intro above, you would have noticed that the amino-acids not found on earth also are mostly "left-handed" which is not how they form in a lab.
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