Re: Did lefty molecules seed life?

"Robert Karl Stonjek" <rstonjek@xxxxxxxxxxxxxx> wrote in message news:gprb5b$2jt7$1@xxxxxxxxxxxxxxxxxxxxxx
Did lefty molecules seed life?
Posted by Elie Dolgin
[Entry posted at 16th March 2009 10:00 PM GMT]

The molecular orientation of compounds brought to Earth by meteorites
could have determined the world's chemistry long before life began,
according to a new study published online today (Mar. 16) in the Proceedings
of the National Academy of Sciences.

Amino acids come in left-handed and right-handed forms, which, like a
pair of human hands, are mirror images that cannot be superimposed onto each
other. Yet living organisms use only the left-handed version, which presents
a conundrum: There's no biochemical reason why one mirror image should be
better than the other, so scientists have long debated whether life's
left-handed leaning arose because of random processes or whether rocks from
outer space seeded a southpaw solar system.

The current study argues for the latter possibility by showing that
some extraterrestrial meteorites contain an abundance of left-handed
molecules. "The implications are that all life in our solar system could be
the same handedness as life on Earth," Jeffrey Bada, a geochemist at the
Scripps Institution of Oceanography in La Jolla, Calif., who was not
involved in the research, told The Scientist.

Daniel Glavin and Jason Dworkin, astrobiologists at NASA's Goddard
Space Flight Center in Greenbelt, MD, compared the ratio of left- and
right-handed 5-carbon amino acids found in six primitive, carbon-rich
meteorites that have an elemental composition similar to that presumably
found in the early solar system. Three of these rocks were heavily
left-skewed, while the remaining three showed equal handedness, or
chirality, the researchers found. Of the lefty rocks, the meteorite that
fell on Murchison, Australia, in 1969 -- arguably the most widely studied
carbonaceous meteorite in the world -- contained the largest imbalance ever
observed: a 18.5% excess of the left-handed form of the amino acid
isovaline.

"There really is a large, 15 to 20% excess for this particular amino
acid, and it has important implications for homochirality
[single-handedness] and the origins of life," Glavin told The Scientist.

Very strange. I googled "isovaline" to find out just how it differs from
valine. And the top hit was this paper
http://adsabs.harvard.edu/abs/1975GeCoA..39.1571P

And it says that the isovaline in the Murcheson meteorite is nearly racemic.

"Maybe life was biased toward left-handedness in our solar system"
said Dworkin. The possibility that left-handed amino acids are so prevalent
in our solar system is "bad news in looking for independent origins of
life," he noted, because it decreases the chances of researchers stumbling
upon an organism that uses only right-handed amino acids -- a clear
trademark of alien life. "But it's also good news" for the possibility of a
second origin, because single-handedness is essential for biotic chemistry
as we know it. Thus, a meteorite-driven imbalance could have helped
"jumpstart" early life, he said.

Paul Davies, a theoretical physicist and astrobiologist at Arizona
State University (ASU) in Tempe who was not involved in the study, was not
convinced by this argument. "Even if there's this slight excess at the
outset there has to be some sort of mechanism that's going to amplify that
to make it 100%," he said.

Good for Davies. He may be the first physicist to get the point which
biochemists have realized for decades.

The mechanism that Glavin and Dworkin propose to explain the observed
left-handed excess is that polarized light -- which is twisted and can
rotate molecules -- probably set the imbalance in motion. Then, once the
balance was slightly askew, water within the meteorites further drove an
enrichment of left-handed amino acids in the liquid phase and relegated
right-handed molecules to the solid phase. "The whole amplification is due
to this process of aqueous alteration," said Dworkin.

But the link between water-bearing rocks and a left-handed skew is
just a correlation, said ASU biochemist Sandra Pizzarello, who was also not
involved with the work. "It's just a supposition," she said. "I would have
liked them to back it up with physico-chemical possibilities."

Glavin pointed to the work of Columbia University's Ronald Breslow and
Imperial College's Donna Blackmond, which has demonstrated that this
so-called "enantiomeric enrichment" can occur in a liquid phase, such as the
one found during the melting of ice inside the meteorites' parent asteroid.

Still, even if aqueous alteration can explain the build-up of
left-handed molecules, it doesn't explain the disappearance of their mirror
images, noted Robert Hazen, a geochemist at the Carnegie Institution in
Washington, DC. "Where did the right-handed amino acids go? They had to go
somewhere else... There has to be a destruction process."

Bada also noted that the left-skewed, 5-carbon amino acids described
by the authors are not the same molecules that are used by life on Earth.
The 20-odd protein building blocks that living beings rely on showed no such
bias in the meteorites, the NASA researchers found, so to get homochirality
in life there would need to be some mechanism of transferring the
single-handedness between different types of amino acids.

The origins of life remain "one of those bewildering things," Bada
said. "If there was a straight-forward answer for the homochirality of amino
acids, I think we would have found it."

We have found it. But idiots like Bada who cling to the Miller experiment
and the theory of heterotrophic abiogenesis just can't accept it.

.

Relevant Pages

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