[meteorite-list] Meteorite Source for Life's Handeness

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 10 Apr 2008 16:16:12 -0700 (PDT)
Message-ID: <200804102316.QAA05309_at_zagami.jpl.nasa.gov>

http://www.rsc.org/chemistryworld/News/2008/April/08040801.asp

Meteorite source for life's handedness
Mark Peplow
Chemistry World
08 April 2008

Scientists have long speculated that life's preference for left-handed
amino acids may have been triggered by compounds brought to Earth by
meteorites. Now they've shown exactly how two crucial steps in this
process could happen.

Carbonaceous chondrites, a relatively rare type of meteorite, are rich
in carbon compounds including amino acids. One set of meteorite
fragments, which fell on Murchison in Victoria, Australia, in 1969,
contains more than 70 different amino acids. In 1997, Sandra Pizzarello
of Arizona State University found that there was a small excess of the
left-handed form, or enantiomer, of four amino acids in Murchison samples.

Since then, scientists have been trying to prove how this handedness, or
chirality, could have been transferred to all the other amino acids and
sugars in biology; and how that small excess could have been amplified
into a total preference for one enantiomer over another.

Ronald Breslow of Columbia University, New York, and colleagues now
claim to have convincing answers to both questions. 'I think we've come
up with a credible story about how it happened,' says Breslow. He
presented his work at the American Chemical Society meeting in New
Orleans, US, on 6 April.

Handy transfer

The team studied how several amino acids - including alpha-methyl
valine, alpha-ethyl alanine and alpha-methyl isoleucine - behaved when
mixed with a primordial brew of other organic compounds and copper
sulfate. The structure of these alpha-substituted amino acids means that
they are unable to racemise, locking in their chirality.

They found that the chiral amino acids reacted with ketoacids to form
imines, which then lose carbon dioxide and fall apart to generate a new
amino acid. Crucially, these new amino acids took on the handedness of
the original chiral amino acids. Breslow says this is the first time
that this kind of chirality transfer has been shown to be possible in
plausible prebiotic conditions.

Copper ions, a common constituent of meteorites, seem to play a critical
role in shepherding the imine intermediate so that it adopts the correct
handedness, says Breslow. And although ketoacids have not been found in
meteorites, hydroxyacids and iron oxides - both present in meteorite
samples - react readily together to form ketoacids, he adds.

Once a slight excess of left-handed enantiomers had been transferred
from meteorites, the chirality imbalance could have been amplified,
Breslow's team then showed. Pairs of left- and right-handed molecules
crystallise together out of solution, leaving behind a solution that
contains more than a 90 per cent excess of the left-handed molecules.

This procedure has also been developed by Donna Blackmond's group
at Imperial College, London [1, 2], and Hiroyuki Koshino at RIKEN's
Discovery Research Institute in Japan [3]. But Breslow points out that
his team have also shown that pouring water through the mixture -
mimicking rain falling into a pool - can enhance the amplification even
further.

Universal origins

The amino acids present in the Murchison samples may have originally
formed from reactions between ketones, ammonia and hydrogen cyanide -
the kinds of simple compound that have already been found in
protoplanetary disks circling young stars.

Scientists have speculated that right-handed forms of amino acids in the
meteorites could then be preferentially destroyed by circularly
polarised light, generated by particles accelerated around neutron stars.

Both processes have previously been replicated in the laboratory,
although astronomers are still looking for more convincing evidence of
this in the heavens.

And some scientists are still not convinced that the Murchison samples
were not contaminated after arriving on Earth. However, Breslow points
out that their amino acids are rich in heavy isotopes of hydrogen,
carbon and oxygen, typical of non-terrestrial material. 'That signature
guarantees you're not talking about an Earth origin for the amino
acids,' Breslow told Chemistry World.

What's more, Pizzarello reported in February that a meteorite recovered
from the pristine conditions of Antarctica also held a similar excess of
left-handed amino acids [4].

Breslow's team is now trying to achieve the same chirality transfer and
amplification effects using the nucleosides that make up RNA - thought
by some scientists to be the first molecule on Earth to carry the
information needed to make life.
Received on Thu 10 Apr 2008 07:16:12 PM PDT