[meteorite-list] Space Radiation May Select Amino Acids for Life

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed Aug 24 11:54:16 2005
Message-ID: <200508241553.j7OFr9728825_at_zagami.jpl.nasa.gov>

http://www.newscientistspace.com/article/dn7895-space-radiation-may-select-amino-acids-for-life.html

Space radiation may select amino acids for life
Maggie McKee
New Scientist
24 August 2005

Space radiation preferentially destroys specific forms of amino acids,
the most realistic laboratory simulation to date has found. The work
suggests the molecular building blocks that form the "left-handed"
proteins used by life on Earth took shape in space, bolstering the case
that they could have seeded life on other planets.

Amino acids are molecules that come in mirror-image right- and
left-handed forms. But all the naturally occurring proteins in organisms
on Earth use the left-handed forms - a puzzle dubbed the "chirality
problem".

"A key question is when this chirality came into play," says Uwe
Meierhenrich, a chemist at the University of Nice-Sophia Antipolis in
France. One theory is that proteins made of both types of amino acids
existed on the early Earth but "somehow only the proteins of left-handed
amino acids survived", says Meierhenrich.

Meierhenrich and colleagues have a different theory. "We say the
molecular building blocks of life were already created in interstellar
conditions," he told New Scientist.

The team believes a special type of "handed" space radiation destroyed
more right-handed amino acids on the icy dust from which the solar
system formed. This dust, along with the comets it condensed into, then
crashed into Earth and other planets, providing them with an
overabundance of left-handed amino acids that went on to form proteins.

Magnetic alignment

The radiation is called circularly polarised light because its electric
field travels through space like a turning screw, and comes in right-
and left-handed forms.

It is thought to be produced when dust grains become aligned in the
presence of magnetic fields threading through regions of space much
larger than our solar system. Circularly polarised light is estimated to
make up as much as 17% of the radiation at any given point in space.

In 2000, an experiment showed that when circularly polarised ultraviolet
light of a particular handedness was shone on an equal mix of right- and
left-handed amino acids, it produced an excess of 2.5% by preferentially
disintegrating one type.

But that experiment was done using amino acids in a liquid solution,
which behave differently than those in the solid conditions of icy dust
in space. To avoid absorption by water molecules, it was also necessary
to use light at a wavelength of 210 nanometres - significantly longer
than the peak of 120 nm radiation actually measured in space.

Biased meteorites

Now, Meierhenrich's team has performed a similar experiment. The group
shone circularly polarised light at a wavelength of 180 nm on a solid
film of both right- and left-handed forms of the amino acid leucine. It
found that left-handed light produced an excess of 2.6% left-handed
amino acids.

"Going towards greater realism by exploring another wavelength of light
and solid samples is definitely a good thing and a logical step
forward," says chemist Max Bernstein of NASA's Ames Research Center in
California, US, who is not part of the team.

He says the research adds to previous measurements of an excess of
left-handed amino acids in two meteorites. "If it is thanks to
meteorites that our amino acids are left handed, then the same bias
should exist at least across our solar system", he told New Scientist.

Alien life

But other solar systems may harbour right-handed amino acids if they are
subjected to the other type of circularly polarised light, says
Meierhenrich.

"The chiral amino acids might have been delivered to other planets, to
other solar systems," he adds. "The probability that life arose
somewhere else is increased with this experimental result."

Meierhenrich will continue to reduce the wavelength of the experimental
radiation by using a synchrotron facility, due to begin operating in
2006. But the real test of his theory may come in 2014, when the
European Space Agency's Rosetta spacecraft lands a probe on Comet
67P/Churyumov-Gerasimenko.

He designed an instrument for the lander that will measure the
handedness of any amino acids it finds. "If we identify left-handed
amino acids on the cometary surface, this would underline the hypothesis
that the building blocks of proteins were created in interstellar space
and were delivered via comets or micrometeorites to early Earth," he says.

Journal reference: Angewandte Chemie International Edition (vol 44, p 2)
Received on Wed 24 Aug 2005 11:53:08 AM PDT