Posted on 08/24/2005 10:16:24 PM PDT by LibWhacker
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.
The answer may lie in fundamental physics: the parity-violating weak neutral current produces a very slight energy difference between left and right handed molecules, which may become amplified over an evolutionary timescale, and our calculations of this energy difference show that the natural L-amino acids are indeed more stable than their "unnatural" D mirror images.
Anybody ever observed this? Any evidence indicating a bias toward left handed circular polarized UV light at wavelengths producing this result in our galaxy?
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How did space radiation get to be so smart?
Bartender! I'll have one of those.
Excellent question. My best candidate is our Milky Way galaxy. This has a gravitational field, so an outwards/inwards asymmetry; a magnetic field, so a north/south asymmetry; and a rotation, so a spinward/antispinward asymmetry. That means it does, indeed, exhibit chirality. Unfortunately, a very rough calculation suggests the combined effect would be about one part in 10,000, so it's a rotten candidate!
Any other thoughts?
I am 90% sure I remember that bacteria use both right and left-handed A.A.s.
It is evolutionarily consistent that eukaryotes only use left, and that's an interesting article proposing a reason.
Yes, but what they don't mention is that this difference is far too small even to measure. It's about 1 part in 10^16, I believe.
But if true - a very big if - then since the weak force is CP invariant, living things made out of antimatter would be right handed rather than left handed. Not that you'd want to shake hands with them, of course!
You're correct. The most common one is D-alanine, but about a dozen other have been found. The proportions are not even close to 50-50, however - but the evidence does suggest that many bacteria could evolve to live in a right-handed world.
Unfortunately, what we don't know is the critical datum: did the critters start out 50-50, and move left in response to the selection pressure of the later eukaryotic environment; or did they start out all left, and later incorporate D-aminos as an evolutionary response?
The second view says that, just as cacti evolved thorns and fungi evolved nerve toxins - to discourage eaters - maybe bacteria evolved D-amino envelopes for the same reason.
I would imagine that the effects of such an extended, tenuous structure as the galaxy would be minor on atoms and molecules. I would look to characteristics of subatomic particles, which I was told are not at all symmetrical. That is, the electric positive charge is balanced by the electric negative charge overall, but the carrying particles are vastly different otherwise. When corresponding anti-particles are allowed we find symmetry to a high degree, but it appears that anti-particles are not allowed in general even if they do appear now and then for short periods of time. Any chiralty in molecular structure is likely due to the nature of atoms, which are apparently made of subatomic particles, which have many kinds of asymmetric manifestations. While both levulo and dextro molecules exist, one kind dominates, and being relatively common would be the kind most favored as dinner.
" but the evidence does suggest that many bacteria could evolve to live in a right-handed world. "
You have a reference for this?
Nah. Things got flipped around as a result of the Fall.
Well, the Fall's a coming, we'll see if they flip back.
For things that find bacteria delicious, handling D-amino acids is a piece of cake (amino acid racemases, amino acid oxidases, etc.). Try another idea.
"So for now, I fall back on spontaneous symmetry breaking. Start with a world half full of primitive L critters, and half full of their R mirror images. As we know, L must eat L, and R must eat R. But you can't tell them apart - not if you're a primitive cell.
Now a random event - maybe a tsunami - tips the balance to 51% L and 49% R. The L's are now slightly more successful at eating, because 51% of what they find is useful food. This advantage magnifies over time, until all the Rs have starved. "
I'd like to laugh at this, but I think you might actually be serious. I have a bug in a flask right next to me. They are, of course "L" in amino acids. They will eat almost anything and they don't give a crap if it's D or L.
No, life had to start as all D or all L. The question is why L was favored and if there is something inherent in L that makes it better. It has been suggested a number of times, but I don't think the evidence is unambiguous at this time. Besides many sugars are D form.
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