Thursday, July 26, 2012

Amino acids in meteorites

Researchers found amino acids in the Murchison meteorite 4 years ago, and from the C13/C14 ratio figured that they were extra-terrestrial, and not contamination or an old chunk that got blasted loose long ago and finally fell back to Earth.

In a "yet-to-be-published" article (not on archiv, dang it), they find that the ratio of left to right handed amino acids isn't the same as on Earth either for the Tagish Lake meteorite. In fact the ratios for aspartic acid were 4:1 left:right, but were only 52:48 for alanine, compared to 1:O(0) for Earthly proteins and enzymes. Random synthesis should result in 1:1. Right-handed amino acids just don't fit in proteins. You can imagine proteins made of nothing but right-handed amino acids, but they just don't exist. Here. Though right-handed acids do rarely show up.

The carbon 13 enrichment, combined with the large left-hand excess in aspartic acid but not in alanine, provides very strong evidence that some left-handed proteinogenic amino acids — ones used by life to make proteins — can be produced in excess in asteroids, according to the team.

They point out that some amino acid crystals will be of one handedness only, and that's one way of getting a pure sample. That's nice, but amino acids aren't usually found in crystal form. I suppose one model might be that there is a pool of mixed amino acids, and a crystal of right-handed cytosine forms, leaving the left-handed in the pool to react and get turned into the first proto-proteins. Or something. Alternatively, an crystal of left-handed survives some chemical insult better than the loose pool of right-handed, and is still around to form proto-proteins. Eh.

This process only amplifies a small excess that already exists. Perhaps a tiny initial left-hand excess was created by conditions in the solar nebula. For example, polarized ultraviolet light or other types of radiation from nearby stars might favor the creation of left-handed amino acids or the destruction of right-handed ones, according to the team. This initial left-hand excess could then get amplified in asteroids by processes like crystallization.

Um. Let's suppose we have something like 100% left-handed in a sample that splashes away from Earth. Over time despite the small rate we can get C13 from p+C12->N13->C13+γ, with the solar wind or low energy cosmic rays as a source for the protons. There should be C14 as well, though I'm not sure of the ratio, and some of the transmutations might disrupt the molecule. So chemicals of terrestrial origin can start to look cosmic. If they were in solution I'd expect nuclear recoil to break the molecule, but in a rock matrix there's not much place for the loose carbon to go, and it might as well recombine after a while.

At what rate will ionized amino acids in a rock matrix spontaneously shift handedness configuration? Is it different for different acids?

That sounds like a research project for somebody...


Assistant Village Idiot said...

With very long periods of time involved, processes which have never been observed to occur - or at least, not in quantity - become plausible, simply because they aren't actually impossible.

In this instance, there are reasons to regard most explanations as impossibly unlikely - and yet there it is, so we have to scramble and wonder "gee, I suppose if we started with ratio X but there was a rare but consistent movement in the direction of eliminating the right-handed amino acids, it maybe just could...darn, that seems unlikely..."

We keep trying on ridiculous hats, hoping that one will go with the outfit.

james said...

Yes. I've heard suggestions that the asymmetry of weak interactions in electron spin could introduce a bias--though nobody has come up with a mechanism that I've ever heard of. All other interactions are symmetric. When all you have is a hammer...

Texan99 said...

It could be simply that life appeared only once on Earth, and happened to use left-handed amino acids, and every kind of life since then has inherited that preference. If that's the case, then living processes selected for them, even though natural processes have no preference.

james said...

Yep. But "it was just an accident" explanations are uncomfortable, and people look around for patterns. Sometimes fruitfully, and sometimes coming up with conspiracy theories.

Texan99 said...

But the appearance of life, bringing with it even the most primitive early purposefulness, was a cosmic game-changer. All of a sudden you've got these little systems that do things deliberately, and that try to amass resources to replicate their own special way of doing things deliberately. It's not "random" any more, in the usual sense in which we apply that word to physical systems. Nothing's ever going to look the same after that. Look how oxygen remade the world.

It doesn't remind me of a conspiracy, just a natural result. It's "accidental" only in the sense that it does appear that all the life we see now started with the same beginning. It would be too coincidental to imagine that it started numerous times, but in each case just happened to land on the exact same suite of tricks.

So it fills me with wonder to imagine how it might have started differently somewhere else. People usually get hung up on the necessity of its involving carbon or water, but forget about the other possibilities: something other than L-amino acids, something other than DNA, some completely different kind of code.

james said...

Magnetic vortices in plasma?