Once in a while, you come across a paper whose basic premise (and length) is so short that you can read it in 10 mins. You can talk about this paper to a college student, and its conciseness and simplicity provides a refreshing change from the usual technical articles that assail your faculties every day.
Such is the latest missive from Ronald Breslow. One of the big puzzles in the origin of life is the origin of homochirality (why L-amino acids?). Breslow does not solve this problem, but provides a possible solution for the amplification of existing homochirality that is simplicity exemplified.
The premise? Racemates of amino acids have a lower solubility than the pure enantiomers.
The experiment? Evaporate a solution of a slightly enantiomerically enriched solution of amino acids two times.
The result? The ee increases from 10% to about 91%, and from 1% to about 87%.
Here are all the results:
These experiments have been inspired in part by the remarkable discovery of slightly enantiomerically enriched non-racemizable amino acids in meteorites, most famously the Murchison meteorite (eg. S-alpha methyl valine) (See the linked ACR 2006 review)
These L-amino acids in sufficient concentration could then catalyse the reactions of other organic and biomolecules, such as has been demonstrated with sugars.
Very nice, and could have been entirely plausible on the early earth. The problem of course is how the chiral excess, no matter how small, could have arisen in the first place. The real problem is; how can chirality arise from random processes, which produce either enantiomer in equal parts?
When I started thinking about this question, I realised what a fickle beast the word 'random' is. Random processes of course cannot give rise to a particular bias. But now, consider a random process which happens extremely rarely. For example, the odds of getting heads and tails for a coin are 50:50. However, flip a coin only 10 times. Random variables such as air flow could very reasonably give you six heads and four tails. Now, if there were some process that could take advantage of whichever face lands more often (the heads in this case), then that process could essentially start off with more heads, and then capitalise on the even more heads produced and so on...if this process were auto-catalytic, as is an important condition for early life forming processes, then it would immediately take advantage of the excess heads, and then produce even more heads and...there; you have your L-amino acids. Of course, it can be noted that there was no particular reason to choose L-amino acids. That choice was entirely random, but the above analysis shows that such a choice can be made in the first place.
Based on this argument, I can envisage a process where, say a rock surface forms in a biased way, that is, its surface is more suitable for anchoring the L-amino acids rather than the racemates or the D-amino acids. In an amino acid 'soup' formed in a crater in such a rock then, it is quite plausible that evaporation would take away the racemate simply because it cannot hold on to the surface as well as the L. In such a case, the final 'soup' will then be enriched in L-amino acids.
Again, the hypothesis breaks down if one imagines millions of such rocks, in which case there is no reason for any bias. But if our rock is formed by a cometary impact for example, then I can imagine a comet with amino acids slamming into a giant rock and producing a skewed surface. Planetary impacts are very rare as we know. So by the time the next comet comes around, there already exists an asymmetrical surface which can introduce a bias in the evaporation of Breslow's amino acid pool. So by the time the comet comes back, the L-amino acids have taken over, aided by the biased structure produced by the 'random' impact of the comet.
It's all about natural selection. Even a random process can produce bias if it happens only once, and then allows nature to capitalise on the asymmetry produced by that one time event. Statistics be praised.
* Reference: Ronald Breslow and Mindy S. Levine, Amplification of enantiomeric concentrations under credible prebiotic conditions, PNAS 2006 103: 12979-12980
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