Amino acid crystallisation and the origin of life

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ResearchBlogging.orgRecently I came across a number of attempts to explain the “handedness” of life – the fact that proteins consist only of L-amino acids – by the crystallization behavior of amino acids. The general idea is that something that happens at the transition between solution and crystal that favors one of the enantiomers over the other. I have to admit that it sounds a lot less esoteric than the idea that polarized starlight somehow influenced the formation of interstellar amino acids sufficiently to create such an effect, but frankly, I don’t buy it.

Take, for example, this paper recently published by F.L. Falcon on arxiv.org: He claims that L-Ala crystals are slightly larger than D-Ala crystals. In his scenario there is a saturated solution of racemic alanine in a hydrothermal setting, with a gentle temperature gradient. Forming Ala crystals are pushed by currents into colder areas, where the solution is even more saturated, and grow. But L-Ala crystals are larger, so they get pushed further into the cold regions and grow even more.

Then, Ostwald ripening takes place, enhancing crystal size further and depleting the solution of L-Ala, and that in turn leads to all D-Ala turned into L-Ala by epimerization. Neat, eh? Of course you see the problem immediately: Where’s all that alanine to come from to create a saturated solution? The stuff is highly soluble, on the order of 300 g/L depending on the temperature. This seems highly unlikely to me.

The other problem is the claim that L-Ala crystals are larger, based on X-ray diffraction data. Falcon thinks that parity violation influences the strength of hydrogen bonding, thus explaining the crystal size. I, on the other hand, can think of a number of possibilities much closer to home: Different growth conditions, traces of water, other impurities, temperature differences… As one colleague observed, it may be difficult to get this paper past the reviewers.

The second paper by Lee and Lin about the crystallization of aspartic acid actually made it to publication. In my opinion it is rather weak nevertheless, and this is why:

The authors get very excited about the fact that they did dissolve a mixture of D- and L-Asp crystals and, depending on conditions, sometimes got back a mixture of the separately crystallized enantiomeres and sometimes additionally found racemic crystals, which are thermodynamically more stable.

And yet, only a 10-min, 25 °C incubation treatment of the 1.0 mL equal volume mixture of an unsaturated aqueous solution of conglomerate aspartic acid (i.e., prepared by dissolving 5.0 mg of d-aspartic acid and 5.0 mg of l-aspartic acid in 1.3mL of water) and an unsaturated aqueous solutionof racemic aspartic acid (i.e., prepared by dissolving 10.0 mg of dl-aspartic acid in 1.7 mL of water) produced a mixture of conglomerate crystals and a racemic compound.

From this they boldly conclude

There was some sort of molecular complexation or “memory” existing in those aqueous solutions which could not be erased by instant mixing.

Yeah, right. My money, on the other hand, is on seed crystals getting everywhere and enthusiastic grad students trying again when it didn’t quite work. I could be wrong, though, so read it yourself.

Finally, what does all this have to do with the origin of life? The authors claim that something on early earth led to the separation of the racemic solution into a D/L-solution. The idea then seems to be that due to the low solubility, L-Asp could have acted as a template inducing other amino acids to assume the same configuration. Somehow.

I was rather enthusiastic about the whole idea, at least at the beginning. A discermible chemical mechanism explaining the chirality of proteins would feel far more satisfying to me than cosmic rays or pure accident, but crystallization, it seems, isn’t it…

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Lee, T., & Lin, Y. (2010). The Origin of Life and the Crystallization of Aspartic Acid in Water Crystal Growth & Design, 10 (4), 1652-1660 DOI: 10.1021/cg901219f

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6 Responses to “Amino acid crystallisation and the origin of life”

  1. AK Says:

    Actually, the answer’s easy: life uses left-handed amino acids because they’re easier for ribozymes formed from right-handed RNA to handle. Now as to the RNA…

    Actually, that one’s also pretty easy, too. Assume that at some earlier point, there was a mixture of processes and catalysts roughly divided between right- and left-handed RNA (or whatever came before RNA). These all had to produce a complete (if very simple) metabolism. Mutations would provide improvements, randomly, and as soon as the number of right-handed processes became significantly larger than the number of left-handed, the chance of a new beneficial mutation occurring among the right-handed processes became larger than among left-handed. The system was unstable, and it was certain that it would flop completely to one side or the other, but right-handed RNA (or DNA>RNA, IMO) was the outcome of a simple (single) coin toss.

  2. Lars Fischer Says:

    I don’t Think it’s that easy. Why should left-handed amino acids be easier to handle for right-handed ribozymes? What ribozymes can handle depends on the shape of their active center, not on handedness.

  3. AK Says:

    Because they are? I don’t know that they are, but certainly the ease (probability) of forming chiral active centers of a certain shape and specific handedness will usually be different between left- and right-handed RNA. That is, it will be easier for right-handed ribozymes to form the left-handed version of a particularly shaped chiral active center than the right. (And vice versa for other shapes.) The most parsimonious explanation, pending actual research, is that the left-handed centers were easier (more likely) than the right.

    Of course, if research should show that centers of either handedness are equally likely (which I consider intuitively unlikely), than it comes down to another coin toss: the necessary mutations to form left-handed proteins occurred, by chance, before those for right-handed.

    This whole search for determinism is chasing a red herring, IMO.

  4. federico falcon Says:

    Mr. Fisher says:
    1-“Where’s all that alanine to come from to create a saturated solution?”
    Reply:
    Assumptions 2 and 3 in the paper can be naturally achieved taking into account that, during the Azoic Age, Earth’s surface water reservoirs were for a long time doped with the same aminoacids composition due to Urey-Miller atmospheric reaction.
    On the other hand, the slow increase of aminoacids concentration can be accomplished by the cooling that Earth’s suffered at the end of the Azoic Age, which caused the partial freezing of liquid water in surface water reservoirs. The decreasing of liquid water volume and the segregation of amino acid molecules by the solidified water (ice) led to the slow increase of concentration of dissolved aminoacids in these water reservoirs. This concentration increase could reach such levels that crystallization may have taken place.
    This supposition is in full accordance with the nowadays growing opinion that life could be originated under freezing conditions.

    2-“Falcon thinks that parity violation influences the strength of hydrogen bonding, thus explaining the crystal size.
    I, on the other hand, can think of a number of possibilities much closer to home: Different growth conditions, traces of water, other impurities, temperature differences…”
    Reply:
    The presented “possible explanation” about parity violation needs to be confirmed, but in the case that the unitary cell volume difference was due to other causes (e.g. the different capacity of l and d-alanine to occlude disordered water into the crystal) the soundness of the proposed model would be maintained.

    Please, read the additions that will be made in the paper.

    The author.

  5. Lars Fischer Says:

    Dear Dr. Falcon,

    thanks for your reply. I will certainly watch with interest what comes out of this.

  6. federico falcon Says:

    Dear Dr. Fischer:
    At present I am looking for a laboratory that would be interested to conduct some experiments with the aim to verify my model. I have some ideas about the design of such experiments.
    Please, do you know about a possible interested laboratory?
    With regards,
    Federico Falcon

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