August 24, 2006 | David F. Coppedge

Origin of Left-Handed Proteins Solved?

As noted in prior entries here (09/06/2003, 11/19/2004) and in our online book, the origin of left-hand proteins is recognized as one of the most formidable challenges to naturalistic origin-of-life research.  Occasionally researchers develop lab techniques for getting slight excesses of one hand over the other.  Astrobiologists agree, however, that 100% purity in a protein chain is biologically useful (01/28/2005).  When they go looking on other planets, they usually regard pure one-handedness as foolproof evidence for life (07/13/2005).
    This week in PNAS,1 two researchers at Columbia University published a short but striking paper that claimed they may have found a way the early earth separated the two types.  By twice wetting and evaporating one particular kind of amino acid, they were able to separate out the mixture almost completely because of differing solubilities of the two hands (enantiomers).  Their abstract was not without some understated glee about what this could mean:

Solutions with as little as 1% enantiomeric excess (ee) of D- or L-phenylalanine are amplified to 90% ee (a 95/5 ratio) by two successive evaporations to precipitate the racemate [mixture].  Such a process on the prebiotic earth could lead to a mechanism by which meteoritic chiral {alpha}-alkyl amino acids could form solutions with high ee values that were needed for the beginning of biology.

Since some of the amino acids found in meteorites arrive with a slight enantiomeric excess to begin with, they feel this simple evaporative mechanism might amplify the excess to the point where a pure one-handed solution could arise purely by chance and natural law.
    Another problem remains, though.  A homochiral (one-handed) protein is of no use without a homochiral sugar to match with in the genetic code.  They referred to other researchers who have found possible ways this might have happened, though so far with only 10% success at best, and not under plausible prebiotic conditions.
    Nevertheless, they feel they are on the way to finding how the chemical soup separated out these otherwise chemically identical molecules that only differ by their mirror-image configuration:

We propose that such a process could occur and may have occurred under prebiotic conditions.  As a water solution of one of the meteoritic amino acids or of one of the products from it dried, there would be an increased enantiomeric concentration in the solution.  If that solution ran off from the solid racemate, or indeed if subsequent chemistry simply occurred in the solution, the modest ee’s from the meteoritic components or their subsequent products could be amplified in solution to start the processes leading to our observed homochiralities in life today….

Breslow and Levine, “Amplification of enantiomeric concentrations under credible prebiotic conditions,” Proceedings of the National Academy of Sciences USA, 10.1073/pnas.0605863103, published online before print August 22, 2006. 1

These two apparently hoped their brief paper would start the herald angels of astrobiology sounding their trumpets.  Nothing really new, however, was shown, and the problems are still staggering.  Keeping in mind that molecules do not care anything about forming life, consider just a few of the problems:

  1. It only worked with the alpha-alkyl amino acids (so far, although they said they are testing other kinds).
  2. All the amino acid types would have to conspire to be left- or right-handed.  For the 20 different amino acids in living organisms, this is astronomically improbable.
  3. In realistic prebiotic conditions, nothing is going to prevent the next wave, meteorite or current from mixing the two hands all up again.
  4. The need for repeated evaporations severely limits the physical space where all required ingredients could form and process their blind, random walk through configuration space.
  5. This scenario rules out the deep-sea vent and open-ocean scenarios.  Champions of those views are undoubtedly going to find ways to shoot it down.
  6. Evaporative environments expose the amino acids to ultraviolet radiation that will destroy them all within a few hours anyway.
  7. There is still the problem of getting amino acids to link up into polypeptides.
  8. There is still the problem of getting a nascent polypeptide with a functional sequence.  Remember, only chance arrangements are permitted.
  9. One mis-handed amino acid in a growing chain will still render it useless for biology.  The chance for this happening, even if amino acids did link up somehow, grows with the length of the chain.
  10. Even if, under the most wildly improbable strokes of luck, a pure polypeptide formed, it would be the end of the line without a genetic code that is also homochiral and able to make backup copies.

That’s just a first draft of the possible problems.  The fact remains that amino acids, enantiomeric excess or not, cannot care what happens.  Nobody is cheering them on.  Nobody will give them a gold medal if they purify themselves.  Molecules have no personality, except that, according to the Law of Perversity of Inanimate Objects, they love to frustrate chemistry students and astrobiologists.
    It is a logical fallacy to assume evolution to demonstrate evolution.  You have to be realistic and put yourself at the edge of the primordial soup as a detached observer, unable to assist with your lab equipment and intelligent design.  The molecules are only going to obey the laws of mass action, valence bonding and thermodynamics.  It will be a long, boring, hopeless wait when you realize that you cannot interfere.
    Despite the glee in the subtext of this short paper, there is nothing left but vanity and despair.  The mythical primordial soup did not have a Columbia chemistry lab with PhDs present to help.  Without guidance, molecules just bounce and stick in careless ways.  All these researchers did was illustrate again the power of intelligent design to overcome the innate tendencies of mindless matter to never mind about such matters.

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Categories: Origin of Life

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