"We eat optically active bread & meat, live in houses, wear clothes, and read books made of optically active cellulose. The proteins that make up our muscles, the glycogen in our liver and blood, the enzymes and hormones, the sugars in DNA and RNA and in the metabolic pathways … are all optically active. Naturally occurring substances are optically active because the enzymes which bring about their formation … are optically active. As to the origin of the optically active enzymes, we can only speculate." (from Morrison and Boyd, 1987)


The handedness is complete mistery to evolutionists
(is homochirality in the monomeric building-blocks of biopolymers a condition for the initiation of a prebiotic chemistry?)


The alternative to an evolutionary origin of homochirality is that it was achieved prior to the origin of life. In the primordial soup, D- and L-amino acids would likely have been present in nearly equal amounts. The preference for L-amino acids has been explained differently: a) certain chemical processes were favored energetically leading to increased levels of L-isomers of amino acids on the early earth; b) meteorites brought L-amino acids to earth thus provifing an imbalance that may have favored the formation of life; c) it is related to the creationistic theory of origin of life.


The mystery has not been solved yet!


According to the LOH-theory (hydrate hypothesis of living matter origination), amino acids originated within the methane-hydrate localizations from natural gas, niter, and included sulfur-containing substances at temperatures close to those of living organisms. The geometric differences of the L- and D-forms are rather significant: accordingly, the mechanisms of the sets of elementary steps that lead to their formation from the source methane and nitrate are rather different.

At 270-290 K the reaction proceeded slowly: the direction of the synthesis and the final optical structure of any amino acid was dictated not by thermodynamic but by kinetic! Racemization is expected but proceeds extremely slow.

The primary AAs (and proteins) were produced from natural gas and niter at a low temperature and within solid/semiliquid gas-hydrate structures: under these conditions, L-AAs only were produced that could not transform into the D-form because of the kinetic nature. Each AA was formed within some cavity and was not in touch with other AA molecules: L-AA is produced that can not be tranformed into the D-form.

The reaction of formation of L-and D-isomers of AAs proceed with different rates under the same ambient conditions. Racemization is thermodynamically expected but kinetically inhibited as a result of differences in the rates of formation of different optical isomers because of the complexity of the intramolecular rearrangements (racemization of asparagine acid ester has a rate of 0.1% per year!). It seems very probable that certain D-AAs present on the primitive earth had enantiomeric excess and that these excesses were large enough to be susceptible to amplification (e.g, by polymerization).

Monod (1970): homochirality of terrestrial life was a necessity and not a matter of chance selection.

Origin of homochirality

1. Spontaneous symmetry breaking:

    a) Spontaneous separation of a racemic mixture into its constituents enantiomers during crystallization (difficult on the primitive Earth)

    b) Selective adsorption of one enantiomer from a racemic mixture at the surface of a chiral crystal (i.e. quartz crystals)

2. Homochirality arose from statistical fluctuation from the equimolar condition

The energy difference between enantiomers is called the parity-violating energy difference (PVED, 10-14 J/mol, corresponding to e.e. of 10-15%. The violation of parity constitutes a symmetry breaking at the level of the basic laws of atomic physics.


  • Blackmond group: compounds can strongly influence the e.e. in solution under solid-liquid equilibrium conditions (e.g., valine e.e. increase from 47% to 99% in the presence of fumaric acid).
  • Polymerization of a Glu derivative occurs 20-fold faster starting with molecules of the same chirality (L-L or D-D reactions) rather than with a racemic mixture (L-D or D-L reactions). Biopolymers result from the polymerization of chiral monomers.
  • HIV-1 protease was synthesized using D-AAs: it exhibits catalytic activity identical to that of the native enzyme except that it is specific for the opposite enantiomer of the substrate.

References:

  • Dolgin E. (2009). "Did lefty molecules seed life?" - The Scientist Magazine
  • Doolittle, R. Probability and the origin of life (1983). In: Godfrey, L.R., ed., Scientists Confront Creationism, W.W. Norton, NY.
  • Kadyshevich E.A. and Ostrovskii V.E. "Natural Mechanism of Origination and Conservation of Monochirality of Amino Acids - Chirality 2015
  • Klussmann, M., Toshiko, I., White, A.J.P., Armstrong, A., Blackmond, D.G., Emergence of solution-phase homochirality via crystal engineering of amino acids (2007), J. Am. Chem. Soc. 129: 7657-7660
  • Morrison, R.T. and Boyd, R.N. (1987). Organic Chemistry, 5th ed. (Allyn & Bacon Inc.), page 150
  • Riddle M. (2008). Can natural processes explain the origin of life? -  - The New Answers Book 2, ed, Ken Ham (Green Forest, AR: Master Books), pp. 15-24
  • The Origin of Life: A Critique of Current Scientific Models (www.creation.com/origin-of-life-critique)

Authors:

Loredano Pollegioni, Università degli Studi dell'Insubria