Published online on May 1, 2001, 10.1073/pnas.101085998
PNAS | May 8, 2001 | vol. 98 | no. 10 | 5487-5490

Geology
Selective adsorption of L- and D-amino acids on calcite: Implications for biochemical homochirality
Robert M. Hazen*,, Timothy R. Filley*,, and Glenn A. Goodfriend§

The emergence of biochemical homochirality was a key step in the origin of life, yet prebiotic mechanisms for chiral separation are not well constrained. Here we demonstrate a geochemically plausible scenario for chiral separation of amino acids by adsorption on mineral surfaces. Crystals of the common rock-forming mineral calcite (CaCO3), when immersed in a racemic aspartic acid solution, display significant adsorption and chiral selectivity of D- and L-enantiomers on pairs of mirror-related crystal-growth surfaces. This selective adsorption is greater on crystals with terraced surface textures, which indicates that D- and L-aspartic acid concentrate along step-like linear growth features. Thus, selective adsorption of linear arrays of D- and L-amino acids on calcite, with subsequent condensation polymerization, represents a plausible geochemical mechanism for the production of homochiral polypeptides on the prebiotic Earth.

Chem Commun (Camb). 2005 Apr 21;(15):2047-9. Epub 2005 Mar 1.

Plausible origins of homochirality in the amino acid catalyzed neogenesis of carbohydrates.

Córdova A, Engqvist M, Ibrahem I, Casas J, Sundén H.
Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, Sweden. acordova1a@netscape.net

The intrinsic ability of amino acids to catalyze the asymmetric formation of carbohydrates, which enzymes have mediated for millions of years, with significant amplification of enantiomeric excess suggests a plausible ancient catalytic process for the evolution of homochirality.

Proc Natl Acad Sci U S A. 2006 Aug 29;103(35):12979-80. Epub 2006 Aug 22.

Amplification of enantiomeric concentrations under credible prebiotic conditions.

Breslow R, Levine MS.
Department of Chemistry, Columbia University, New York, NY 10027, USA. rb33@columbia.edu

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. 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.

However, there is still another step that must be accomplished before even this can take place that complicates the process even further.

Before the Long chains of amino acids can become a protein, they must first be joined into "short chains" called peptides. In or order for this to even begin to take place, an amino acid molecule must react with another and become joined through an amide linkage.

This just does not occur naturally, let me qualify that by saying that it has never been observed or demonstrated to occur naturally.

Science. 2004 Oct 8;306(5694):283-6.

Carbonyl sulfide-mediated prebiotic formation of peptides.

Leman L, Orgel L, Ghadiri MR.
Department of Chemistry and Skaggs Institute for Chemical Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, CA 92037, USA.

Almost all discussions of prebiotic chemistry assume that amino acids, nucleotides, and possibly other monomers were first formed on the Earth or brought to it in comets and meteorites, and then condensed nonenzymatically to form oligomeric products. However, attempts to demonstrate plausibly prebiotic polymerization reactions have met with limited success. We show that carbonyl sulfide (COS), a simple volcanic gas, brings about the formation of peptides from amino acids under mild conditions in aqueous solution. Depending on the reaction conditions and additives used, exposure of alpha-amino acids to COS generates peptides in yields of up to 80% in minutes to hours at room temperature.

A possible primordial peptide cycle.Huber C, Eisenreich W, Hecht S, Wächtershäuser G.

Department for Organic Chemistry and Biochemistry, Technische Universität München, Lichtenbergstrabetae 4, D-85747 Garching, Germany.

alpha-Amino acids can undergo peptide formation by activation with carbon monoxide (CO) under hot aqueous conditions in the presence of freshly coprecipitated colloidal (Fe,Ni)S. We now show that CO-driven peptide formation proceeds concomitantly with CO-driven, N-terminal peptide degradation by racemizing N-terminal hydantoin and urea derivatives to alpha-amino acids. This establishes a peptide cycle with closely related anabolic and catabolic segments. The hydantoin derivative is a purin-related heterocycle. The (Fe,Ni)S-dependent urea hydrolysis could have been the evolutionary precursor of the nickelenzyme urease. The results support the theory of a chemoautotrophic origin of life with a CO-driven, (Fe,Ni)S-dependent primordial metabolism.

A theory that life on earth began at hydrothermal (hot water) vents in the ocean floor has been proved false by recent experiments. “This is probably the most unlikely area for the origin of life to occur,” said chemist Jeffrey L. Bada of the University of California.

The theory had been advanced after the discovery of thriving bacteria and other organisms, such as giant clams and worms, around the hydrothermal vents.

Simulating the temperatures and pressures of the vents, Bada and his colleague, Stanley L. Miller, found that amino acids, the building blocks of life, decomposed rapidly under such conditions. “The combination of amino acids into larger peptide molecules, known as polymerization, was found to be impossible in the presence of water at any temperature,” notes The New York Times. “And more complex molecules carrying the genetic code, a requirement for living organisms, did not last long in the extreme heat.”
According to the Times, the researchers concluded “that the hot waters in the primitive oceans would have destroyed rather than created organic compounds in the primitive oceans.”