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The exogenous delivery of cometary and asteroidal material is observed today and undoubtedly has showered the Earth through its prior history (1). Because carbonaceous meteorites contain amino acids displaying asymmetry that, if not as extensive, has the same sign (L) as terrestrial amino acids (2), it is reasonable to ask whether these chiral compounds, acquired upon delivery to the early Earth, could have played a role in the origin of homochirality by transferring their asymmetry to other prebiotic building blocks, such as sugars.
To assess this possibility, we examined the catalytic influence of two nonracemic amino acids, alanine and isovaline, on a water-based prebiotic model of sugar syntheses from glycolaldehyde and formaldehyde (3). Alanine is a common protein amino acid and isovaline is the most abundant chiral amino acid in meteorites. Results show a catalytic asymmetric effect.
Two series of sugar syntheses were studied; in one, the reagent was glycolaldehyde alone, and in the second, formaldehyde and glycolaldehyde were reacted in equal amounts. Reactions were carried out in an aqueous triethylammonium acetate buffer (pH 5.4 at 50°C) in the presence of either amino acid in various enantiomeric excesses (ee). The duration was 10 hours to limit sugar production to the four-carbon species, threose and erythrose, which are formed by aldol condensation of two glycolaldehyde molecules. Reaction products were analyzed by gas chromatography-mass spectrometry, using a chiral phase that separated all tetrose enantiomers (fig. S1).
The catalyzed aldol condensation of glycolaldehyde in water produces tetroses whose chiral configuration is affected...