A likely possible origin of homochirality in amino acids and sugars on prebiotic earth

For life to start on earth and elsewhere, it is critical that the building blocks—amino acids and sugars—be in predominant homochiral form. Over the past century, the origin of terrestrial prebiotic homochirality has been the subject of many speculations. In this Letter I summarize the experimental evidence for ways in which some meteoritic components could have led to the dominance of L amino acids and D sugars on earth, and the most likely way in which the original chiral excesses in the meteorites were formed. 2010 Elsevier Ltd. All rights reserved. The homochirality of amino acids is critical to their function in proteins. If proteins with L amino acids had occasional random placements of the D enantiomers they would have varying and random conformations. While this is no problem in current biology, where the L amino acids are produced by the action of specific enzymes, the mode of formation of dominant L amino acids on prebiotic earth before the existence of such enzymes is less obvious. This problem has excited interest and speculation for at least 100 years, but the field has lacked convincing experimental support for the various theories until recently. The same problem exists for sugars, which have the D configuration in modern biochemistry. The amino acids generally have a single center of chirality whose three-dimensional geometry at a particular carbon defines the L configuration—the carbon bearing an amino group, a carboxyl group, a hydrogen atom, and a side-chain group (which only in threonine and leucine has an additional chiral center). However, the sugars such as ribose and glucose have several chiral centers; they are classified as D sugars, as in D-ribose and D-fructose, based on the configuration of the chiral center furthest from the carbonyl group of the sugar. Thus, for the sugars, the question is: how did this particular carbon become preferentially formed with the D configuration on prebiotic earth? A striking relevant recent result was the discovery that some meteorites have landed on Earth containing organic compounds, including some amino acids that are in proteins today and also some special amino acids with a methyl group in place of the hydrogen on the chiral center of normal L amino acids. The protein (2H) amino acids are racemic; they could have started with an excess of the L enantiomer, by the processes to be described below, d. All rights reserved. but over time they could have been converted to the racemate by reversible loss of a proton on the chiral center. However, no racemization is possible if that hydrogen is replaced by a methyl group. In the meteorite that landed near Murchison Australia in 1969, five a-methyl amino acids were found (Fig. 1), all of which had a small but real excess of what were described as the L enantiomers; in modern chemical notation these are the S enantiomers—structures with the methyl group attached where the hydrogen atom would be in L amino acids. This raises two questions: why do they have this excess of the S enantiomers, and how could they have played a role in generating the normal L amino acids and D sugars on earth? For the first question, the best evidence is the finding by astronomers that there is an excess of right circularly polarized light in this sector of the universe. Bonner showed that irradiating a racemic mixture of a normal amino acid with right circularly polarized light in the ultraviolet region led to selective destruction of the D enantiomer, resulting in a mixture with a few percent excess of the L enantioFigure 1. a-Methyl amino acids discovered in the Murchison meteorite, all of which have the S configuration that was described as L.