So called biotic or selection theories were developed in the 1950s in order to explain the biomolecular asymmetry. According to these theories, two kinds of mirror-image organisms could have co-existed next to each other at a cellular or protocel-lular level in a very early phase of the biochemical or biological evolution. One type consisted of proteins (and nucleic acids) composed of L-amino acids (and d-sugars) and the other one of D-amino acid (and L-sugar) enantiomers. Both antipodal organisms co-existed for a certain time and the more favourable mirror image form, having a certain evolutionary advantage such as the development of "metabolic interdependence", could have been selected by interacting with other non-racemic substances (Wald 1957). This theory suggests that life arose in a racemic environment, and homochirality as such only developed later (Bonner 1995a). Wald suggested that the a-helix, formed during peptide growth, may function as the stereoselectivity agent, favouring the incorporation of one amino acid enantiomer over the other.
At the University of New Orleans, in the US, Stanley Goldberg and his team intensely tried to discover experimental evidence for Wald's theory. Goldberg et al. undertook an investigation in an effort to assess the tendency of an arbitrarily chosen system of a peptide assembly to display biased stereoselectivity. Derivatives of a-alanine, aspartic acid, and glycine were chosen for the formation of 34 different structures of di-, tri-, and tetrapeptides. These amino acids had been known for their probable presence in prebiotic environments. And indeed, each of the competitive peptide-forming processes was found to be stereoselective. Goldberg et al. (1987) concluded that these results supported the notion that reaction conditions may be found to allow or promote the stepwise assembly of chiral monomers into even small chains with significant levels of stereoregularity.
More evidence was compiled for the biotic theory by the fact that in contrast to "normal" proteins, D-amino acids were discovered in a number of specific biological samples such as bacterial cells (Nagata et al. 1998), vertebrate brains (Nagata et al. 1994), and others, which might be interpreted as relicts of mirror-image life and which might emphasize the biological selection process of specific antipodes during evolution.
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