A Sequential Scenario for the Origin of Biological Chirality
A sequential model is proposed regarding the origin of biological chirality. Three major stages are presumed: a symmetry breaking (prebiotic chiral disruption in enantiomeric mixtures of monomers), a chiral amplification (prebiotic increase of the chiral character of the monomers affected first by the symmetry breaking), and a chiral expansion (proto biological increase of the chiral character and spread of the chirality to molecules which were less affected by prebiotic chiralizations). As a symmetry-breaking mechanism, the model proposed by Deutsch (1991) is used, which involves a dissymmetric exposure of amino acids (AA) to ultraviolet circularly polarized light (UV-CPL) on evaporative seashores. It is presumed that the chiral amplification, up to a protobiologic significance, was influenced by a periodic overlapping of two abiotic events, a synchronization between tidal-based hydrous-anhydrous cycles, and littoral asymmetric photolysis cycles. This long-term astronomic asymmetry acted around 3.8-4.2 billion years ago and was unique to the Earth in our solar system. It is also presumed that the abiotic symmetry breaking is heterogenous, that only a few l-AAs were used in the beginning, and that the chirality expanded later to all 20 AAs based on a coevolutionary strategy of the genetic code and on a physiological relationship between AAs. In this scenario the d-chirality of pentoses in polynucleotides was attributed to both d-pentose/l-AA relationships and to a structural evolution.
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Department of Biology, The American University, 4400 Mass. Avenue, N.W., Washington, DC 20016, USA