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<p>At an early stage of evolution only four amino acids (Gly, Asp, Val, Ala) were encoded by the genetic code (GCo). Using ternary phase diagrams, we show that among racemic compound amino acids, solution enantiomeric excess (<i>ee</i>) of Asp, Val and Ala would be amplified preferentially by "self-disproportionation of enantiomers (SDE)" in a scalemic sample and thus become homochiral before others. For any seed solution-<i>ee</i> of a racemic compound amino acid, an equation is derived for minimum required threshold supersaturation for this <i>ee</i> to be amplified to its eutectic value. The supersaturation threshold is lowest for Asp, Val and Ala, resulting in amplification of their <i>ee</i> before other amino acids, a reason for their early incorporation in GCo that has hitherto been overlooked. The <i>ee</i> amplification factor is a linearly increasing function of (C<sub>0</sub>/C<sub>E</sub>), the ratio of equilibrium racemic concentration (C<sub>0</sub>) and that at the eutectic point (C<sub>E</sub>). This ratio is 0.96, 0.89, 0.80, 0.62, 0.58, 0.41, 0.13 for Asp, Val, Ala, Pro, Ile, Leu, Ser respectively. Similar reasoning is used to show that among the four nucleosides that make the backbone of the GCo, solution-<i>ee</i> of guanosine (G) was preferentially amplified before other three and could explain why GGN is a RNA codon for Gly, the achiral amino acid that was probably the first to be encoded by the GCo.</p>

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Early genetic code is explained by preferential amplification of enantiomeric excess of amino acids and nucleosides

  • 期刊论文

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  • 发表日期：

    2025年2月14日

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  • A. Sharma

摘要

At an early stage of evolution only four amino acids (Gly, Asp, Val, Ala) were encoded by the genetic code (GCo). Using ternary phase diagrams, we show that among racemic compound amino acids, solution enantiomeric excess (ee) of Asp, Val and Ala would be amplified preferentially by "self-disproportionation of enantiomers (SDE)" in a scalemic sample and thus become homochiral before others. For any seed solution-ee of a racemic compound amino acid, an equation is derived for minimum required threshold supersaturation for this ee to be amplified to its eutectic value. The supersaturation threshold is lowest for Asp, Val and Ala, resulting in amplification of their ee before other amino acids, a reason for their early incorporation in GCo that has hitherto been overlooked. The ee amplification factor is a linearly increasing function of (C0/CE), the ratio of equilibrium racemic concentration (C0) and that at the eutectic point (CE). This ratio is 0.96, 0.89, 0.80, 0.62, 0.58, 0.41, 0.13 for Asp, Val, Ala, Pro, Ile, Leu, Ser respectively. Similar reasoning is used to show that among the four nucleosides that make the backbone of the GCo, solution-ee of guanosine (G) was preferentially amplified before other three and could explain why GGN is a RNA codon for Gly, the achiral amino acid that was probably the first to be encoded by the GCo.