Factors of protobiomonomer selection for the emergence the genetic code

Abstract

The natural selection of specific protobiomonomers during abiogenic development of the prototype genetic code is hindered by the diversity of structural, spatial, and rotational isomers. They have identical elemental composition and molecular mass (M), but can vary significantly in their physicochemical characteristics, such as melting point Tm, the Tm : M ratio, and solubility in water, due to the different arrangement of atoms in the molecule. These parameters differ between cis- and trans-isomers of dicarboxylic acids, spatial monosaccharide isomers, and structural isomers of α-, β-, and γ-amino acids. The stable planar heterocyclic molecules of the major nucleobases comprise four (C, H, N, O) or three (C, H, N) elements and contain a single -C=C bond and two nitrogen atoms in each heterocycle involved in C-N and C=N bonds. They exist as isomeric resonance hybrids of single and double bonds and as a mixture of tautomer forms due to the presence of -C=O and/or -NH2 side groups. They are thermostable, insoluble in water, and exhibit solid-state stability, which is of central importance for DNA molecules as carriers of genetic information. In M Tm diagrams, proteinogenic amino acids and the corresponding codons are distributed in a rather ordered manner relative to the distinct clusters of purine and pyrimidine bases, reflecting the correspondence between codons and amino acids that was established in different periods of genetic code development. The body of data on the evolution of the genetic coding system indicates that the elemental composition and molecular structure of protobiomonomers, and their M, Tm, photostability, and aqueous solubility determined their selection in the emergence of the standard genetic code.