Intramolecular interactions in aminoacyl cyclic-3′,5′-nucleotides

Polymerization of amino-acid acyl cyclic-3′,5′-nucleotides is postulated to be the origin of RNA and associated protein in prebiotic molecular evolution. The enthalpy change in the intramolecular interaction between the nucleotide base and the amino-acid side chain determines the stability of the particular complex, resulting in a preferred association (or coding) of a base for a particular amino acid. The compounds studied were glycine acyl cyclic-3′,5′-guanylate where the strong hydrogen bond between protonated glycine and guanine N7 gives an enthalpy change of −0.05 h. Similarly, hydrogen bonds in l-lysine acyl cyclic-3′,5′-adenylate give an enthalpy change of −0.06 h. Hydrophobic interactions in l-phenylalanine acyl cyclic-3′,5′-uridylate give an enthalpy change of −0.02 h and the corresponding value for l-proline acyl cyclic-3′,5′-cytidylate is −0.01 h. These interactions were expected to be modified as the genetic code became a duplet and finally a triplet code. The interactions have been shown to be feasible from the overall enthalpy changes in the ZKE approximation at the MP2/6-31G* level.