D-RNA selects L-amino acids in a newly characterized aminoacyl-RNA loop ligation.

Abstract

Biological life adheres to molecular asymmetry to an extent that chirality is considered a requisite for the earliest biochemical systems. Particularly, L-amino acids and D-sugars (in nucleic acids) are almost exclusive throughout biology, while purely chemical synthesis pathways for these molecules (which necessarily predate biological synthesis pathways) yield an even split of both enantiomers. Thus, the origins-of-life field has outstanding interest in physical and chemical mechanisms for symmetry breaking. Numerous investigations on the origins of homochirality have yielded explanations including magnetic fields, spin-polarized electrons, crystallization, and the Soai reaction. However, proposed mechanisms tend to target molecules of a single species or those resulting from a particular synthesis pathway. These mechanisms are significant only because chemical reactions with such enantioenriched species are assumed to propagate asymmetry throughout chiral molecules. We experimentally suggest such proliferation of asymmetry through an enantiopreferential and self-amplifying reaction between amino acids and RNA - loop ligation. The loop ligation reaction takes place with two RNA strands in a nicked stem-loop, in which the 3’ hydroxyl of one strand is aminoacylated by an amino acid chemically activated with imidazole, forming an aminoacyl ester. The amino acid ligates to the imidazole-activated 5’ phosphate of a second RNA strand, forming a phosphoramidate linkage. The final product is a closed stem loop with an amino acid-bridged backbone. Our results demonstrate five-fold preference for incorporation of L-amino acids in a backbone of D-RNA. We determined that enantioselectivity occurs most prominently in the loop-closing step based on experiments which isolated individual kinetic steps of the loop ligation process. Because aminoacylation is the cornerstone of RNA-coded protein synthesis, the enantiopreference observed in prebiotically plausible loop ligation offers a chemical axis for the selection of L-amino acids in a D-RNA world.