Template-Directed Nonenzymatic Primer Extension Using 2‑Methylimidazole-Activated Morpholino Derivatives of Guanosine and Cytidine

Efforts to develop self-replicating
nucleic acids have led to insights
into the origin of life and have also suggested potential pathways
to the design of artificial life forms based on non-natural nucleic
acids. The template-directed nonenzymatic polymerization of activated
ribonucleotide monomers is generally slow because of the relatively
weak nucleophilicity of the
primer 3′-hydroxyl. To circumvent this problem, several nucleic
acids based on amino-sugar nucleotides have been studied, and as expected,
the more-nucleophilic amine generally results in faster primer extension.
Extending this logic, we have chosen to study morpholino nucleic acid
(MoNA), because the secondary amine of the morpholino-nucleotides
is expected to be highly nucleophilic. We describe the synthesis of
2-methylimidazole-activated MoNA monomers from their corresponding
ribonucleoside 5′-monophosphates and the synthesis of an RNA
primer with a terminal MoNA nucleotide. We show that the activated
G and C MoNA monomers enable rapid and efficient extension of the
morpholino-terminated primer on homopolymeric and mixed-sequenced
RNA templates. Our results show that MoNA is a non-natural informational
polymer that is worthy of further study as a candidate self-replicating
material.