Synthesis of 2′-Deoxy-α‑l‑threofuranosyl Nucleoside Triphosphates

α-l-Threofuranosyl nucleic acid (TNA) is an artificial genetic polymer in which the natural five-carbon ribose sugar found in RNA has been replaced with an unnatural four-carbon threose sugar. Despite a different sugar–phosphate backbone, TNA is capable of forming stable, antiparallel Watson–Crick duplex structures with itself and with complementary strands of DNA and RNA. This property of intersystem base pairing, coupled with the chemical simplicity of threose relative to ribose, provides support for TNA as a candidate RNA progenitor in the evolution of life. In an effort to evaluate the functional properties of TNA by in vitro evolution, engineered polymerases have been developed that are capable of copying information back and forth between DNA and TNA. However, the current generation of TNA polymerases function with reduced activity relative to their natural counterparts, which limits the evaluation of TNA as a primordial genetic material. Here, we describe the chemical synthesis and polymerase recognition of 2′-deoxy-α-l-threofuranosyl nucleoside 3′-triphosphates (dtNTPs) as chain-terminating reagents in a polymerase-mediated TNA synthesis reaction. The synthesis of dtNTPs should make it possible to investigate the mechanism of TNA synthesis by X-ray crystallography by trapping the polymerase in the catalytically active conformation.