Threose Nucleic Acid

Threose Nucleic Acid (TNA), formally designated α-L-threofuranosyl-(3'→2') nucleic acid, is a synthetic xeno-nucleic acid polymer in which the five-carbon β-D-ribose sugar of ribonucleic acid (RNA) is replaced by a four-carbon tetrose sugar, α-L-threose, connected by 3'→2' phosphodiester linkages. Synthesized and characterized in 2000 by Kai-Uwe Schöning, Peter Scholz, Sreenivasulu Guntha, Xiaohua Wu, Ramanarayanan Krishnamurthy, and Albert Eschenmoser at the Scripps Research Institute and ETH Zürich, TNA emerged from Eschenmoser's chemical etiology of nucleic acid structure—an experimental program designed to test whether five-carbon ribofuranosyl nucleic acids were chemically predetermined by Hadean prebiotic chemistry or selected from a broader library of sugar-phosphate isomers.

Despite having a five-bond repeating phosphodiester backbone unit rather than the six-bond repeating unit of canonical RNA and deoxyribonucleic acid (DNA), α-L-TNA strands form stable, antiparallel Watson–Crick homoduplexes and cross-hybridize with complementary single-stranded RNA and DNA templates. Because the four-carbon threose sugar contains fewer stereocenters than ribose, forms in a single C₂ + C₂ aldol dimerization of glycolaldehyde, and lacks a free vicinal hydroxyl group adjacent to the phosphodiester bridge, TNA exhibits ~100-fold greater hydrolytic resistance than RNA and is a model polymer for a pre-RNA world stage of abiogenesis.

Tetrose vs. Pentose Prebiotic Synthesis and Stereoisomer Count

In prebiotic sugar synthesis via the formose reaction, assembling a five-carbon pentose such as ribose requires sequential aldol condensations combining a C₂ unit (glycolaldehyde) with a C₃ unit (glyceraldehyde). Because pentoses possess three asymmetric carbon atoms in their open-chain aldopentose series (and four chiral centers in the cyclized furanose ring), non-selective synthesis generates eight stereoisomers (D/L-ribose, D/L-arabinose, D/L-xylose, and D/L-lyxose) that equilibrate with pyranose six-membered rings.

By contrast, a four-carbon aldotetrose contains two asymmetric carbon atoms in its open-chain form (and three in the cyclized furanose ring), cutting the number of stereoisomers in half to four: D- and L-threose plus D- and L-erythrose. Furthermore, tetroses are the product of a single C₂ + C₂ aldol dimerization between two molecules of glycolaldehyde (HOCH₂–CHO), eliminating cross-condensation with glyceraldehyde. Because a tetrose lacks the exocyclic 5'-CH₂OH hydroxymethyl group of ribose, cyclization can only yield a five-membered furanose ring; competing six-membered pyranose ring isomers cannot form.

Five-Bond Repeat and Hydrolytic Stability

Prior to Eschenmoser's work, structural biologists assumed that Watson–Crick cross-pairing with RNA and DNA required a six-bond repeating unit along the sugar-phosphate backbone. Eschenmoser and colleagues demonstrated that in α-L-threofuranosyl-(3'→2') oligonucleotides, the two oxygen atoms at C2' and C3' sit in a trans configuration on the furanose ring. When the threofuranose ring adopts a C4'-exo pucker, the 2'- and 3'-phosphate substituents project in a quasi-trans-diaxial orientation that stretches the inter-phosphorus distance to ~0.57–0.59 nm, matching the axial rise and 11-base-pair-per-turn A-form helical twist of RNA despite containing five covalent bonds per monomer repeat. Note the stereochemical relationship: α-L-threofuranose (derived from L-threose) forms a right-handed helix that pairs antiparallel with natural β-D-RNA and β-D-DNA.

In canonical RNA, the 3'→5' phosphodiester linkage leaves the 2'-hydroxyl group unesterified, allowing deprotonated 2'-OH to attack the adjacent phosphorus atom and cleave the RNA chain via a 2',3'-cyclic phosphate intermediate. In TNA, both the 2'-OH and 3'-OH groups are esterified into the 3'→2' phosphodiester backbone, and C4' carries only hydrogen atoms. Lacking a free hydroxyl nucleophile on the ring, TNA phosphodiester bonds are ~100-fold more resistant to alkaline hydrolysis than RNA and resist biological RNases.

Engineered TNA Polymerases and In Vitro Selection

For TNA to function as a pre-RNA genetic polymer, it must fold into three-dimensional structures capable of ligand recognition and catalysis. In 2012, John C. Chaput and coworkers (simultaneously with Philipp Holliger's laboratory) engineered mutants of the archaeal Thermococcus kodakarensis (Kod) B-family DNA polymerase—specifically Kod-RI and Kod-RSGA—that transcribe DNA templates into α-L-TNA using α-L-threofuranosyl nucleoside 3'-triphosphates (tNTPs) at ~1 nt/s with ~99% fidelity, while Bacillus stearothermophilus (Bst) DNA polymerase or Tgo RT521 reverse-transcribes TNA back into DNA. Using this cycle for SELEX, Chaput's group isolated TNA aptamers against human α-thrombin, HIV-1 reverse transcriptase, and ochratoxin A with dissociation constants (K_d) of 0.2 to 50 nM.

Catalytic Threozymes and Genetic Takeover by RNA

In 2020–2022, Hanyang Yu, John Chaput, and colleagues evolved catalytic TNA oligomers (threozymes) that accelerate bond formation and cleavage. The evolved TNA ligase enzyme T8-6 catalyzes template-directed formation of a phosphodiester bond between an RNA substrate and a 5'-triphosphorylated RNA fragment (k_obs = 0.015 min^-1, a ~10⁶-fold rate enhancement), while endonuclease threozymes such as Tz1 cleave target RNA transcripts in vivo. The existence of catalytic threozymes and bidirectional TNA↔RNA base pairing provides a chemical mechanism for genetic takeover in abiogenesis, whereby an early C₂+C₂ tetrose genetic system transferred its hereditary sequences to RNA by cross-templating.

Why is α-L-threose used in TNA instead of β-D-threose to pair with natural D-RNA and D-DNA?

Although natural RNA and DNA use β-D-ribofuranose and β-D-2'-deoxyribofuranose, the C4' carbon of a pentose ring carries a –CH₂OH group that determines D/L nomenclature. In a four-carbon threofuranose ring, C4' carries only hydrogen atoms, so the C3' hydroxyl group determines the D/L configuration. When the nucleobase is attached in the α-configuration on L-threofuranose (α-L-TNA), the spatial orientation of the C1' glycosidic bond relative to the C2' and C3' phosphodiester groups mirrors the right-handed stereochemistry of β-D-RNA, allowing antiparallel Watson–Crick base pairing with natural D-nucleic acids.

How can TNA have a 5-bond backbone repeat and still base-pair with 6-bond RNA and DNA?

Canonical RNA and DNA contain six covalent bonds per monomer repeat along the 5'→3' phosphodiester backbone (P–O5'–C5'–C4'–C3'–O3'). Because TNA lacks a C5' carbon and links monomers directly across the ring via 3'→2' phosphodiester bonds (P–O3'–C3'–C2'–O2'), its repeat contains five covalent bonds. However, steric and electrostatic repulsion between the trans-oriented 2'- and 3'-phosphate groups forces the threofuranose ring into a C4'-exo pucker with quasi-diaxial phosphates, stretching the five-bond unit to a 0.58 nm phosphorus-phosphorus distance that matches the 11-base-pair-per-turn A-form helix of RNA.

Why is Threose Nucleic Acid 100 times more resistant to hydrolysis than RNA?

RNA degrades in water at neutral to alkaline pH or in the presence of divalent cations (such as Mg²⁺) because every ribose ring possesses a free 2'-hydroxyl (2'-OH) group located adjacent to the 3'→5' phosphodiester bond. This 2'-OH group functions as an internal nucleophile that attacks the phosphorus atom to cleave the backbone via 2',3'-cyclic phosphate formation. In TNA, both the 2'-OH and 3'-OH groups are esterified into the 3'→2' phosphodiester backbone, leaving zero free hydroxyl groups on the sugar ring to initiate intramolecular cleavage.

1. Schöning, K.-U., Scholz, P., Guntha, S., Wu, X., Krishnamurthy, R., & Eschenmoser, A. (2000). "Chemical etiology of nucleic acid structure: The α-threofuranosyl-(3'→2') oligonucleotide system." Science, 290(5495), 1347–1351. https://doi.org/10.1126/science.290.5495.1347
2. Yu, H., Zhang, S., & Chaput, J. C. (2012). "Darwinian evolution of an alternative genetic system provides support for primordial genetic polymers." Nature Chemistry, 4(3), 183–187. https://doi.org/10.1038/nchem.1241
3. Ebert, M.-O., Mang, C., Krishnamurthy, R., Eschenmoser, A., & Jaun, B. (2008). "The structure of a TNA–TNA duplex in solution: NMR study of the octamer duplex derived from α-(L)-threofuranosyl-(3'-2')-CGAATTCG." Journal of the American Chemical Society, 130(45), 15105–15115. https://doi.org/10.1021/ja8041959
4. Wang, Y., Nguyen, K., Spitale, R. C., & Chaput, J. C. (2021). "A biologically stable DNAzyme that efficiently silences gene expression in cells." Nature Chemistry, 13(4), 319–326. https://doi.org/10.1038/s41557-021-00645-x