Selective Prebiotic Synthesis of α-Threofuranosyl Cytidine by Photochemical Anomerization

Abstract

The structure of life's first genetic polymer is a question of intense ongoing debate. The "RNA world theory" suggests RNA was life's first nucleic acid. However, ribonucleotides are complex chemical structures, and simpler nucleic acids, such as threose nucleic acid (TNA), can carry genetic information. In principle, nucleic acids like TNA could have played a vital role in the origins of life. The advent of any genetic polymer in life requires synthesis of its monomers. Here we demonstrate a high-yielding, stereo-, regio- and furanosyl-selective prebiotic synthesis of threo-cytidine 3, an essential component of TNA. Our synthesis uses key intermediates and reactions previously exploited in the prebiotic synthesis of the canonical pyrimidine ribonucleoside cytidine 1. Furthermore, we demonstrate that erythro-specific 2',3'-cyclic phosphate synthesis provides a mechanism to photochemically select TNA cytidine. These results suggest that TNA may have coexisted with RNA during the emergence of life.

Keywords: TNA; nucleotides; origin of life; photochemistry; prebiotic chemistry.

Figure 1

Nucleic acid backbones of TNA, RNA (R=OH, R¹=H), ANA (R=H, R¹=OH), and DNA (R=H, R¹=H). Truncated 3′‐2′‐phosphodiester of TNA (blue) compared to natural 5′‐3′‐phosphodiesters (red). In TNA the α‐anomer of l‐threose nucleic acid is observed to Watson–Crick base pair with RNA/DNA. B=nucleobase.

Figure 2

Divergent synthesis of RNA and TNA cytidines through the reaction of cyanamide with prebiotically plausible aldehydes. The five‐carbon RNA backbone requires sequential reaction of glycolaldehyde 5 (blue) and glyceraldehyde (red) generating four furanosyl pentose aminooxazolines (RAO, AAO, XAO, and LAO) and one pyranosyl pentose aminooxazolines (p‐LAO). The four‐carbon TNA backbone requires only glycolaldehyde 5 (blue) and only generates two furanosyl isomers (EAO and TAO).

Figure 3

Synthesis of tetrose nucleosides by photochemical anomerization. Tetrose aminooxazolines (EAO and TAO) react with cyanoacetylene (HC₃N) to yield tetrose 2,2′‐anhydrocytidines 6 and 7. Reversible thiolysis of 6 and 7, followed by selective photoanomerization of the 1′,2′‐cis‐nucleosides 9 and 10 yields 1′,2′‐trans‐nucleosides 8 and 11. Oxidative hydrolysis of trans‐1′,2′‐thiocytidine 8 and 11 furnishes cytidines 3 and 15, whereas oxidation of cis‐1′,2′‐thiocytidines 9 and 10 furnishes anhydrocytidines 6 and 7.

Figure 4

¹H NMR (700 MHz, noesygppr1d, H₂O/D₂O (9:1), 6.1–8.3 ppm) spectra showing the irradiation (254 nm) of 9 (18 mm) and 10 (16 mm) at pH 7 with H₂S (113 mm): a) before the irradiation; and after b) 1 day; c) 2 days; d) 3 days irradiation. Spectrum e) following addition of authentic 8. Spectrum f) following addition of authentic 11. (C6)−H and (C1′)−H resonances are labelled: 9 (▾), 10 (▴), 8 (•), and 11 (✶).

Figure 5

Photochemical selection of threo‐thiocytidine induced by erythro‐specific 2′,3′‐cyclic phosphate synthesis. A) Urea‐mediated phosphorylation of erythro‐thiocytidine 10 yields cyclic phosphate 13 in near quantitative yield. 13 is observed to undergo rapid and complete photodegradation to free base 12. B) Urea‐mediated phosphorylation of β‐threo‐thiocytidine 9 yields a mixture of β‐threo‐thiocytidine phosphates that photoanomerize to their α‐anomers. Annulation of 9 is also observed during urea‐mediated phosphorylation to yield anhydronucleoside 6, which phosphorylates and rearranges to yield photostable erythro‐cytidine 14. R=H or (PO₃ ⁻)_n.

Figure 6

¹H NMR (700 MHz, noesygppr1d, H₂O/D₂O (9:1), 5.85–8.3 ppm) spectra showing the irradiation (254 nm) of 9 (12 mm) and 13 (12 mm) at pH 7 with H₂S (109 mm): a) before the irradiation; and after b) after 1 day; c) 2 days; d) 3 days; e) 4 days irradiation. Spectrum f) following addition of authentic 8. Spectrum g) following addition of authentic 12. (C6)−H and (C1′)−H resonances are labelled for 9 (▾), 13 (▴), and 8 (•). (C6)−H and (C5)−H resonances are labelled for 12 (✶).