Abiotic synthesis of purine and pyrimidine ribonucleosides in aqueous microdroplets

Abstract

Aqueous microdroplets (<1.3 µm in diameter on average) containing 15 mM d-ribose, 15 mM phosphoric acid, and 5 mM of a nucleobase (uracil, adenine, cytosine, or hypoxanthine) are electrosprayed from a capillary at +5 kV into a mass spectrometer at room temperature and 1 atm pressure with 3 mM divalent magnesium ion (Mg²⁺) as a catalyst. Mass spectra show the formation of ribonucleosides that comprise a four-letter alphabet of RNA with a yield of 2.5% of uridine (U), 2.5% of adenosine (A), 0.7% of cytidine (C), and 1.7% of inosine (I) during the flight time of ∼50 µs. In the case of uridine, no catalyst is required. An aqueous solution containing guanine cannot be generated under the same conditions given the extreme insolubility of guanine in water. However, inosine can base pair with cytidine and thus substitute for guanosine. Thus, a full set of ribonucleosides to generate the purine-pyrimidine base pairs A-U and I-C are spontaneously generated in aqueous microdroplets under similar mild conditions.

Keywords: microdroplet chemistry; origin of life; prebiotic chemistry; purine ribonucleosides; pyrimidine ribonucleosides.

Fig. 1.

Reaction mechanism for the prebiotic synthesis of ribonucleosides by an abiotic salvage pathway using Rib-1-P as the intermediate.

Fig. 2.

Free energy diagrams for the abiotic synthesis of the ribonucleosides (A) adenosine, (B) cytidine, (C) inosine, and (D) uridine. The condensation occurs between ribose and phosphoric acid to generate Rib-1-P. The salvage reaction occurs between Rib-1-P and nucleobase to generate the corresponding ribonucleoside. The ΔG values in bulk solution (blue lines) are positive, whereas in microdroplets (red lines) they are negative.

Fig. 3.

Mass spectra of reactions in microdroplets containing 15 mM d-ribose, 15 mM phosphoric acid, and 5 mM of the nucleobase (A) adenine, (B) cytosine, and (C) hypoxanthine, with and without 3 mM Mg²⁺. The red boxes show the detected ion signals of ribonucleosides with the Mg²⁺ catalyst, and the red letters denote the detected m/z peaks of (A) adenosine, (B) cytidine, and (C) inosine.

Fig. 4.

The synthetic path of adenosine in microdroplets with Mg²⁺ catalyst. (A) Mass spectra for the products of reaction between adenine and Rib-1-P and (B) the mechanism of salvage reaction between adenine and Rib-1-P to give adenosine. Red letters denote the detected m/z peak of the adenosine from an abiotic salvage reaction. (C) Mass spectrum for the products of reaction between d-ribose and adenine in the absence of phosphate and (D) the previously assumed mechanism for the direct ribosylation of adenine. C₆H₁₃N and P denote a fragment of adenine (cyclohexylamine) and phosphoric acid.