Building the uracil skeleton in primitive ponds at the origins of life: carbamoylation of aspartic acid

Abstract

A large set of nucleobases and amino acids is found in meteorites, implying that several chemical reservoirs are present in the solar system. The “geochemical continuity” hypothesis explores how protometabolic paths developed from so-called “bricks” in an enzyme-free prebiotic world and how they affected the origins of life. In the living cell, the second step of synthesizing uridine and cytidine RNA monomers is a carbamoyl transfer from a carbamoyl donor to aspartic acid. Here we compare two enzyme-free scenarios: aqueous and mineral surface scenarios in a thermal range up to 250 °C. Both processes could have happened in ponds under open atmosphere on the primeval Earth. Carbamoylation of aspartic acid with cyanate in aqueous solutions at 25 °C gives high N-carbamoyl aspartic acid yields within 16 h. It is important to stress that, while various molecules could be efficient carbamoylating agents according to thermodynamics, kinetics plays a determining role in selecting prebiotically possible pathways.

Ring‐Closure on the Rocks in a Prebiotic Environment

“…We continued our exploration with a series of carbonates. In our previous attempts at synthesizing NCA, [35] we observed the easy formation of a carbonate layer on basic oxides. For instance, in CO 2 -containg atmosphere, brucite (Mg(OH) 2 ) [59,60] or periclase (MgO) [61] are carbonated into magnesite (MgCO 3 ).…”

“…[34] However, we proved that cyanate, itself a possible product of CP decomposition, is a good alternate carbamoylating agent to perform NCA synthesis. [35] We explored NCA synthesis via the reaction between cyanate and L-aspartic acid under alkaline aqueous conditions and in a mineral surface scenario involving a drying step. Scenarios in aqueous solution and on mineral surfaces are both efficient, provided that the mineral surface does not exhibit acidic sites, e. g., on magnesium carbonate.…”

“…[52,53] Amorphous silica may catalyse hydrolysis reactions rather than the reverse condensations, if the reagents already contain labile amide bonds, as we have observed in a previous study. [35] The interactions between zinc ions and the NCA substrate are clearly important in vivo, so the candidate geochemical scenarios should explore the effects of zinc. We considered stoichiometric zinc minerals such as zincite (ZnO) and zinc carbonate.…”

“…Magnesium carbonate, marketed as MgCO 3 , is actually hydromagnesite (Mg 5 (CO 3 ) 4 (OH) 2 • 4H 2 O). [35] The hydration water is only lost at 217 °C (see DTG trace in Figure S33), while NCA activation occurs at 200 °C; thus, the hydration water of the support is still available and can take part in amide bond hydrolysis. Zinc carbonate also appears to retain constitution water up to 210 °C.…”

“…Hence, there is a low probability that CP itself was available for a prebiotic carbamoylation corresponding to the second step of the biosynthesis [34] . However, we proved that cyanate, itself a possible product of CP decomposition, is a good alternate carbamoylating agent to perform NCA synthesis [35] . We explored NCA synthesis via the reaction between cyanate and L‐aspartic acid under alkaline aqueous conditions and in a mineral surface scenario involving a drying step.…”