CaCO3 Polymorphs as Mineral Catalysts for Prebiotic Phosphorylation of Uridine

Establishing plausible routes for the abiotic formation of nucleotides is a challenging problem because the phosphorylation of organic molecules is thermodynamically unfavorable in water, and because common phosphorous-containing minerals such as apatite are highly insoluble. Reactions of reduced phases such as the meteoritic mineral schreibersite with ammonia containing solutions can form stable amino-derivatives of phosphates/phosphite, and carbonate-rich lakes have been suggested as environments where phosphate species and organic molecules could accumulate in significant abundances, thus promoting an ideal environment for abiotic phosphorylation. This work reports the catalytic properties of three CaCO3 polymorphs-calcite, aragonite, and vaterite-on diamidophosphate (DAP)-induced phosphorylation of the uridine nucleoside during a 24-hr dry-down reaction. It is shown that the phosphorylation reaction is accelerated in solutions containing CaCO3 compared to those with no mineral present. For un-buffered solutions with no mineral present, the primary products formed are uridine monophosphates (UMPs), with yields making up 22.3 +/- 3.9% of the total detected species, while solutions containing calcite and aragonite formed primarily UMP dimers (yields of 15.3 +/- 1.1% and 14.8 +/- 1.3%, respectively). Vaterite showed a strong preference for forming cyclic UMP (cUMP) (26.3 +/- 0.3% yield), and no higher order polymers were observed using any carbonate mineral. Reactions containing CaSO(4)middot2H(2)O (gypsum) showed a preference for forming cUMP, though not as strong as vaterite, while those containing CaCl2 (calcium chloride) and CaWO4 (scheelite) did not yield any phosphorylated products other than UMPs. These results suggest that CaCO3 minerals could have played an important role in facilitating prebiotic phosphorylation in aqueous environments that undergo drying cycles.