Ultraviolet-Driven Deamination of Cytidine Ribonucleotides Under Planetary Conditions.

A previously proposed synthesis of pyrimidine ribonucleotides makes use of ultraviolet (UV) light to convert beta-d-ribocytidine-2',3'-cyclic phosphate to beta-d-ribouridine-2',3'-cyclic phosphate, while simultaneously selectively degrading synthetic byproducts. Past studies of the photochemical reactions of pyrimidines have employed mercury arc lamps, characterized by narrowband emission centered at 254 nm, which is not representative of the UV environment of the early Earth. To further assess this process under more realistic circumstances, we investigated the wavelength dependence of the UV-driven conversion of beta-d-ribocytidine-2',3'-cyclic phosphate to beta-d-ribouridine-2',3'-cyclic phosphate. We used constraints provided by planetary environments to assess the implications for pyrimidine nucleotides on the early Earth. We found that the wavelengths of light (255-285 nm) that most efficiently drive the deamination of beta-d-ribocytidine-2',3'-cyclic phosphate to beta-d-ribouridine-2',3'-cyclic phosphate are accessible on planetary surfaces such as those of the Hadean-Archaean Earth for CO2-N2-dominated atmospheres. However, continued irradiation could eventually lead to low levels of ribocytidine in a low-temperature, highly irradiated environment, if production rates are slow.