An efficient prebiotic synthesis of cytosine and uracil

IN contrast to the purines^{1 3}, the routes that have been proposed for the prebiotic synthesis of pyrimidines from simple precursors give only low yields. Cytosine can be synthesized from cyano-acetylene and cyanate^4,5; the former precursor is produced from a spark discharge in a CH₄/N₂ mixture^4,5 and is an abundant interstellar molecule⁶. But this reaction requires relatively high concentrations of cyanate (>0.1 M), which are unlikely to occur in aqueous media as cyanate is hydrolysed rapidly to CO₂ and NH₃. An alternative route that has been explored7 is the reaction of cyanoacetaldehyde (formed by hydrolysis of cyanoacetylene⁸) with urea. But at low concentrations of urea, this reaction produces no detectable quantities of cytosine⁷. Here we show that in concentrated urea solution-such as might have been found in an evaporating lagoon or in pools on drying beaches on the early Earth-cyanoacetaldehyde reacts to form cytosine in yields of 30-50%, from which uracil can be formed by hydrolysis. These reactions provide a plausible route to the pyrimidine bases required in the RNA world⁹.