Photochemical etiology of promising ancestors of the RNA nucleobases.

RNA is a product of chemical and biological evolution and the identification of its heterocyclic ancestors is essential for understanding the molecular origins of life. Among a diverse array of selection pressures thought to have shaped the composition of the nucleobases on prebiotic Earth, protection against intense ultraviolet radiation must have been essential. In this contribution, a detailed spectroscopic and photophysical investigation of barbituric acid and 2,4,6-triaminopyrimidine, two promising candidates for the prebiotic ancestors of RNA nucleobases, is presented in aqueous solution. It is shown that although these pyrimidine derivatives absorb ultraviolet radiation strongly, both compounds possess efficient electronic relaxation mechanisms for dissipating most of the absorbed ultraviolet energy to their aqueous environment as heat within hundreds of femtoseconds, thus safeguarding their chemical integrity. In fact, these two heterocyclic compounds rival the photostability observed in the canonical nucleobases in aqueous solution, thus supporting the recent proposal that both barbituric acid and 2,4,6-triaminopyrimidine are promising ancestors of the RNA nucleobases.