Photochemical steps in the prebiotic synthesis of purine precursors from HCN.

cis-2,3-diaminomaleonitrile (cis-DAMN, 1), which may be converted photochemically into an imidazole intermediate (4amino-1H-imidazole-5-carbonitrile, AICN, 2). Although this reaction has been investigated in detail since its discovery by Ferris and Orgel in 1966, the mechanism of the photochemical steps remains unresolved. Herein, we address this issue from a theoretical perspective: by the use of computational chemistry and chemical kinetics we show that among a number of possibilities, including all those previously proposed, there is only one sequence of steps that is thermodynamically and kinetically compatible with the experimental conditions. One of the most appealing features of the DAMN! AICN reaction is its robustness. It was observed in a large variety of solvents (polar and nonpolar), with several enaminonitrile derivatives, and at diverse concentrations and temperatures. The imidazole derivative 2 is photostable (5% reduction in absorbance after irradiation at 254 nm for 3 h; see also Ref. [7] on imidazole photostability) and resistant to hydrolysis (lifetime: 2000 years at pH 8). These characteristics imply that different prebiotic environments, either terrestrial or extraterrestrial, could have been the source of AICN (2) in the prebiotic world. The accumulation of AICN, however, requires relatively large HCN concentrations (> 10 m). This requirement sets a first relevant environmental constraint: such high HCN concentrations are only possible in low-temperature environments, such as ice and eutectic water–HCN phases. Therefore, any realistic mechanism cannot count on high thermal energy in addition to the photon energy. The number of photons and intermediates involved in the photochemical steps is unknown (Scheme 2). The process