Solvent effects on the photochemistry of 4-aminoimidazole-5-carbonitrile, a prebiotically plausible precursor of purines

4-Aminoimidazole-5-carbonitrile (AICN) was suggested as a prebiotically plausible precursor of purine nucleobases and nucleotides. Although it can be formed in a sequence of photoreactions, AICN is immune to further irradiation with UV-light. We present state-of-the-art multi-reference quantum-chemical calculations of potential energy surface cuts and conical intersection optimizations to explain the molecular mechanisms underlying the photostability of this compound. We have identified the N–H bond stretching and ring-puckering mechanisms that should be responsible for the photochemistry of AICN in the gas phase. We have further considered the photochemistry of AICN–water clusters, while including up to six explicit water molecules. The calculations reveal charge transfer to solvent followed by formation of an H3O+ cation, both of which occur on the 1πσ* hypersurface. Interestingly, a second proton transfer to an adjacent water molecule leads to a 1πσ*/S0 conical intersection. We suggest that this electron-driven proton relay might be characteristic of low-lying 1πσ* states in chromophore–water clusters. Owing to its nature, this mechanism might also be responsible for the photostability of analogous organic molecules in bulk water.