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 (1)pi sigma* hypersurface. Interestingly, a second proton transfer to an adjacent water molecule leads to a (1)pi sigma*/S-o conical intersection. We suggest that this electron-driven proton relay might be characteristic of low-lying lice 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.