Conical-Intersection Topographies Suggest That Ribose Exhibits Enhanced UV Photostability.

Carbohydrates are essential building blocks of life that assume a multitude of biological functions in all living organisms found on Earth. It was recently reported that ribose was identified in UV-irradiated interstellar ice analogs, which suggests that it can be found on comets and that it may have been transported to Earth via the impact of comets. Herein, we present computational results obtained with multiconfigurational ab initio quantum-chemical methods showing that various photochemical processes for radiationless deactivation are available for photoexcited ribose. These processes are driven by nσ* states and involve either O-H- or endocyclic or exocyclic C-O-bond elongation whereby a conical intersection with the electronic ground state becomes accessible. The local topography of the potential-energy surfaces around these conical intersections suggests that these intersections mediate efficient radiationless deactivation and favor regeneration of the initially photoexcited ground-state reactant. These findings indicate that ribose found in interstellar space can be expected to be highly photostable upon irradiation with UV starlight, which could be of relevance in the field of astrobiology.