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Mechanisms of photoreactivity in hydrogen-bonded adenine–H2O complexes

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The mechanisms of photoinduced reactions of adenine with water molecules in hydrogen-bonded adenine–water complexes were investigated with ab initio wave-function-based electronic-structure calculations. Two excited-state electron/proton transfer reaction mechanisms have been characterized: H-atom abstraction from water by photoexcited adenine as well as H-atom transfer from photoexcited adenine or the (adenine+H) radical to water. In the water-to-adenine H-atom transfer reaction, an electron from one of the p orbitals of the water molecule fills the hole in the n (π) orbital of the nπ* (ππ*) excited state of adenine, resulting in a charge-separated electronic state. The electronic charge separation is neutralized by the transfer of a proton from the water molecule to adenine, resulting in the (adenine+H)⋯OH biradical in the electronic ground state. In the adenine-to-water H-atom transfer reaction, πσ* states localized at the acidic sites of adenine provide the mechanism for the photoejection of an electron from adenine, which is followed by proton transfer to the hydrogen-bonded water molecule, resulting in the (adenine–H)⋯H3O biradical. The energy profiles of the photoreactions have been computed as relaxed scans with the ADC(2) electronic-structure method. These reactions, which involve the reactivity of adenine with hydrogen-bonded water molecules, compete with the well-established intrinsic excited-state deactivation mechanisms of adenine via ring-puckering or ring-opening conical intersections. By providing additional decay channels, the electron/proton exchange reactions with water can account for the observed significantly shortened excited-state lifetime of adenine in aqueous environments. These findings indicate that adenine possibly was not only a photostabilizer at the beginning of life, but also a primordial photocatalyst for water splitting.

Graphical abstract: Mechanisms of photoreactivity in hydrogen-bonded adenine–H2O complexes

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Publication details

The article was received on 20 Aug 2018, accepted on 04 Dec 2018 and first published on 07 Dec 2018

Article type: Paper
DOI: 10.1039/C8CP05305G
Phys. Chem. Chem. Phys., 2019, Advance Article

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    Mechanisms of photoreactivity in hydrogen-bonded adenine–H2O complexes

    X. Wu, J. Ehrmaier, A. L. Sobolewski, T. N. V. Karsili and W. Domcke, Phys. Chem. Chem. Phys., 2019, Advance Article , DOI: 10.1039/C8CP05305G

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