Issue 26, 2019

Mechanisms of photoreactivity in hydrogen-bonded adenine–H2O complexes

Abstract

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

Supplementary files

Article information

Article type
Paper
Submitted
09 ذو الحجة 1439
Accepted
26 ربيع الأول 1440
First published
29 ربيع الأول 1440

Phys. Chem. Chem. Phys., 2019,21, 14238-14249

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, 21, 14238 DOI: 10.1039/C8CP05305G

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