Excited-State Deactivation of Adenine by Electron-Driven Proton-Transfer Reactions in Adenine-Water Clusters: A Computational Study

• Published in: Chemphyschem Vol. 17; no. 9; pp. 1298 - 1304
• Main Authors: Wu, Xiuxiu, Karsili, Tolga N. V., Domcke, Wolfgang
• Format: Journal Article
• Language: English
• Published: Germany Blackwell Publishing Ltd 04.05.2016
• Subjects: ab initio calculations; adenine; Adenine - chemistry; Computer Simulation; electron transfer; Electrons; photochemistry; proton transfer; Protons; Water - chemistry; ab initio calculations; proton transfer; electron transfer; photochemistry; adenine
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.201501154

The reactivity of photoexcited 9H‐adenine with hydrogen‐bonded water molecules in the 9H‐adenine–(H2O)5 cluster is investigated by using ab initio electronic structure methods, focusing on the photoreactivity of the three basic sites of 9H‐adenine. The energy profiles of excited‐state reaction paths for electron/proton transfer from water to adenine are computed. For two of the three sites, a barrierless or nearly barrierless reaction path towards a low‐lying S1–S0 conical intersection is found. This reaction mechanism, which is specific for adenine in an aqueous environment, can explain the substantially shortened excited‐state lifetime of 9H‐adenine in water. Depending on the branching ratio of the nonadiabatic dynamics at the S1–S0 conical intersection, the electron/proton transfer process can enhance the photostability of 9H‐adenine in water or can lead to the generation of adenine‐H⋅ and OH⋅ free radicals. Although the branching ratio is yet unknown, these findings indicate that adenine might have served as a catalyst for energy harvesting by water splitting in the early stages of the evolution of life. Signs of life: The reactivity of photoexcited adenine in a pentahydrated hydrogen‐bonded cluster is investigated by using ab initio calculations. The theoretical photoreactivities of the three basic sites of 9H‐adenine indicate that early DNA or RNA nucleobases might have played a role in the origin of life as catalysts for harvesting energy by water splitting.