Kinetics and Thermodynamics of the Reaction between the ^•OH Radical and Adenine: A Theoretical Investigation

The accessibility of all possible reaction paths for the reaction between the nucleobase adenine and the ^•OH radical is investigated through quantum chemical calculations of barrier heights and rate constants at the ωB97X-D/6-311++G­(2df,2pd) level with Eckart tunneling corrections. First the computational method is validated by considering the hydrogen abstraction from the heterocyclic N₉ nitrogen in adenine as a test system. Geometries for all molecules in the reaction are optimized with four different DFT exchange-correlation functionals (B3LYP, BHandHLYP, M06-2X, and ωB97X-D), in combination with Pople and Dunning basis sets, all of which have been employed in similar investigations in the literature. Improved energies are obtained through single point calculations with CCSD­(T) and the same basis sets, and reaction rate constants are calculated for all methods both without tunneling corrections and with the Wigner, Bell, and Eckart corrections. In comparison to CCSD­(T)//BHandHLYP/aug-cc-pVTZ reference results, the ωB97X-D/6-311++G­(2df,2pd) method combined with Eckart tunneling corrections provides a sensible compromise between accuracy and time. Using this method, all subreactions of the reaction between adenine and the ^•OH radical are investigated. The total rate constants for hydrogen abstraction and addition for adenine are predicted with this method to be 1.06 × 10^–12 and 1.10 × 10^–12 cm³ molecules^–1 s^–1, respectively. Abstractions of H₆₁ and H₆₂ contribute the most, while only addition to the C₈ carbon is found to be of any significance, in contrast to previous claims that addition is the dominant reaction pathway. The overall rate constant for the complete reaction is found to be 2.17 × 10^–12 cm³ molecules^–1 s^–1, which agrees exceptionally well with experimental results