The Effect of Solvation on the Radiation Damage Rate Constants for Adenine

It is well known that water plays an important part in almost all biological systems and that inclusion of solvation effects might therefore be of utmost importance in studies of radiation damage to DNA. In the present investigation, we have studied the effect of different solvation models in calculations of Gibbs free energies and reaction rates for the reaction between the OH radical and the DNA nucleobase adenine by conducting density functional theory calculations at the ωB97X-D/6-311++G(2df,2pd) level with the Eckart tunnelling correction. The solvent, water, was included through either the implicit polarizable continuum model (PCM) or through explicit modelling of micro-solvation by a single water molecule at the site of reaction as well as by the combination of both. Scrutiny of the thermodynamics and kinetics of the individual sub-reactions suggests that the qualitative differences introduced by the solvation models do not significantly alter the conclusions made based solely on simple gas-phase calculations. Abstraction of the amine hydrogen atoms H₆₁ and H₆₂ and addition onto C₈ remain the most likely reaction pathways.