Electron Attachment to DNA: The Protective Role of Amino Acids.

We have studied the effect of amino acids on the electron attachment properties of a DNA nucleobase, with cytosine as a model system. The equation of motion coupled cluster theory with an extended basis set has been used to simulate the electron-attached state of the DNA model system. Arginine, alanine, lysine, and glycine are the four amino acids considered to investigate their role in electron attachment to a DNA nucleobase. The electron attachment to cytosine in all the four cytosine-amino acid gas-phase dimer complexes follows a doorway mechanism, where the electron gets transferred from the initial dipole-bound doorway state to the final nucleobase-bound state through the mixing of electronic and nuclear degrees of freedom. When cytosine is bulk-solvated with glycine, the glycine-bound state acts as the doorway state, where the initial electron density is localized on the bulk amino acid and away from the nucleobase, thus leading to the physical shielding of the nucleobase from the incoming electron. At the same time, the presence of amino acids can increase the stability of the nucleobase-bound anionic state, which can suppress the sugar-phosphate bond rupture caused by dissociative electron attachment to DNA.