  We have studied the effect of amino acids on electron attachment properties
of DNA nucleobases, taking 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. Four selected amino acids,
Arginine, Alanine, Lysine, and Glycine which form a major component of histone
proteins are considered to investigate their role in electron attachment to DNA
nucleobase. The electron attachment to cytosine in all the cytosine-amino acid
dimer complexes follows a doorway mechanism, where the electron gets
transferred from initial dipole-bound doorway state to the final
nucleobase-bound state through the mixing of electronic and nuclear degrees of
freedom. In higher amino acid concentration, amino acid-bound state acts as the
doorway state, where the initial electron density is localized on the amino
acid, away from the nucleobase. This leads to the physical shielding of
nucleobase from the incoming extra electron. At the same time, the presence of
amino acids can increase the stability of nucleobase-bound anionic state, which
can suppress the dissociative electron attachment induced sugar-phosphate bond
breaking.
