On the Enhanced Stability of the Guanine−Cytosine Base-Pair Radical Cation

Ab initio (UHF/6-31G*) and density functional
(Becke3LYP/D95*) calculations have been used to
investigate the structures and stabilities of the radical cations of
the DNA bases and base pairs. The calculated
structures of the base pairs show excellent agreement with
crystallographic data. The most easily oxidizable
base,
guanine, forms a particularly stable radical cation base pair with
cytosine, so that the calculated adiabatic ionization
potential for the guanine−cytosine hydrogen-bonded complex is about
0.75 eV lower than that of guanine itself.
UBecke3LYP/D95*//UHF/6-31G* calculations show that the shift of
the central hydrogen-bonded proton at N1 of
guanine to N3 of cytosine is only slightly endothermic (+1.6 kcal
mol-1). The product of the corresponding
proton
shift in the adenine−thymine system is unfavorable by +14.1 kcal
mol-1. These results suggest that the
guanine−cytosine radical cation represents even more of a thermodynamic sink in
oxidized DNA than might be concluded
from the ionization potentials of the individual bases, and that it
enjoys about 7.3 kcal mol-1 extra
stabilization from
the central low-barrier hydrogen bond.