Structures and energetics of adenosine radicals:: (2′-dAdo-H)

The radicals and anions generated from the 2'-deoxyriboadenosine molecule were studied using the carefully calibrated computational scheme described in Chem. Rev. 2002, 102, 231. The method employs three different density functionals in connection with a double-zeta plus polarization and diffuse functions, DZP++, basis set. For all species considered, the optimized geometries, energies, and second derivatives were obtained. The adiabatic electron affinities (AEAs), vertical electron affinities (VEAs), and vertical detachment energies (VDEs) were determined. The nature of the radicals and anions was investigated, analyzing, respectively, the spin densities and the highest occupied molecular orbitals. Natural population analysis was applied to examine the electron distributions. Prediction of substantial positive AEAs in the range of 0.99-3.47 eV was supported by the analysis of the VEAs (0.08-2.44 eV) to correlate geometry perturbations with the AEA values. Vertical detachment energies are in the range of 1.78-3.65 eV. The energetic order found for the radicals is C(aliphatic) < N(amino) < O(hydroxyl) < C(aromatic). The radical created at C-1, was found to be the most stable, due to delocalization of the unpaired electron on the adenine moiety. The anion energies follow the trend N(amino) < O(hydroxyl) < C(aliphatic) < C(aromatic). Four of the five aliphatic anions are subject to dissociative behavior. This finding has several biological implications and may explain the possible degrees of damage to the DNA double strand.