The binding of first-row transition metal monocations (Sc +-Cu+) to N7 of gu̇anine and N7 or N3 of adenine nucleobases has been analyzed using the hybrid B3LYP density functional theory (DFT) method. The nature of the bonding is mainly electrostatic, the electronic ground state being mainly determined by metal-ligand repulsion. M +-guanine binding energies are 18-27 kcal/mol larger than those of M+-adenine, the difference decreasing along the row. Decomposition analysis shows that differences between guanine and adenine mainly arise from Pauli repulsion and the deformation terms, which are larger for adenine. Metal cation affinity values at this level of calculation are in very good agreement with experimental data obtained by Rodgers et al. (J. Am. Chem. Soc. 2002, 124, 2678) for adenine nucleobases. © 2007 American Chemical Society.