The first and second substitution reactions between activated (hydrolyzed) cisplatin, Pt(NH3)(2)(H2O)(2)(2+), and purine bases guanine and adenine are explored using the B3LYP hybrid functional, IEF-PCM solvation models, and large basis sets. The computed free energy barrier for the first substitution is 19.5 kcal/mol for guanine (exptl value = 18.3 kcal/mol) and 24.0 kcal/mol for adenine. The observed predominance toward guanine in the first substitution is explained in terms of significantly larger stabilization energy for the initially formed complex, compared with adenine, in combination with favored kinetics, and represents a revised view of the proposed mechanism for cisplatin binding to DNA. For the second substitution, the computed barrier for Pt(NH3)(2)G(2)(2+) head-to-head formation is 22.5 kcal/mol, in very good agreement with experimental data for adduct closure (23.4 kcal/mol). Again, a higher stability in complexation with G over A is ascribed as the main contributing factor favoring G over A substitution. The calculations provide a first explanation for the predominance of 1,2-d(GpG) over 1,2-d(ApG) intrastrand didentate adducts, and the origin of the 5'-3' direction specificity of the 1,2-d(ApG) adducts.