SINGLE VERSUS DOUBLE PROTON-TRANSFER REACTIONS IN WATSON-CRICK BASE-PAIR RADICAL CATIONS - A THEORETICAL-STUDY

Single and double proton-transfer reactions in Watson-Crick Guanine-Cy tosine (GC) and Adenine-Thymine (AT) radical cations have been studied using the hybrid density functional B3LYP method. Calibration calcula tions for the formamidine-formamide dimer: a model system of AT, have shown that B3LYP compares well to the high level ab initio correlated method CCSD(T), both for the neutral and cationic systems. The single proton-transfer reaction is favorable in both the GC and AT radical ca tions; it takes place from the ionized monomer (guanine and adenine, r espectively), which increases its acidity, to the neutral fragment. Fo r the two systems, GC and AT, the nonproton transferred and single pro ton transferred structures are almost degenerate (Delta E = 1.2 kcal/m ol), and the process presents low energy barriers (4.3 kcal/mol for GC and 1.6 kcal/mol for AT). The double proton-transfer reaction is less favorable than the single one, in contrast to what is observed for th e neutral systems. The relative stability of the different structures can be understood considering two factors: the relative stability of t he asymptotes from which they derive and the number and sequence of th e strong and weak hydrogen bonds formed. Fur the same number of strong short hydrogen bonds, the most stable structures are those in which t he strong Ii-bonds are neighbors. Based on these considerations, a pre diction for other pairings is reported.