MOLECULAR-ORBITAL STUDY OF THE PROTONATION OF DA, DG, DC AND DT 2'-DEOXYRIBONUCLEOSIDES

Semiempirical AM1 quantum-chemical computations of the most stable tau tomers of dA, dG, dC and dT 2'-deoxyribonucleosides were carried out t o determine the energetic and geometrical properties of the protonatio n process. The results show that the geometrical parameters near the p rotonation sites are affected by the protonation. The relative proton affinities follow the order dC > dA > dG >> dT if the most stable taut omer of the neutral system is considered. If other low-energy tautomer s are also considered, the proton affinity trend is more similar to th e experimental data.