MULTIPLE-SITE PROTON AFFINITIES OF METHYLATED NUCLEIC-ACID BASES

Proton affinities of nucleic acid base molecules methylated at the sit e where the sugar connects to the base in the DNA helix are calculated by the use of ab initio methods. Geometry optimizations and frequency calculations are performed at the HF/6-31G(d) level, while the final energies and proton affinities are calculated with the MP2/6-311(2d,2p ) method. The proton affinities are reported for temperatures of 0 and 298.15 K. The proton affinity values for the most basic protonation s ites are: 891.5 kJ/mol for 1-methyluracil, 982.8 kJ/mol for 1-methylcy tosine, 961.3 kJ/mol for 9-methyladenine and 980.5 kJ/mol for 9-methyl guanine. The calculated proton affinity values of the singly methylate d nucleic acid bases are in agreement with the experimental values for the sugarylated bases, the nucleosides.