Multiple site proton affinities of methylated nucleic acid bases

Proton affinities of nucleic acid base molecules methylated at the site 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 sites are: 891.5 kJ/mol for 1-methyluracil, 982.8 kJ/mol for 1-methylcytosine, 961.3 kJ/mol for 9-methyladenine and 980.5 kJ/mol for 9-methylguanine. The calculated proton affinity values of the singly methylated nucleic acid bases are in agreement with the experimental values for the sugarylated bases, the nucleosides.