Calculated pKa’s of the DNA Base Radical Ions

For the DNA bases the pKa's of the neutral molecules have been measured, and a review article by Steenken provides a convenient summary of the experimental pKa's of the DNA radical ions. The purpose of the present work is to attempt to calculate the pKa's of the DNA radical ions and to compare these results with the experimental values. The agreement between the calculated and experimental pKa's for the guanine, cytosine, and thymine cations is very good but the pKa calculation on the adenine cation is not close to the experimental value. It is tempting to suggest that the pKa of the adenine cation is not actually ≤1 but is more likely somewhere near the calculated value of 3.9. Problems were encountered in calculating the pKa's of the one-electron reduced nucleobases because optimizations tend to produce nonplanar structures. The calculations presented show close agreement between the calculated and experimental pKa's for the thymine, cytosine, and adenine anions. Although there are no experimental data for the pKa of the guanine reduction product, the calculated value of 17.6 seems to be too high, given that for thymine the protonation is also at oxygen and results in a rather low pKa.