net charge nucleobases at alkaline conditions
The conjugate acids of the adenine and guanine have pKa's between 9-10. Guanine also has a nitrogen with a pKa of 12.3, which would be mostly protonated at pH 12. The pKa for the phosphoric acid proton of a nucleotide is significantly lower than 7. 

Therefore, in a pH 8 buffer, the basic nitrogens of each adenine and guanine are protonated, while the phosphate is deprotonated. This gives an overall net charge closer to zero than you would see at pH 12.

In a pH 12 buffer, where the pH is above the pKa of both the phosphoric acid protons and the nitrogen conjugate acid protons (except the last one on each guanine), all of these positions would be deprotonated. Therefore, the formal charge on the nitrogens would be zero (again, except the most basic nitrogen on guanine), but the phosphates would still have a charge of -1 each. This would lead to a net charge of significantly higher charge magnitude because there would be very few positive charges to balance out the negative charges.


At pH 12, the net charge should be approximately: $$(n_N +n_u) - (n_c + n_g).$$
Approximate net charge at pH 8: $$n_N - (n_a + n_c + 2n_g).$$

nN= number of nucleotides

nu= number of uracils/thymines

nc= number of cytosines

ng= number of guanosines

na= number of adenosines