The bases in DNA are able to form Watson-Crick base pairs at neutral pH. However, when the pH is raised above pH 10, the duplex comes apart and base pairing is disrupted. Here are the pKa values for the functional groups on the DNA and RNA bases.

If you don't remember the numbering system for the nucleotide bases, you might check here [local].

Are these pKa values constant? No. Like the amino acid residues in proteins, the tertiary structure of the macromolecule can provide an environment that promotes or deters deprotonation and changes the pKa values. For example the adenine pKa value of 3.8 is modified to 7.6 for the catalytic residue involved in peptide bond formation in rRNA (T. Cech [2000] Science 289: 878-879). This change in pKa is essential to the acid-base catalysis in the translation mechanism.