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Sites of protonation and pKa's of the four nucleobases and the ribose-phosphate backbone. Note that all unperturbed nucleobases are primarily in their neutral form at biological pH (∼7.2). All nucleobase ionizations are on the Watson-Crick imino nitrogens, which are numbered, except for the left-hand ionization of guanosine, which is on the Hoogsteen face N7.
