Focus 6 Exercises Question #21

[Jonathan Sautter 1J]

The two strands of the nucleic acid DNA are held together by hydrogen bonding between four organic bases. The structure of one of these bases, thymine, is shown below. (a) How many protons can this base accept? (b) Draw the structure of each conjugate acid that can be formed. (c) Mark with an asterisk any structure that can show amphiprotic behavior in aqueous solution.

I am a bit confused on how to determine the answer to this question when looking at the Lewis structure of the molecule. For example, why wouldn't the oxygen molecules be able to accept a proton?

[jennaj3k]

It normally would make sense for the oxygen to accept protons, but you have to think about electronegativities here.
Oxygen is more electronegative than Nitrogen so it's unlikely to share its electrons. If it did, then thymine would have a positive charge which isn't stable. Nitrogen is less electronegative and more likely to share its electrons, so its going to accept those protons.