Textbook 6.21

[Gwendolyn Hill 2F]

Hi,

I had a question about textbook problem 6.21. The problem is as follows:
"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 pro- tons 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."
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In the solutions manual it says that the base can accept 2 protons, one on each Nitrogen atom. I understand that part, because the nitrogen atoms each have a lone pair. However, why would it not be 6 protons; 2 H atoms for each oxygen with the 2 sets of lone pairs? Is it because oxygen is electronegative...?

Also, amphiprotic behavior means acting like an acid or a base given a certain situation right? So would it be the two NH structures since they can give off an H+, or act as a base with the e-s?

[705383815]

The oxygens would not be able to accept H's probably because those are the sites that are reserved for hydrogen bonding when there are other nucleic acids present (particularly adenine). And yes, the nitrogen sites should be amphiprotic.