I'm sure this is a dumb question, but here it is. All the representations I can find of Adenine show C2 as only being bound to N1 and N3. One of those is a single bond and the other is a double bond. So 3 bonds. Shouldn't it have 4 bonds? So, why isn't C2 protonated in Adenosine? If the answer is that there is a Hydrogen there, but Hydrogens are often omitted, then why don't canonical pairings of A-T have 3 hydrogen bonds. The third one being between Adenosine's C2 Hydrogen and Thymidine's C2 Oxygen . Thanks!