# Why aren't the amine groups in the nucleic acid bases protonated?

Nucleobases G, C and A carry an amine group ($\ce{-NH2}$). In amino acids, the amine groups are considered protonated at physiological pH. Why isn't it the case for the amine groups of nucleobases?

I found some references discussing the protonation of nucleobases, but I am not enough expert in either DFT (http://www.ncbi.nlm.nih.gov/pubmed/19049279) or Mass Spectrometry (http://www.ncbi.nlm.nih.gov/pubmed/17920289/) to get a rational from this. Plus, neither they nor the references they cite and that I have briefly examined actually consider protonation of the amine groups.

• Amines are basic because of the lone pair on the nitrogen. In the nucleobases, the amine is adjacent to a double bond, which allows the lone pair to be delocalized. This reduces the basicity of the amine.
– f''
Jul 23, 2016 at 16:42