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May
6
comment Histidine aromaticity
I was referring to the basicity of the side chain itself. Why is its pKa neutral?
May
6
comment Histidine aromaticity
Rather, the protonation of imidazole generates a cation where the positive charge is delocalized, which contributes to its stability. I still do not see why it is a weak base.
May
6
comment Histidine aromaticity
But the protonation occurs on the pyridine-like nitrogen. It is in sp2 configuration and its lone pair is not involved in the aromatic pi system.