I understand that the imidazole ring in histidine is aromatic. I also realize that it retains it's aromaticity when protonated. I am wondering why it is not mentioned at all in basic text books such as Lehninger? Also, across the web there are a number of places claiming that histidine isn't aromatic in all cases. Another reason I am a little confused is, if protonation doesn't harm the aromatic nature, then why is histidine such a weak base? Am I missing something?

I understand that the imidazole ring in histidine is aromatic. I also realize that it retains its aromaticity when protonated. I am wondering why this is not mentioned at all in basic text books such as Lehninger? Also, across the web there are a number of places claiming that histidine isn't aromatic in all cases. Another reason I am a little confused is, if protonation doesn't alter the aromatic nature, then why is histidine such a weak base?