Histidine

My guess is that the Nitrogen atom with the double bond will accept the Hydrogen ion, because the high electron density within the double bond will attract the positively-charged Hydrogen ion...is this correct?

http://wps.prenhall.com/wps/media/object...0110f.html

Answer (from source above): I believe delocalization confers a formal positive charge on the sigma bonded nitrogen and results in the other nitrogen to have a more basic lone pair.




Pandering:I would think that delocalization (if caused by conjugation) would have delocalized the sigma-pi bonded nitrogen rather than the sigma nitrogen as the sigma nitrogen is not in conjugation with the pi-system. Hmm. This amino group does undergo a lot of rearrangement. Perhaps that could be a factor in this electropositive / electronegative designation. Anyway, was fun. Please post an explanation when you get one. Would be much appreciated.