0
pK1 = 2.3, pKr = 6, pK2 = 9.7 (all approximate)
at physiological pH, histidine has no net charge.
at pH 1, below all pKas and charge is +2
at pH 3, amine group proton pops off, so +1 charge
at pH 5, still below 6 and above 2.3 so +1 charge
if had pH above 6, for instance... at pH of 8 net charge is zero, or neutral... such that it is neutral at physiological pH (a bit above a pH of 7)
at pH 11, exceeds all pKas of amine, acid group, and R group. So net charge of -1
amine deprotonation, carboxylic acid deprotonation, and R group deprotonation happen pH 11 because it exceeds all pKs
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