Answer
The structure of the major form of histidine at pH values of 1, 4,
7.6, and 11 are shown in the picture below.
The nitrogen in the histidine ring is a weaker base than the α-amino group because the imidazole ring is aromatic (with pyrrole-type N that is not basic and a pyridine-type N that is weakly basic). Aromatic amines like pyridine are weaker bases than aliphatic amines by several pK units.
Work Step by Step
The general rule when determining if a group will be protonated is: at solution pH below the group's pKa value, the group will be protonated; at pH higher than the group's pKa value, it will not be protonated.
