 The shielding at N3 is substantially reduced due to the second-order paramagnetic effect, which involves a symmetry-allowed interaction between the nitrogen lone pair and the excited π* states of the aromatic ring.At pH > 9, the chemical shifts of N1 and N3 are approximately 185 and 170 ppm.The imidazole sidechain of histidine commonly serves as a ligand in metalloproteins.