A neutron crystallographic analysis of a cubic porcine insulin at pD 6.6

In the case of HisB5, both Nπ and Nτ of an imidazole ring are protonated at pD 6.6, but at pD 9 only Nπ is protonated. In contrast, for HisB10, both Nπ and Nτ are protonated at pD 6.6 as well as at pD 9. The ionization states of several amino acids in porcine insulin have been obtained at pD 6.6 and they are compared with those at pD 9 obtained by neutron diffraction as well as those at pH 6.50 and 6.98 obtained by X-ray diffraction. In this manuscript, the difference between these forms will be discussed.