The Magnetic Properties and Structure of the Hemochromogens and Related Substances.

In the course of a general study of the structure of hemoglobin and its derivatives we have investigated the magnetic properties of ferriheme, ferroheme, several hemochromogens and nickel protoporphyrin. The results of this investigation and their structural interpretation are discussed in the following paragraphs. The prosthetic group of hemoglobin, ferrous protoporphyrin, we call ferroheme, ferriheme being ferric protoporphyrin combined with a negative ion (chloride in hemin, hydroxide in hematin). On denaturation hemo-globin forms a deep red substance called hemochromogen.1 It was shown by Anson and Mirsky2 in 1925 that hemochromogen is not ferroheme itself, as had been thought before, but a compound of ferroheme and denatured globin, and that hemochromogens in general are compounds of ferroheme and other substances, usually containing nitrogen. The characteristic property of the hemochromogens is their strong absorption spectrum of two sharp bands, a at about 5600 A and ,3 at about 5200 A, with a stronger than (B. The magnetic measurements reported in the experimental part of this paper correspond to the following numbers of unpaired electrons per molecule of the The presence of five and four unpaired electrons in ferriheme and ferro-heme, respectively, shows that in these substances the iron atom is attached to the four adjacent nitrogen atoms of the porphyrin not by covalent bonds but by ionic bonds;' that is, iron is present essentially as ferric ion in ferriheme and ferrous ion in ferroheme. The observed magnetic moments are incompatible with the assumption that four dsp2 covalent bonds are formed, directed toward the corners of a square, whether or not additional bonds to the iron atom are present. On the other hand, we have found that the four hemochromogens which we have investigated contain no unpaired electrons. This observation shows that two 3d orbitals of the ferrous iron atom are involved in the formation of covalent bonds, and that the iron atom is accordingly attached by essentially covalent bonds not only to the four porphyrin nitrogen atoms but also to two other atoms, giving an octahedral (d2sp3)