Magnetism as a Tool in Biology

Abstract

Magnetism is a property of atoms. Sometimes it makes itself macroscopically evident when the magnetic fields of the atoms in a piece of matter line up in one direction and a natural or artificial magnet results. More often the magnetism of atoms is not grossly evident, but can be sensed by appropriate means. When magnetism is applied to biology both the atomic and gross aspects of it can come into play. The magnetism of atoms and their nuclei can be used to study the physics and chemistry of biologically important substances; the gross magnetism of sizable bodies can be used as an aid in investigating physiological processes or in carrying out medical therapy. One example of the use of magnetism in physiochemical studies is the investigation of hemoglobin, myglobin and various enzymatic proteins with the Mossbauer effect. Dr. C. E. Johnson of the University of Liverpool in England described some of these experiments at a recent Conference on Magnetism and Magnetic Materials in Miami Beach. Mossbauer effect is the name for the so-called recoilless emission or absorption of gamma rays by atomic nuclei. Nuclei emit or absorb gamma rays when their internal energy changes from one value to another. If the atom involved is in a gas, it will recoil when the gamma ray is emitted or absorbed. This is because the principle of conservation of momentum requires that the momentum of the emitted ray be balanced by a momentum of the atom in the opposite direction and that the momentum of the absorbed ray be preserved as a momentum of the absorbing nucleus. Some of the energy involved in the interaction will go into the momentum of the atom, and since this may vary, it is not possible to use the absorption or emission spectra of gamma rays from gases to determine the exact amounts of energy involved in the internal changes of the nucleus. But if the atom is bound in a crystal, the recoil involves the whole crystal. Since the crystal is massive compared with a single atom, the recoil necessary to conserve momentum is slight, almost nothing. Thus nearly all the energy in the interaction passes between the gamma ray and the internal energy of the nucleus, and the gamma-ray spectra of solids can be used to determine the exact energies involved in the nuclear transitions. Since the energy states of the nucleus are influenced by the magnetic state of the atom, Mbssbauer studies can be used to determine magnetic properties of the atom. And where magnetic changes are involved in their chemical activity much can be learned about that. Many biological substances, such as hemoglobin and myoglobin, form large crystals. The important chemical activity of these substances is carried on not by the whole molecule but by a few active atoms. In hemoglobin, for example, four iron atoms do all the work involved in taking up oxygen and transporting it to cells. Similar characteristics are found in the so-called iron-sulfur proteins, whose structures play roles in photosynthesis, metabolism and nitrogen fixation. One of these proteins that has been extensively studied is spinach ferridoxide, a substance found in spinach leaves. In spinach ferridoxide the enzymatic activity is concentrated on the iron atoms, of which there are two per molecule. Investigation can therefore concentrate on the state and structure of the iron atoms, and much can be learned even without knowing the detailed structure of the whole molecule, which is extremely difficult to determine. It was found, for example that in the oxidized state the molecule is nonmagnetic; in the reduced or deoxydized state it is magnetic. In reduction, one electron is transferred to the two iron atoms, and the question was whether it belonged entirely to one of them or was shared by both and how this affected the magnetism. Study shows that the electron goes to one, not both, of the atoms and that the magnetic fields of the two atoms are pointed in opposite directions. Thus in the oxidized state, when both atoms have equal numbers of electrons, 4~~~~~~~~~~~~~~~J