21] Magnetic susceptibility applied to metalloproteins

Abstract

This chapter describes the application of magnetic susceptibility measurement method in studying metalloproteins. The single-crystal biological susceptibility measurement can be done using heme protein, myoglobin. Because of the anisotropy of g-values in the high-spin ferric forms, the susceptibility varies greatly with the angle between applied field and molecular axis. This variation has been used to map the orientation of protein heme groups relative to the crystal axes, and it has been used as a valuable constraint on interpretation of X-ray diffraction patterns in determining total molecular structure. The magnetic measurements on hemoglobin have been extended to many heme proteins. The linkage between iron spin state and the nature of the ligands bonded perpendicular to the heme plane made the physical susceptibility measurements a standard chemical tool. The concept is broadened to include the notion of mixed spin states, in which chemical bonding of an intermediate character, between those typified as ionic or covalent, is indicated by susceptibilities intermediate between those defined by the alternative pairings of the electron spins. Measurements over wide temperature ranges showed that the spin states could exist in mixtures in thermal equilibrium, and that the exact details of the thermal mixtures could become a fine indicator of the chemical bond. The notion is broadened further with the realization that some ionic spin states could exist in quantum mixtures, in which the individual ionic wave function is not a pure spin state in thermal mixture with other ions, but was itself quantum mixture of wave functions characterizing distinct spin states.