Magnetization curves of haemoproteins measured by low-temperature magnetic-circular-dichroism spectroscopy

Abstract

The magnetic-circular-dichroism (MCD) spectra of metmyoglobin cyanide and oxidized horse heart cytochrome c were measured in the region of the Soret band over a range of temperatures from 1.5 to 50.degree. K and in fields from 0 to 5T. A similar study was made with reduced bovine heart cytochrome c oxidase, which contains 1 high-spin ferrous heme, namely a3. MCD magnetization curves are presented. The 2 oxidized hemoproteins give magnetization curves characterictic of an isolated Kramer''s ground state with spin S = 1/2. These curves contrast with the magnetization curve of the high-spin ferrous heme with spin S = 2. The electronic ground state of the latter compound contains 0-field components that are thermally accessible over the temperature range of the experiment. The magnetization curves are a complex nested set. The magnetization curves of the S = 1/2 proteins were analysed, and it is possible to make estimates of the ground-state g-factors even in the presence of rhombic anisotropy, provided that some knowledge of the polarizations of the electronic transitions is available. The difference between MCD magnetization curves of a simple S = 1/2 paramagnet and a magnetically complex ground state should prove useful when MCD spectroscopy is used to probe the magnetic properties of metal centres in proteins, and should have wide application beyond the field of hemoproteins.