A novel, general method of analyzing magnetic circular dichroism spectra and magnetization curves of high-spin metal ions: Application to the protein oxidized rubredoxin, Desulfovibrio gigas

Abstract

We have developed a general theoretical approach for analyzing the intensities of magnetic circular dichroism (MCD) spectra of paramagnetic species with S>1/2 in the nonlinear regions of temperature and magnetic field. The method takes full advantage of the irreducible tensor method in order to obtain maximum simplification from symmetry. The approach, which is based on a detailed treatment of spin-orbit coupling and Zeeman interaction in terms of the symmetry properties of basis sets of wave functions, factorizes contributions into bands with Gaussian and derivative shapes in order to extend earlier treatments based on the so-called linear field limit. The method is applied to analyze and fit the form of the MCD spectra and the MCD magnetization curves of pseudo-tetrahedral high-spin Fe(III), S=5/2, in the protein rubredoxin from Desulfovibrio gigas, a representative of a family of iron–sulphur proteins. This treatment provides for the first time a satisfactory fit of these curves over a temperature range between 1.6 and 10 K and up to magnetic fields of 5 T. We show that the forms of the magnetization curves are strongly dependent on the polarizations of the optical transitions and on both the sign and magnitude of D, the ground state axial zero-field parameter. The sign and magnitude of D is determined to be −0.6 cm−1 with a fixed value of E/D=0.25 obtained from an analysis of electron paramagnetic resonance data. This shows that earlier, simpler fitting procedures were inadequate.