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.

Magnetic Circular Dichroism of Paramagnetic Species

“…[14][15][16] Since MCD spectroscopy is orientation selective, only molecules that are suitably aligned in the magnetic field contribute to MCD intensity. For large ZFS, the S ¼ 3/2 system can be approximated as a collection of non-interacting doublets.…”

Magnetic circular dichroism spectroscopy of weakly exchange coupled transition metal dimers: A model study

“…However, while the formalism for the analysis of individual thermally isolated spin multiplets is reasonably well understood [9,10], it is less clear how to treat exchange coupled systems in which more than one multiplet is thermally accessible. In most previous studies, problems of data interpretation have been circumvented by working in the linear region of the MCD response and only studying the variation of the signal with temperature [11].…”

Magnetic circular dichroism study of GaIIICoII and FeIIICoII complexes

“…The MCD saturation behavior of the signal from the randomly oriented mononuclear complexes with the orbitally degenerate ground state can be simulated as [5]:…”

“…In the case of mononuclear complexes with the orbitally nondegenerate ground state the MCD signal appears as a result of spinorbit admixture of some states to the ground and/or excited states [5,6]. The signal behavior depends strongly on the polarization of the corresponding transitions and can be simulated with the use of Eqs.…”