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.

We have surveyed the chemical utility of the near-edge structure of molybdenum x-ray absorption edges from the hard x-ray K-edge at 20,000 eV down to the soft x-ray M 4,5 -edges at ~230 eV. We compared, for each edge, the spectra of two tetrahedral anions, MoO 4 and MoS 4 2-. We used three criteria for assessing near-edge structure of each edge: (i) the ratio of the observed chemical shift between MoO 4 2-and MoS 4 2-and the linewidth, (ii) the chemical information from analysis of the near-edge structure and (iii) the ease of measurement using fluorescence detection. Not surprisingly, the K-edge was by far the easiest to measure, but it contained the least information. The L 2,3 -edges, although harder to measure, had benefits with regard to selection rules and chemical speciation in that they had both a greater chemical shift as well as detailed lineshapes which could be theoretically analyzed in terms of Mo ligand field, symmetry, and covalency. The soft x-ray M 2,3 -edges were perhaps the least useful, in that they were difficult to measure using fluorescence detection and had very similar information content to the corresponding L 2,3 -edges.Interestingly, the soft x-ray, low energy (~230 eV) M 4,5 -edges had greatest potential chemical sensitivity and using our high resolution superconducting tunnel junction (STJ) fluorescence detector they appear to be straightforward to measure. The spectra were amenable to analysis using both the TT-multiplet approach and FEFF. The results using FEFF indicate that the sharp near-edge peaks arise from 3d → 5p transitions, while the broad edge structure has predominately 3d → 4f character. A proper understanding of the dependence of these soft x-ray spectra on ligand field and site geometry is necessary before a complete assessment of the utility of the Mo M 4,5 -* Corresponding author: sjgeorge@lbl.gov, +1 510 486 6094, FAX +1 510 486 5664. Supporting Information(1) Crystallographic data of Na 2 MoS 4 in CIF format.

High-resolution molybdenum K-edge X-ray emission spectroscopy (XES) was used to characterize the K beta(4) and K beta' ' valence-to-core transition bands in the oxo-Mo compounds K(2)MoO(4), MoO(S(2)CNEt(2))(2), and MoO(2)(S(2)CNEt(2))(2). The K beta(4) and K beta' ' emission bands are attributed to transitions to the Mo 1s core hole from molecular orbitals possessing primarily molybdenum 4d and oxygen 2s character, respectively. This communication describes the first assignment of the K beta' ' interatomic band in the emission spectra of molybdenum complexes. Additionally, the K beta(4) and K beta' 'transitions are shown to be sensitive to the chemical and electronic environment of the metal, suggesting that high-resolution XES might be an effective method for elucidating the nature of the molybdenum centers in biological systems.

Tobacco peroxidase (36 kDa, pI 3n5) exhibits unique catalytic and spectral properties that are modulated by pH, calcium and magnesium ions. It catalyses the oxidation of veratryl alcohol by hydrogen peroxide over a wide pH range (1n5-5n0) in the presence of these metal ions with a pH optimum of 1n8. This is the only example of a holoperoxidase described so far that is active and comparatively stable at such a low pH. The enhancement of tobacco peroxidase activity by magnesium ions is to our knowledge the first example of a magnesium-induced peroxidase activation. UV\visible spectra of tobacco peroxidase showed that the Soret band shifted and its absorption coefficient increased upon the addition of calcium or magnesium ions and on lowering