Spin-polarised SCC-Xα calculations for electronic- and magnetic-structure properties of (2Fe-2S) ferredoxin models

The spin-polarised SCC-X alpha method is used for the evaluation of electronic- and magnetic-structure properties (i.e., electron densities at the iron nuclei, electric-field gradients, g factors and magnetic hyperfine tensors) of oxidised and reduced (2Fe-2S) ferredoxin models, represented by the structure formula ((SR)₂FeS₂Fe(SR)₂) (n=-2, -3), with R=H, CH₃ and (C₆H₄(CH ₂)₂)₁2/. A strong geometry dependence of charge and spin densities, and their related properties, is found for minimal cluster size (R=H) on the position of the terminal hydrogen atoms. A Fe-S-H bond angle of about 90' turned out to be preferable to 180'. Experimental hyperfine data of synthetic analogues and of ferredoxins are in reasonable agreement with corresponding calculated properties obtained for the (2Fe-2S) ferredoxin models mentioned above. Comparing experimental and calculated g factors of reduced (2Fe-2S) centres indicates that the model clusters used are of g_av approximately 1.91 type. Finally, the exchange-coupling constant J between the two iron ions in the oxidised (2Fe-2S) ferredoxin model is estimated from the difference in the total energy between the high-spin and broken-spin symmetry states of the model.