CONFORMATIONAL DEPENDENCE OF THE ELECTRONIC-PROPERTIES OF [FE(SCH3)(4)](-,2-)

The electronic properties of [Fe(SCH3)(4)](-2-), a model for the iron- sulfur site in rubredoxin, and the conformational dependence of these properties were investigated with ab initio calculations. The calculat ions were done at the unrestricted Hartree-Fock level with various bas is sets, and the effects of electron correlation were tested in some c alculations using second-order perturbation theory. The conformational dependence was studied using five conformations of the model complex that differed by rotations about the Fe-S and C-S bonds. Geometry opti mizations showed that these bond lengths changed little among the conf ormations examined, except where steric crowding existed. The atomic s pin populations and spin contamination values (S-2) were also homogene ous among the conformations. The relative energies of all but one of t he conformations were very small in both the oxidized and reduced stat es; thus, the total energy changes upon reduction, Delta E-redox, of a ll but one of the conformations were within 30 mV of each other. The e xception was about 100 mV less negative than the other four structures and represents a less likely dihedral rotation transition state. More important, the difference of only 30 mV between conformations similar to that of the iron-sulfur site in rubredoxin and in the Holm-Ibers a nalogue indicates that very little of the 800 mV difference in redox p otential between rubredoxin and in the Holm-Ibers analogue can be attr ibuted strictly to conformationally dependent differences in electroni c structure. In addition, atom-centered partial charges were determine d by Mulliken's method and two different electrostatic potential (ESP) fitting methods: the surface-based geodesic tessellation method and t he grid-based CHELPG method. For all three methods, the partial charge distributions showed little conformational dependence. Moreover, the Mulliken charges, which are not subject to the problems of buried atom s associated with ESP methods, indicate very little conformational dep endence. The CHELPG partial charges were selected as being the most re asonable for ESP calculations; thus, a geometry independent set of CHE LPG partial charges is proposed.