Molecular-Orbital Calculation Of Hyperfine Interactions, Electric-Field Gradient And Quadrupole Splitting In Reduced Rubredoxins

The active site of rubredoxins, the simplest class of iron-sulfur, electron-transfer proteins consists of a single Fe atom surrounded by a distorted tetrahedral array of four cysteine sulfur atoms. In a previous paper on the oxidized rubredoxins, we calculated the electric field gradient at the Fe57 nucleus, the resultant quadrupole splitting and the hyperfine field due to interaction between the Fe nucleus spins and the net electron spin of the molecular complex. In this paper, we are going to present our calculated results of the same set of properties for the reduced state, using the molecular orbitals similarly obtained from an Iterated Extended Hückel calculation. Significant contribution of the anisotropy to the hyperfine field as well as the sign of the electric field gradient, and hence the quadrupole splitting allow us to pin down conformations which give the best agreement with the experimental results. In fact, the conformation (conformer A) which represents a small movement of one of the S atoms from its crystal structure position in such a way that pairs of S atoms lie in ? planes yield remarkably good results for all the physical properties calculated in both oxidation states. This consistency also suggests that there is no appreciable conformational change through the oxidation process.