Synthetic Analogs of the Active Sites of Oxidized and Reduced Rubredoxin

— The bischelate monoanion [Fe((SCH2)2C6H4)2] 1_ contains an unconstrained, near-tetrahedral Fe(III)-S4 coordination unit (similar to the Fe-SU coordination unit occurring in rubredoxin proteins) and exhibits the Fe(III)/Fe(II) redox couple. Mossbauer parameters at 77 K are isomer shift S = 0.13 mm/s (relative to iron metal) and electric quadrupole splitting AZIQ = 0.57 mm/s. Application of an external magnetic field parallel to the direction of observation at 4.2 K induces a saturation magnetic hyperfine field of — 380 kOe. Upon reduction to the dianion [Fe((SCH2)2C6H.t)2] the isomer shift changes to S = 0.61 mm/s and the quadrupole splitting at 4.2 K to A£Q = 3.34 mm/s. From the temperature dependence of AEQ for 1.4 K < T < 295 K we infer the splitting of the ground orbital doublet, e(dz2, dX2-yi), to be A s 900 cm 1 . External magnetic fields up to 80 kOe reveal the sign of Vzz to be negative indicating that the ground orbital has dz% symmetry and induce a magnetic hyperfine interaction of negative sign. For H 0 = 60 kOe, Hm = — 128 kOe. | H M | increases with increasing H 0 , showing that saturation has not been reached up to Ho = 80 kOe, and implying a zero field splitting of several cm 1 of the electronic ground state. These results as well as magnetic moment measurements for the unconstrained model compounds are comparable to measurements in oxidized and reduced rubredoxin and imply that an « entatic state » description of rubredoxin is not valid.