Electronic Paramagnetic Resonance and Magnetic Properties of Model Complexes for Binuclear Active Sites in Hydrolase Enzymes

Abstract

A set of binuclear metal complexes with the formula M_2(Im)_4(OAc)_4(H_2O), where M = Mn, Co, or Ni, designed as mimics of the active sites of hydrolase enzymes, were subjected to EPR and magnetic susceptibility studies. The manganese complex displayed a broad EPR spectrum over the field range 0−5500 G. The 4 K spectrum was simulated as a summation of the S = 1, S = 2, and S = 3 spin manifolds of the coupled dimer, with each manifold weighted by the appropriate Boltzmann factor. Parameters for the best fits of EPR and magnetic susceptibility were g = 1.95, J = −1.29 cm^(-1), and D = 0.095 cm^(-1), with line widths of 350, 450, and 550 G for the S = 1, 2, and 3 manifolds, respectively. An alternative simulation, with g = 1.93, J = −1.28 cm^(-1), and D = 0.081 cm^(-1) provided a better fit for the central peaks but only accounted for six of the seven observed peaks. The cobalt and nickel complexes are EPR-silent. Magnetic susceptibility measurements of these two dimers showed weak antiferromagnetic coupling; variable-temperature magnetic susceptibility plots were fit with the parameters g = 2.22, J = −1.60 cm^(-1) (cobalt) and g = 2.04, J = −2.47 cm^(-1) (nickel). The weak coupling highlights the difficulty in detecting bound water in these binuclear complexes or in enzymes and hence in determining active site structure through the physical techniques used in this work.