A review of spin Hamiltonians applicable to exchange-coupled systems of interest in biomaterials

Abstract

Dinuclear and tetranuclear complexes of spin-coupled manganese-manganese, manganese-iron, and iron-iron ions occur in enzymes which are of importance in biology and photosynthesis. These multinuclear systems contain higher oxidation states of manganese and iron ions than those of the commonly occurring Mn(II) and Fe(III) mononuclear ions, in which there has been a lot of interest recently with regard to their electron paramagnetic resonance (EPR) spectra. In order to aid the interpretation of EPR spectra, the spin Hamiltonians, and the resulting energy levels, characterizing binuclear and tetranuclear manganese ions will be discussed in this presentation. These will include, in particular, Mn(II)−Mn(III), Mn(III)−Mn(IV), and Mn(IV)−Mn(V) systems for the binuclear situation, and the Mn4O2 “butterfly”, cubane, dimer of dimer systems, and diamond structure for the tetramer situation. These coupled systems are characterized by a variety of exchange interactions whose magnitudes and nature, ferromagnetic or antiferromagnetic, affect profoundly the energy levels. In addition, the hyperfine interactions amongst the various55Mn nuclei produce a complex EPR spectrum consisting of a large number of hyperfine lines. In order to interpret EPR spectra properly, one needs to simulate them as accurately as possible. Details of a rigorous technique for the simulation of EPR spectra of these systems with matrix diagonalization and homotopy technique will be provided.