Spin-spin interactions in the reduced [Fe6S6](5+) cluster

We have developed a theoretical model of the Heisenberg exchange interaction and resonance delocalization (also called ''double exchange'') for the Fe/S protein containing six mixed valence iron ions. The model we have worked out is that of the reduced [Fe6S6](5+) cluster in which there are five Fe(III) ions and one Fe(II) ion. This system is the core of the compound belonging to the group of prismane clusters with the general formula [Fe(6)S(6)X(6)](n-) (X = Br, Cl, I, SC6H4-p-CH3, OCH4-p-CH3; n = 0, 1, 2, 3). By means of the above mentioned model we have calculated the energies and described the properties of the spin eigenstates of the investigated system The lowest energies we have presented on the correlation diagrams depending on the Heisenberg exchange coupling parameters J(i) (i = 1, 2, 3, 4) and the b parameter for the double exchange. In our calculations the meaning of the indices i is the following: J(1) J[Fe(II)-Fe(III)](d1), J(2) = J[Fe(III)-Fe(III)](dl). J(3) = J[Fe(III)-Fe(III)]d(2) and J(4) = J[Fe(III)-Fe(III)](d3), where the values d1 < d2 < d3 denote the bonds lengths. The protein containing a [Fe6S6](5+) prismane core exhibits characteristic electron paramagnetic resonance (EPR) spectra indicative of S = 1/2 and S = 9/2 ground states [W.R. Hagen et al., J. Chem. Sec. Faraday Trans. 1 (1989) 4083; AJ. Pierik et al., Eur. J. Biochem. 206 (1992) 4083]. This fact is in a good agreement with our calculations.