HEISENBERG-MODEL FOR TETRAMERIC IRON CLUSTERS IN HIGH-POTENTIAL IRON-SULFUR PROTEINS .2. DOUBLE EXCHANGE IN DISTORTED CLUSTER

The joint Heisenberg and double exchange models for the [Fe3(III)Fe(II )] cluster of the oxidized form of the high-potential proteins are con sidered. The double exchange and Heisenberg interactions form the exch ange-resonance states of this cluster in the case of electron migratio n. In the model with S34 exchange mixing, the increasing of resonance interaction leads to non-linear double exchange behavior of the levels . For the model with electron transfer between two mixed-valence iron ions in the tetramer, the parameter b(ij) of the electron transfer and the Heisenberg term delta1 form one effective exchange-resonance para meter delta(b)+/- = delta1 +/- b(ij)/(2S12+1)(2s(a)+1), which depends on the type of the exchange-resonance multiplet. The linear double exc hange splitting of the Heisenberg exchange states takes place in the c ase of the model with S12 exchange mixing.