The spin-coupling model of zero-field splitting for trimeric [3Fe–4S] and mixed-metal [3FeZn–4S] clusters of ferredoxins from Pyrococcus furiosus Original Research Article

The spin-coupling model of zero-field splitting (ZFS) is developed for trimeric [3Fe–4S] clusters. The correlations between the cluster ZFS parameters DS and ES of the states with total spin S and ZFS parameters Di and Ei of individual ions were obtained for mixed-valent (MV) [3Fe–4S]0 clusters with high-spin ground state Sgr=2, for the MV iron core of the hetero-metal [3FeZn–4S]+ cluster with Sgr=5/2 and for the monovalent [3Fe–4S]+ cluster with Sgr=5/2 of Pyrococcus furiosus ferredoxin (Pf Fd). These correlations and the cluster ZFS parameters DS and ES depend on total spin S, intermediate spin S12 and individual spins si. The spin-coupling model explains the experimentally observed negative cluster ZFS parameters of MV trimers in the [3Fe–4S]0 and [3FeZn–4S]+ clusters and the positive cluster ZFS parameter of the tetrameric MV [4Fe–4S]+ cluster and the monovalent [3Fe–4S]+ trimer of Pf Fd. Single-particle ZFS parameters Di were obtained for the [3Fe–4S] trimers and [4Fe–4S]+ tetramer (Sgr=3/2) of Pf Fd. In distorted trimers, the cluster ZFS parameter DS of anisotropy changes the value and sign under the variation of isotropic Heisenberg exchange or/and double exchange coupling due to the exchange admixture of the excited states. Experimentally observed peculiarities of effective hyperfine constants Ai for the MV trimer with Sgr=5/2 of the hetero-metal [3FeZn–4S]+ cluster were described in the spin-coupling exchange-resonance model with the exchange admixture of the excited states and non-equivalence of the states of different localization.