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

Abstract

The spin-coupling model of zero-field splitting (ZFS) is developed for trimeric [3Fe–4S] clusters. The correlations between the cluster ZFS parameters D S and E S of the states with total spin S and ZFS parameters D i and E i of individual ions were obtained for mixed-valent (MV) [3Fe–4S] 0 clusters with high-spin ground state S gr=2, for the MV iron core of the hetero-metal [3FeZn–4S] + cluster with S gr=5/2 and for the monovalent [3Fe–4S] + cluster with S gr=5/2 of Pyrococcus furiosus ferredoxin ( Pf Fd). These correlations and the cluster ZFS parameters D S and E S depend on total spin S, intermediate spin S 12 and individual spins s i . 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 D i were obtained for the [3Fe–4S] trimers and [4Fe–4S] + tetramer ( S gr=3/2) of Pf Fd. In distorted trimers, the cluster ZFS parameter D S 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 A i for the MV trimer with S gr=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.