Evidence for antisymmetric exchange in cuboidal [3Fe-4S](+) clusters

Iron-sulfur clusters with [3Fe-4S] cores are widely distributed in biologic al systems. In the oxidized state, designated [3Fe-4S](+), these electron-t ransfer agents have an electronic ground state with S = 1/2, and; they exhi bit EPR signals centered at g = 2.01. It has been established by Mossbauer spectroscopy that the three iron sites of the cluster are high-spin Fe3+; a nd the general properties of the S = 1/2 ground state have been described w ith the exchange Hamiltonian H-exch = J(12)S(1).S-2 + J(23)S(2).S-3 + J(13) S(1).S-3 Some [3Fe-4S](+) clusters (type 1) have their g-values confined to the range between g = 2.03 and 2.00 while others (type 2) exhibit a contin uous distribution of g-values down to g approximate to 1.85. Despite consid erable efforts in various laboratories no model has emerged that explains t he g-values of type 2 clusters. The 4.2 K spectra of all [3Fe-4S](+) cluste rs have broad features,which have been simulated in the past by using Fe-57 magnetic hyperfine tensors with anisotropies that are unusually large for high-spin feme sites. It is proposed here that antisymmetric exchange, H-AS = d.(S-1 x S-2 + S-2 x S-3 + S-3 x S-1), is the cause of the g-value shift s in type 2 clusters. We have been able to fit the EPR and Mossbauer spectr a of the 3Fe clusters of beef heart aconitase and Desulfovibrio gigas ferre doxin II by using antisymmetric exchange in combination with distributed ex change coupling constants J(12), J(13), and J(23) (J-strain). While antisym metric exchange is negligible for aconitase (which has a type 1 cluster), f its of the ferredoxin II spectra require \d\ approximate to 0.4 cm(-1). Our studies show that the data of both proteins can lie fit using the same iso tropic Fe-57 magnetic hyperfine coupling constant for th three cluster site s, namely a -18.0 MHz for aconitase and a = -18.5 MHz for the D. gigas ferr edoxin. The effects of antisymmetric exchange and J-strain on the Mossbauer and EPR spectra are discussed.