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

被引:46
作者
Sanakis, Y
Macedo, AL
Moura, I
Moura, JJG
Papaefthymiou, V
Münck, E
机构
[1] Carnegie Mellon Univ, Dept Chem, Pittsburgh, PA 15213 USA
[2] NCSR Demokritos, Inst Mat Sci, Athens 15310, Greece
[3] Univ Nova Lisboa, Fac Ciencias & Tecnol, CQFB, Dept Quim, P-2825114 Caparica, Portugal
[4] Univ Ioannina, Dept Phys, GR-45110 Ioannina, Greece
关键词
D O I
10.1021/ja002658i
中图分类号
O6 [化学];
学科分类号
0703 ;
摘要
Iron-sulfur clusters with [3Fe-4S] cores are widely distributed in biological systems. In the oxidized state, designated [3Fe-4S](+), these electron-transfer agents have an electronic ground state with S = 1/2, and; they exhibit 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+; and the general properties of the S = 1/2 ground state have been described with 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 continuous distribution of g-values down to g approximate to 1.85. Despite considerable efforts in various laboratories no model has emerged that explains the g-values of type 2 clusters. The 4.2 K spectra of all [3Fe-4S](+) clusters 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 shifts in type 2 clusters. We have been able to fit the EPR and Mossbauer spectra of the 3Fe clusters of beef heart aconitase and Desulfovibrio gigas ferredoxin II by using antisymmetric exchange in combination with distributed exchange coupling constants J(12), J(13), and J(23) (J-strain). While antisymmetric exchange is negligible for aconitase (which has a type 1 cluster), fits 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 isotropic Fe-57 magnetic hyperfine coupling constant for th three cluster sites, namely a -18.0 MHz for aconitase and a = -18.5 MHz for the D. gigas ferredoxin. The effects of antisymmetric exchange and J-strain on the Mossbauer and EPR spectra are discussed.
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页码:11855 / 11863
页数:9
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