Magnetic interactions between dysprosium complexes and two soluble iron-sulfur proteins.

ABSTRACT

A tetranuclear ferredoxin from Rhodopseudomonas gelatinosa [4Fe-4S](+2,+3) has been examined by electron paramagnetic resonance techniques. The temperature-dependence and relaxation characteristics of the spectral lines indicate that the spin-lattice interaction is described at low temperatures by a T2 law which gives way to a T9 Raman relaxation as the temperature is raised. At higher temperatures an Orbach process becomes dominant. In the presence of dysprosium complexes the relaxation and line widths are modified. From crystallographic structure determinations of similar proteins we are able to relate the dysprosium effects to the spatial separation between the complex and the tetranuclear cluster. This scale is then tested against a ferredoxin from Clostridium pasteurianum which contains two tetranuclear clusters, [4Fe-4S](+1,+2). We find that for these soluble iron-sulfur proteins the dysprosium complexes form a shell at the protein surface. The magnetic interaction between the clusters and the complexes altering the relaxation time goes as r-6, while the low temperature line broadening is described by an r-3 dipole interaction.