EPR Characteristics and Relaxation Properties of the Centers

In this section, the characteristics of the spectra displayed by the different types of iron—sulfur centers are presented, with special emphasis on how they depend on the geometrical and electronic structure of the centers. The electronic structure is only briefly recalled here, however, and interested readers are referred to the excellent standard texts published on this topic (3, 4). Likewise, the relaxation properties of the centers are described, but the nature of the underlying spin-lattice relaxation processes is not analyzed in detail. However, a short outline of these processes is given in the Appendix. The aim of this introductory section is therefore mainly to describe the tools used in the practical applications presented in Sections III and IV. It ends in a discussion about some of the issues that may arise when EPR spectroscopy is used to identify iron-sulfur centers. 

The simplest iron-sulfur centers, which were first discovered in ru-bredoxins, consist of one iron ion coordinated by a distorted tetrahedron of cysteinyl sulfur atoms. This environment provides a weak ligand field giving a spin equal to and 2 when the ion is Fe(III) and Fe(II), respectively. It also determines the splitting of the ground spin manifold, and consequently the characteristics of the EPR spectrum. This splitting is generally described in the framework of the spin Hamiltonian  

The Fe hyperfine tensor components were determined by Mossbauer spectroscopy in the case of the rubredoxin from Clostridium 

No EPR spectra have yet been reported to our knowledge in the case of a protein containing a well-characterized reduced FeS4 center, although a spectrum has been observed in the case of a model complex (24 ). The lack of EPR signals in the case of proteins is apparently directly related to the D and E values, which are equal to D = +7.5 cm E D = 0.28 in the case of C. pasteurianum rubredoxin (15), and D = 6 cm E D = 0.19 in that of D. gigas desulforedoxin (18), 

In the oxidized form, the weak coupling of the high-spin Fe(III) ion to its surroundings amd the very large ligand-field energy of about 10,000 cm (12) are not liable to give rise to very efficient relaxation processes (see Appendix). However, the S = 5/2 manifold provides a set of transitions for multiple direct processes that may be efficient 

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