title: Spectroscopic and redox studies of valence-delocalized [Fe2S2](+) centers in thioredoxin-like ferredoxins.
abstract: Reduced forms of the C56S and C60S variants of the thioredoxin-like Clostridium pasteurianum [Fe2S2] ferredoxin (CpFd) provide the only known examples of valence-delocalized [Fe2S2](+) clusters, which constitute a fundamental building block of all higher nuclearity Fe-S clusters. In this work, we have revisited earlier work on the CpFd variants and carried out redox and spectroscopic studies on the [Fe2S2](2+,+) centers in wild-type and equivalent variants of the highly homologous and structurally characterized Aquifex aeolicus ferredoxin 4 (AaeFd4) using EPR, UV-visible-NIR absorption, CD and variable-temperature MCD, and protein-film electrochemistry. The results indicate that the [Fe2S2](+) centers in the equivalent AaeFd4 and CpFd variants reversibly interconvert between similar valence-localized S = 1/2 and valence-delocalized S = 9/2 forms as a function of pH, with pKa values in the range 8.3-9.0, because of protonation of the coordinated serinate residue. However, freezing high-pH samples results in partial or full conversion from valence-delocalized S = 9/2 to valence-localized S = 1/2 [Fe2S2](+) clusters. MCD saturation magnetization data for valence-delocalized S = 9/2 [Fe2S2](+) centers facilitated determination of transition polarizations and thereby assignments of low-energy MCD bands associated with the Fe-Fe interaction. The assignments provide experimental assessment of the double exchange parameter, B, for valence-delocalized [Fe2S2](+) centers and demonstrate that variable-temperature MCD spectroscopy provides a means of detecting and investigating the properties of valence-delocalized S = 9/2 [Fe2S2](+) fragments in higher nuclearity Fe-S clusters. The origin of valence delocalization in thioredoxin-like ferredoxin Cys-to-Ser variants and Fe-S clusters in general is discussed in light of these results.
authors: †Department of Chemistry and Center for Metalloenzyme Studies, University of Georgia, Athens, Georgia 30602, United States.
authors: Sowmya
authors: S
authors: Subramanian
authors: †Department of Chemistry and Center for Metalloenzyme Studies, University of Georgia, Athens, Georgia 30602, United States.
authors: Evert C
authors: EC
authors: Duin
authors: ‡Department of Chemistry, University of Oxford, South Parks Road, Oxford, OX1 3QR, United Kingdom.
authors: Sarah E J
authors: SE
authors: Fawcett
authors: ‡Department of Chemistry, University of Oxford, South Parks Road, Oxford, OX1 3QR, United Kingdom.
authors: Fraser A
authors: FA
authors: Armstrong
authors: §1, Allée Docteur Calmette, 38130 Échirolles, France.
authors: Jacques
authors: J
authors: Meyer
authors: †Department of Chemistry and Center for Metalloenzyme Studies, University of Georgia, Athens, Georgia 30602, United States.
authors: Michael K
authors: MK
authors: Johnson
mesh_terms: D029063:Aquifoliaceae; D003013:Clostridium; D005288:Ferredoxins / Q000737:chemistry* / Q000378:metabolism; D007501:Iron / Q000737:chemistry* / Q000378:metabolism; D010084:Oxidation-Reduction; D013057:Spectrum Analysis; D013455:Sulfur / Q000737:chemistry* / Q000378:metabolism; D013879:Thioredoxins / Q000737:chemistry*
chemical_list: D005288:Ferredoxins; D013879:Thioredoxins; D013455:Sulfur; D007501:Iron