Ligand K-Edge X-ray Absorption Spectroscopy of [Fe 4 S 4 ] 1+,2+,3+ Clusters: Changes in Bonding and Electronic Relaxation upon Redox Abhishek Dey, ² Thorsten Glaser, ²,‡ Manon M.-J. Couture, § Lindsay D. Eltis, ⊥ R. H. Holm, | Britt Hedman,* ,# Keith O. Hodgson,* ,²,# and Edward I. Solomon* ,² Contribution from the Department of Chemistry and Stanford Synchrotron Radiation Laboratory, Stanford UniVersity, Stanford, California 94305, Department of Microbiology and Biochemistry, UniVersity of British Columbia, VancouVer, Canada, V6T 1Z3, Department of Biochemistry, UniVersite ´ LaVal, Quebec City, Canada, and Department of Chemistry and Chemical Biology, HarVard UniVersity, Cambridge, Massachusetts 02138 Received March 15, 2004; E-mail: edward.solomon@stanford.edu; hodgson@ssrl.slac.stanford.edu; hedman@ssrl.slac.stanford.edu Abstract: Sulfur K-edge X-ray absorption spectroscopy (XAS) is reported for [Fe4S4] 1+,2+,3+ clusters. The results are quantitatively and qualitatively compared with DFT calculations. The change in covalency upon redox in both the [Fe4S4] 1+/2+ (ferredoxin) and the [Fe4S4] 2+/3+ (HiPIP) couple are much larger than that expected from just the change in number of 3d holes. Moreover, the change in the HiPIP couple is higher than that of the ferredoxin couple. These changes in electronic structure are analyzed using DFT calculations in terms of contributions from the nature of the redox active molecular orbital (RAMO) and electronic relaxation. The results indicate that the RAMO of HiPIP has 50% ligand character, and hence, the HiPIP redox couple involves limited electronic relaxation. Alternatively, the RAMO of the ferredoxin couple is metal-based, and the ferredoxin redox couple involves extensive electronic relaxation. The contributions of these RAMO differences to ET processes in the different proteins are discussed. Introduction Iron-sulfur proteins are ubiquitous in nature, performing many biological functions involving electron transfer and catalysis. The most common iron-sulfur proteins have mono- nuclear, binuclear, trinuclear, and tetranuclear clusters in their active site. 1,2 The four-iron clusters, found in bacterial ferre- doxins and high potential proteins (HiPIPs), have four µ 3 S sulfide , forming a cubane structure (Scheme 1), with one terminal S cysteine ligand for each Fe atom. These clusters generally perform one- electron transfer. The tetranuclear site undergoes two different biologically functional redox couples. 1a The HiPIP proteins cycle between the oxidized [Fe 4 S 4 ] 3+ and the resting [Fe 4 S 4 ] 2+ states (the HiPIP couple), whereas the bacterial ferredoxins cycle between the [Fe 4 S 4 ] 2+ resting form and the reduced [Fe 4 S 4 ] 1+ state (the ferredoxin couple). All three oxidation states have been isolated in proteins and inorganic model complexes. 2 The electronic structures of these states are well-known from detailed spectroscopic and computational studies. 3-7 The reduced [Fe 4 S 4 ] + state has a valence-delocalized [Fe 2 S 2 ] + subcluster antiferromagnetically coupled to an all- ferrous [Fe 2 S 2 ] 0 subcluster. The spin states of these clusters are predominantly S ) 1/2 in the protein active sites, while in the [Fe 4 S 4 ] + model complexes higher spin states, S ) 3/2 and S ) 5/2 are also found. 8,17 The EPR silent resting form [Fe 4 S 4 ] 2+ ² Stanford University. ‡ Present address: Institut fu ¨r Anorganische und Analytische Chemie, Westfaelische Wilhelms-Universita ¨t Muenster, Germany. § Universite ´ Laval. ⊥ University of British Columbia. | Harvard University. # Stanford Synchrotron Radiation Laboratory, SLAC, Menlo Park, CA, 94025. (1) (a) Iron-Sulfur Proteins; Lovenberg, W., Ed.; Academic Press: New York, 1973-1977; Vols. I-III. (b) Iron-Sulfur Proteins; Spiro, T. G., Ed.; Metal Ions In Biology; Wiley-Interscience: New York, 1982; Vol. IV. (c) Iron- Sulfur Proteins; Cammack, R., Ed.; Advances in Inorganic Chemistry; Academic Press: San Diego, CA, 1992; Vol. 38. (d) Iron-Sulfur Proteins; Sykes, A. G., Cammack, R., Eds.; Advances in Inorganic Chemistry; Academic Press: San Diego, CA, 1999; Vol. 47. (e) Flint, D. D. H.; Allen, R. R. M. Chem. ReV. 1996, 96, 2315. (2) Rao, P. V.; Holm, R. H. Chem. ReV. 2004, 104, 527. (3) Beinert, H.; Holm, R. H.; Mu ¨nck, E. Science 1997, 277, 653. (4) Le Pape, L. L.; Lamotte, B. B.; Mouesca, J.-J. M.; Rius, G. J. Am. Chem. Soc. 1997, 119, 9757 (5) Czernuszewicz, R. S.; Macor, K. A.; Johnson, M. K.; Gewirth, A.; Spiro, T. G. J. Am. Chem. Soc. 1987, 109, 7178. (6) Norman, J. G.; Ryan, P. B.; Noodleman, L. J. Am. Chem. Soc. 1980, 102, 4279. (7) Noodleman, L.; Norman, J. G.; Osborne, J. H.; Aizman, A.; Case, D. A. J. Am. Chem. Soc. 1985, 107, 3418. Scheme 1. Schematic Diagram of Fe4S4 Cluster in Bacterial Ferredoxin and HiPIP Proteins Published on Web 06/12/2004 8320 9 J. AM. CHEM. SOC. 2004, 126, 8320-8328 10.1021/ja0484956 CCC: $27.50 © 2004 American Chemical Society