J. Am. Chem. Soc. 1997, 119, 12683-12684
12683
Models for Nonheme Diiron Enzymes. Assembly of a High-Valent Fe2(µ-O)2 Diamond Core from Its Peroxo Precursor Yanhong Dong, Yan Zang, Lijin Shu, Elizabeth C. Wilkinson, and Lawrence Que, Jr.* Department of Chemistry and Center for Metals in Biocatalysis, UniVersity of Minnesota Minneapolis, Minnesota 55455 Karl Kauffmann and Eckard Mu¨nck Department of Chemistry, Carnegie-Mellon UniVersity Pittsburgh, PennsylVania 15213 ReceiVed September 4, 1997 High-valent intermediates have been identified in the redox cycles of the oxygen activating nonheme diiron enzymes1 methane monooxygenase (MMO)2 and ribonucleotide reductase (RNR).3 For MMO, a diiron(IV) species Q has been observed,4 while an iron(III)iron(IV) species X has been characterized for RNR.5 Rapid-freeze-quench EXAFS studies have shown both Q and X to have Fe-Fe distances of ca. 2.5 Å.6,7 This unusually short distance has led us to consider the possibility that these intermediates may have a bis(µ-oxo)diiron diamond core,8 as precedented in synthetic nonheme diiron(III,IV) complexes.9 The latter are obtained from reaction of diiron(III) complexes with H2O2, but no studies of the mechanism of assembly of the Fe2(µ-O)2 diamond core have yet been reported. In this communication, we present evidence that a precursor to the bis(µoxo)FeIIIFeIV complex is a (µ-oxo)(µ-1,2-peroxo)diiron(III) complex; this conversion can serve as a synthetic precedent for the formation of high-valent intermediates from their peroxo precursors in the mechanisms of RNR and MMO. The diiron(III) complex [Fe2(µ-O)2(6-Me3-TPA)2](ClO4)210 (1), the only crystallographically characterized example of a complex with the Fe2(µ-O)2 diamond core, exhibits a visible spectrum is shown in Figure 1.11 Addition of 2 equiv of H2O2 to 1 at -40 °C in CH3CN generates a transient dark green (1) Wallar, B. J.; Lipscomb, J. D. Chem. ReV. (Washington, D.C.) 1996, 96, 2625-2657. (2) (a) Rosenzweig, A. C.; Frederick, C. A.; Lippard, S. J.; Nordlund, P. Nature 1993, 366, 537-543. (b) Rosenzweig, A. C.; Nordlund, P.; Takahara, P. M.; Frederick, C. A.; Lippard, S. J. Chem. Biol. 1995, 2, 409418. (c) Elango, N.; Radhakrishnan, R.; Froland, W. A.; Wallar, B. J.; Earnhart, C. A.; Lipscomb, J. D.; Ohlendorf, D. H. Protein Sci. 1997, 6, 556-568. (3) (a) Nordlund, P.; Eklund, H. J. Mol. Biol. 1993, 232, 123-164. (b) Logan, D. T.; Su, X.-D.; Åberg, A.; Regnstro¨m, K.; Hajdu, J.; Eklund, H.; Nordlund, P. Structure 1996, 4, 1053-1064. (4) (a) Lee, S.-K.; Nesheim, J. C.; Lipscomb, J. D. J. Biol. Chem. 1993, 268, 21569-21577. (b) Lee, S.-K.; Fox, B. G.; Froland, W. A.; Lipscomb, J. D.; Mu¨nck, E. J. Am. Chem. Soc. 1993, 115, 6450-6451. (c) Liu, K. E.; Wang, D.; Huynh, B. H.; Edmondson, D. E.; Salifoglou, A.; Lippard, S. J. J. Am. Chem. Soc. 1994, 116, 7465-7466. (d) Liu, K. E.; Valentine, A. M.; Wang, D.; Huynh, B. H.; Edmondson, D. E.; Salifoglou, A.; Lippard, S. J. J. Am. Chem. Soc. 1995, 117, 10174-10185. (5) (a) Bollinger, J. M.; Edmondson, D. E.; Huynh, B. H.; Filley, J.; Norton, J.; Stubbe, J. Science (Washington, D.C.) 1991, 253, 292-298. (b) Ravi, N.; Bollinger, J. M.; Huynh, B. H.; Edmondson, D. E.; Stubbe, J. J. Am. Chem. Soc. 1994, 116, 8007-8014. (c) Sturgeon, B. E.; Burdi, D.; Chen, S.; Huynh, B.-H.; Edmondson, D. E.; Stubbe, J.; Hoffman, B. M. J. Am. Chem. Soc. 1996, 118, 7551-7557. (6) Shu, L.; Nesheim, J. C.; Kauffmann, K.; Mu¨nck, E.; Lipscomb, J. D.; Que, L., Jr. Science 1997, 275, 515-518. (7) Riggs-Gelasco, P. J.; Shu, L.; Chen, S.; Burdi, D.; Huynh, B. H.; Que, L., Jr.; Stubbe, J. J. Am. Chem. Soc. In press. (8) (a) Que, L., Jr. J. Chem. Soc., Dalton Trans. 1997, 3933-3940. (b) Que, L., Jr.; Dong, Y. Acc. Chem. Res. 1996, 29, 190-196. (9) (a) Dong, Y.; Fujii, H.; Hendrich, M. P.; Leising, R. A.; Pan, G.; Randall, C. R.; Wilkinson, E. C.; Zang, Y.; Que, L., Jr.; Fox, B. G.; Kauffmann, K.; Mu¨nck, E. J. Am. Chem. Soc. 1995, 117, 2778-2792. (b) Dong, Y.; Que, L., Jr.; Kauffmann, K.; Mu¨nck, E. J. Am. Chem. Soc. 1995, 117, 11377-11378.
S0002-7863(97)03115-6 CCC: $14.00
Figure 1. Visible spectra of 1 (- -) and 2 (s) in CH3CN. Inset: Resonance Raman spectra of 2 obtained in CH3CN using 628-nm excitation with H216O2 (bottom) and H218O2 (top).
Figure 2. Mo¨ssbauer spectra of 2 (A) and its decomposition products (B). Spectra were recorded at 4.2 K in zero field (A) and in a parallel applied field of 8.0 T (B). The solid line in part A is a simulated spectrum with ∆EQ ) 1.68 mm/s and δ ) 0.54 mm/s. The solid line in part B is a spectral simulation (representing 35% of Fe) outlining the contributions of the Fe(IV) and Fe(III) sites of the S ) 1/2 complex 3; the parameters used are the same as those published for [FeIIIFeIV(µ-O)2(6-Me-TPA)2]3+.9b The triangles mark contributions of the monomeric Fe(III) species (
