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Modeling the active site of non-heme iron proteins: Synthesis and characterization of carboxylate-bridged diiron(II) complexes
NON-HEME IRON PROTEINS 399
H40
MODELING THE ACTIVE SITE OF NON-HEME IRON PROTEINS: SYNTHESIS A N D C H A R A C T E R I Z A T I O N OF CARBOXYLATEBRIDGED DIIRON(II) COMPLEXES Susanna Herold and Stephen J. Lippard
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA. Carboxylate-bridged diiron centers are found in the active sites of several nonheme iron proteins [1]. We have been particularly interested in the study of one of these enzymatic systems, methane monooxygenase (MMO), which catalyzes the reaction of methane with dioxygen to form methanol and water. The X-ray structure of the oxidized form of the MMO hydroxylase (MMOH) reveals two iron centers in an oxygen-rich coordination sphere, bridged by two carboxylate groups and an hydroxide ion [2]. A variety of diiron(III) model complexes have contributed to afford considerable insight into the structural and physical properties of this enzymatic system. However, diferrous model compounds are needed to understand, and possibly mimic, the dioxygen reactivity found in the biological system. We describe the synthesis and the X-ray characterization of a series of diferrous complexes bridged by the FIGURE 1. dinucleating bis-carboxylate XDK (H2XDK = m-xylenediamine bis(Kemp's triacid)imide). Particularly relevant is the benzoate-bridged d e r i v a t i v e [Fe 2(~t-OBz)(XDK)(ImH)2(OBz)(MeOH)] (FIGURE 1) which was assembled u n d e r anaerobic conditions by reacting a 2:1:2:2:3 ratio of [Fe(H20)6](BF4)2, H2XDK, Et3N, ImH (imidazole) and (Me4N)(OBz). This complex, with an oxygen-rich coordination sphere and only one N-donor per iron center, represents a very close model for the reduced form of the diiron core in MMOH. In the presence of fluoride, chloride or triflate ions the complexes [Fe2(~t-F)(XDK)(N-MeIm)2(MeOH)3](BF4), [Fe2 (~t-OTf)(XDK)(N-MeIm)3(MeOH)(H20)](OTf) a n d [Fe2(~t-C1)(XDK)(N-Melm)2(MeOH)3](BF4) w e r e assembled by a similar procedure. X-ray studies of these complexes reveal an asymmetric arrangement of the ligands, with 5- and 6-coordinate Fe(II) ions bound only to terminal monodentate N- and O-donors. Preliminary magnetic measurements show that the two iron centers are weakly antiferromagnetically coupled (0 < J < -5 cm -1) in all the prepared complexes, regardless of the nature of the bridging ligand. The reactivity of these complexes with dioxygen and other reagents is currently under investigation. This work was supported from the National Institute of General Medical Sciences and AKZO. S. H. thanks the Swiss National Foundation and the Ciba-GeigyJubil~iumsstiftung for postdoctoral fellowships. 1. A. L. Feig and S. J. Lippard Chem. Rev., 94, 759 (1994). 2. A. C. Rosenzweig, C. A. Frederick, S. J. Lippard, P. Nordlund, Nature, 366, 537 (1993).
H40
MODELING THE ACTIVE SITE OF NON-HEME IRON PROTEINS: SYNTHESIS A N D C H A R A C T E R I Z A T I O N OF CARBOXYLATEBRIDGED DIIRON(II) COMPLEXES Susanna Herold and Stephen J. Lippard
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA. Carboxylate-bridged diiron centers are found in the active sites of several nonheme iron proteins [1]. We have been particularly interested in the study of one of these enzymatic systems, methane monooxygenase (MMO), which catalyzes the reaction of methane with dioxygen to form methanol and water. The X-ray structure of the oxidized form of the MMO hydroxylase (MMOH) reveals two iron centers in an oxygen-rich coordination sphere, bridged by two carboxylate groups and an hydroxide ion [2]. A variety of diiron(III) model complexes have contributed to afford considerable insight into the structural and physical properties of this enzymatic system. However, diferrous model compounds are needed to understand, and possibly mimic, the dioxygen reactivity found in the biological system. We describe the synthesis and the X-ray characterization of a series of diferrous complexes bridged by the FIGURE 1. dinucleating bis-carboxylate XDK (H2XDK = m-xylenediamine bis(Kemp's triacid)imide). Particularly relevant is the benzoate-bridged d e r i v a t i v e [Fe 2(~t-OBz)(XDK)(ImH)2(OBz)(MeOH)] (FIGURE 1) which was assembled u n d e r anaerobic conditions by reacting a 2:1:2:2:3 ratio of [Fe(H20)6](BF4)2, H2XDK, Et3N, ImH (imidazole) and (Me4N)(OBz). This complex, with an oxygen-rich coordination sphere and only one N-donor per iron center, represents a very close model for the reduced form of the diiron core in MMOH. In the presence of fluoride, chloride or triflate ions the complexes [Fe2(~t-F)(XDK)(N-MeIm)2(MeOH)3](BF4), [Fe2 (~t-OTf)(XDK)(N-MeIm)3(MeOH)(H20)](OTf) a n d [Fe2(~t-C1)(XDK)(N-Melm)2(MeOH)3](BF4) w e r e assembled by a similar procedure. X-ray studies of these complexes reveal an asymmetric arrangement of the ligands, with 5- and 6-coordinate Fe(II) ions bound only to terminal monodentate N- and O-donors. Preliminary magnetic measurements show that the two iron centers are weakly antiferromagnetically coupled (0 < J < -5 cm -1) in all the prepared complexes, regardless of the nature of the bridging ligand. The reactivity of these complexes with dioxygen and other reagents is currently under investigation. This work was supported from the National Institute of General Medical Sciences and AKZO. S. H. thanks the Swiss National Foundation and the Ciba-GeigyJubil~iumsstiftung for postdoctoral fellowships. 1. A. L. Feig and S. J. Lippard Chem. Rev., 94, 759 (1994). 2. A. C. Rosenzweig, C. A. Frederick, S. J. Lippard, P. Nordlund, Nature, 366, 537 (1993).