Diiron carboxylate-bridged

Hydroxylase in the mixed-valent Fe(II)Fe(III) oxidation state (Hmv) is readily accessible by one-electron reduction of the dinuclear center. Mossbauer data indicate the presence of one Fe(III) and one Fe(II) (39). Hmv has a rhombic EPR signal with gav = 1.83 (27, 37) and J -30 cm1 (38,39), properties characteristic of other mixed-valent nonheme carboxylate-bridged diiron centers such as that in semimet hemerythrin (J = -15 cm-1) (32). ENDOR spectroscopic studies of Hmv... 

The Mossbauer spectrum of L consists of a symmetric doublet which was fitted with parameters 8 = 0.66 0.02 mm/s and AE = 1.51 0.03 mm/s (71). The isomer shift is larger than values (0.45-0.55 mm/s) generally seen for carboxylate-bridged diiron(III) clusters,... 

The final class of iron proteins that we consider here are a family that contain a carboxylate-bridged diiron centre. They carry out a variety of functions, which have the common link that they react with dioxygen as part of their functional processes. The dimetallic centre is incorporated into a four-helix bundle domain (see Figure 3.9a), which seems to represent... 

The best characterized of the BMMs are the sMMOs (Figure 13.24), which are the only members of the family capable of activating the inert C-H bond of methane, one of the most difficult reactions in nature to achieve. Like most members of the BMM superfamily, sMMO requires three protein components, the hydroxylase MMOH, which contains the carboxylate-bridged diiron centre, a regulatory protein MMOB and a [2Fe-2S]- and FAD-containing reductase (MMOR) which shuttles electrons from NADH to the diiron centre. 

Figure 13.26 Dioxygen-utilizing carboxylate-bridged diiron centres (a) Oxidized (top) and reduced (bottom) MMOH (b) oxidized (top) and Mnn-reconstituted ToMOH (bottom) (c) oxidized (top) and reduced (bottom) RNR-R2 (d) oxidized (top) and reduced (bottom) rubryerythrin (e) reduced stearoyl-acyl carrier protein A9 desaturase (f) reduced bacterioferritin (g) methaemerythrin. Fel is on the left and Fe2 on the right. (Reprinted with permission from Sazinsky and Lippard, 2006. Copyright (2006) American Chemical Society.)...

Because of the ubiquitous presence of bridging carboxylato groups, they are also known as carboxylate-bridged non-heme iron proteins. They are known for a large number of nuclearities which range from one iron center to many iron centers the most common contain the diiron unit. Several of these metalloenzymes are involved... 

Tolman, W. B., Liu, S., Bentsen, J. G., and Lippard, S. J., 1991, Models of the reduced forms of polyiron-oxo proteins An asymmetric, triply carboxylate bridged diiron(ll) complex and its reactions with dioxygen, J. Am. Chem. Soc. 113 1529164. 

A new member of the family of nonheme diiron enzymes recently discovered is called rubrerythrin. This metaUoprotein is formally classified as an oxidoreductase (rubredoximoxygen oxidoreductase). The diiron(III,in) active site structure is displayed in Figure 2(f). This biomolecule possesses two histidines coordinated to one iron and one histidine coordinated to the second iron. A carboxylate bridges the two irons and there are carboxylate ligands also coordinated to each iron. The purpose of this enzyme in the strict anaerobe is to safely reduce oxygen to water. 

Other Fe complexes are known to epoxidize olefins with peracetic acid [244]. For example, Jacobson and coworkers report a Fe(mep) complex (mep=N,N -dimethyl-N,N -bis(2-pyridylmethyl)-ethane) (Fig. 1.3c) which self-assembles under reaction conditions to form a /z-oxo, carboxylate-bridged diiron complex similar to that found in the core of the hydroxylase active site of oxidized methane monooxygenase (MMO) [236]. 

E. Y. Tshuva, S. J. Lippard, Synthetic models for non-heme carboxylate-bridged diiron metaUo-proteins Strategies and tactics, Chem. Rev. 104 (2004) 987. 

Feig, A. L. Masschelein, A. Bakac, A. Lippard, S. J. Kinetic studies of reactions of dioxygen with carboxylate-bridged diiron(II) complexes leading to the formation of ( l-oxo)diiron(III) complexes. J. Am. Chem. Soc. 1997, 119 (2), 334-342. 

Zhao, M. Song, D. Lippard, S. J. Water induces a structural conversion and accelerates the oxygenation of carboxylate-bridged non-heme diiron enzyme synthetic analogues. Inorg. Chem. 2006, 45, 6323-6330. 

The loss of one carboxylate bridge does not appear to alter significantly the properties of the ( x-oxo)diiron(III) unit. [Fe20(0Ac)(TPA)2](C104)3 exhibits a visible spectrum very similar to that of [Fe20(0Ac)2(HBpz3)2] (81). The J value for the TPA complex is —114 cm" and LEq is 1.52... 

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