Fe2-center

There are two levels of self-assembly in the formation of tetra-, penta-and hexa-nuclear products from the poly-bipyridyls (L) 20 and 21 and iron(II) salts FeCl2, FeBr2 or FeS04 - the products are anion-dependent. The coordination of three bpy units, from different ligand molecules, to the Fe2+ centers produces a helical structure interaction of these helical strands with anions results in further molecular organization to form the final toroidal product. The discussion draws parallels between the helical and toroidal structures here and secondary and tertiary structure in biological systems (482). Thermodynamic and kinetic intermediates have been characterized in the self-assembly of a di-iron triple stranded helicate with bis(2,2/-bipyridyl) ligands (483). 

SN P spontaneously releases N O both thermally and photochemically [61-65], but is quite stable in the dark and in aqueous in vitro physiological media [66]. This implies that absorption of heat and light energy induces electron transfer from the Fe2+ center to the N 0+ ligand, resulting in weakening of the Fe-N O bond and subsequent release of NO [65]. SNP also decomposes in an aqueous environment in the presence of biological reductants [65, 66] and some transition metal ions to produce nitric oxide. 

As mentioned above, the magnetic data clearly indicate that the overall spin transition is not complete at 300 K, therefore the Fe2 center requires higher temperatures to convert to the HS state. The 7 values for the FesFe2 and Fe3Co2 clusters at 375 K approach the values expected for complete conversion of all equatorial Fe(II) centers to the HS state. 

Purple acid phosphatases (PAPs) catalyze the hydrolysis of phosphate monoesters with mildly acidic pH optima (5-7) utilizing a binuclear metal center containing a ferric ion and a divalent metal ion. PAPs are also characterized by their purple color, the result of a tyrosine (Tyr) to Fe3+ charge transfer transition at about 560nm.113 All known mammalian PAPs are monomeric and have a binuclear Fe3+-Fe2+ center, whereas the kidney bean and soybean enzymes are dimeric and have an Fe3 + -Zn2+ center in each subunit. The X-ray structures for kidney bean PAP114 and the PAP115 from rat bone reveal that despite a sequence similarity of only 18%, they share very similar catalytic sites. The structure of the kidney bean PAP shows the two metal ions at a distance of 3.1 A, with a monodentate bridging Asp-164. These and other residues involved in metal coordination can be seen in Fig. 21. 

Structure of heme. In the hemoglobin molecule, the Fe2+ ion is at the center of an octehedron, surrounded by four nitrogen atoms, a globin molecule, and a water molecule. 

The structure of the [Fe2 / - 82) P o - 61148) )2] complex is presented in Figure 1. The structure we used was obtained from x-ray crystallography, symmetrized such that the environments of the two Fe atoms are the same. The local symmetry of each Fe atom is C3, the overall symmetry of the complex is Ci. To aid in the interpretation of the computed results we oriented the complex such that the z-axis passes through the center of mass and the projections of the two P — Fe- 8 line segments onto the z-axis are maximized. 

Synthetic analog clusters have played a pivotal role in development of Fe-S cluster biochemistry. Indeed, the synthesis and characterization of clusters with [Fe2( t2-S)2], [Fe4( (,3-S)4], and linear [Fe3( t2-S)4l cores by Holm and co-workers (47, 48) were crucial in establishing the properties of these clusters and identifying these types of centers in biological systems. However, the synthesis of a cluster with the physi-... 

A novel polysiloxane, containing the isocyanide group pendent to the backbone, has been synthesized. It is observed to react with the metal vapors of chromium, iron and nickel to afford binary metal complexes of the type M(CN-[P])n, where n = 6, 5, 4 respectively, in which the polymer-attached isocyanide group provides the stabilization for the metal center. The product obtained from the reaction with Fe was found to be photosensitive yielding the Fe2(CN-[P])q species and extensive cross-linking of the polymer. The Cr and Ni products were able to be oxidized on exposure of thin films to the air, or electrochemically in the presence of an electron relay. The availability of different oxidation states for the metals in these new materials gives hope that novel redox-active polymers may be accessible. 

According to Summers and Chang from NASA s Ames Research Center, Moffett Field (1993), the oxidation of Fe2+ to Fe3+ provided a possibility for the reduction of nitrites and nitrates to ammonia. This reaction would have been of great importance, as NH3 is required in many syntheses of biogenesis precursors. The authors assume that nitrogen was converted to NO in a non-reducing atmosphere, and thence to nitrous and nitric acids. These substances entered the primeval oceans in the form of acid rain , and here underwent reduction to NH3 with the help of Fe2+, thus raising the pH of the oceans to 7.3. Temperatures above 298 K favoured this reaction, which can be written as ... 

After reduction in FI2 at 675 K the catalyst consists mainly of metallic iron, as evidenced by the sextet (S= 0.00 mm/s, H = 331 kOe), along with some unreduced iron, which gives rise to two doublet contributions of Fe2+ and Fe3+ in the center. The... 

The metal-centered complexes can also be used as multifunctional initiators. For example, Fe2+(4,4 dichloromethyl-2,2 -bipyridine)3 or the Ru2+ complex have been used as initiators for the living cationic polymerization of 2-ethyl-2-oxazoline [120],... 

Figure 3.5. Continued. The H2-NAD reaction is inhibited neither in air nor in the presence of CO. C,The possible reactions of hydrogen with the Fe-Fe site of active [Fe]-hydrogenases. In the oxidized state, the bimetallic center shows a S = 1/2 EPR signal, presumably due to an Fe -Fe pair (an Fe -Fe pair cannot be excluded). Whether the unpaired spin is localized on iron (Pierik et al. 1998a) or elsewhere (Popescu and Mtlnck 1999) is not known. Hydrogen is presumably reacting at the vacant coordination site on Fe2 (Fig. 3.1C). After the heterolytic splitting, the two reducing equivalents from the hydride are rapidly taken up by the Fe-Fe site (one electron) and the attached proximal cluster (one electron). Subsequently, the electron is transferred from the proximal cluster to the other Fe-S clusters in the enzyme. Under equilibrium conditions, the proximal cluster in the active enzyme appears to be always in the oxidized [4Fe-4S] state (Popescu and Mtlnck 1999). Protons are not shown.

---