Synthetic model complexes

Historically, the first chemical synthesis of urea by Wohler, from ammonium cyanate in 1828, was a milestone that established a bridge between inorganic and organic chemistry. Urease was the first enzyme ever to be crystallized (6), and it was the first protein shown to contain nickel ions in the active site (7). The first X-ray crystal structure of urease became known in 1995 (8). Significant progress was made since then toward an understanding of its catalytic mechanism, as well as toward the structural and functional emulation of its active site by synthetic model complexes (5, 9). 

Biomimetic and bioinspired synthetic model complexes were very useful in providing information about basic functional principles and mechanistic aspects... 

Zunmermann et aL3° have further undertaken high field and variable temperature Mossbauer studies that resulted in a set of parameters (Table 3) which characterize the ferrous state of the enzyme these parameters should prove useful for the design of appropriate synthetic model complexes. 

Reactive Zn OII (n = 1 or 2) species are proposed in the catalytic cycles of several zinc-containing enzymes. In order to gauge the chemical factors that influence the formation and reactivity of Zn OH species, synthetic model complexes have been prepared and systematically examined for biologically relevant stoichiometric and catalytic reactivity. Systems that promote the hydration of C02, the activation and oxidation of alcohols, and amide and phosphate ester cleavage reactions are discussed. 

METAL SITES IN NITROGENASE AND SYNTHETIC MODEL COMPLEXES... 

The spin density distribution and the Mulliken atomic spin densities of the 1V54B model are almost same as those of the 1V540 model, indicating the robust electronic structure of the Cua site and the synthetic model complexes see Fig. 30.6 and Table 30.1. 

The Mo enzyme consists of two components (1) the Fe protein (molecular weight 57,000 daltons), which contains iron and sulfur (4 atoms of each per protein) and (2) the MoFe protein (220,000 daltons, Uz subunits), which contains both metals (1 atom Mo, 32 atoms Fe). Each also contains S ions (ca. one per iron), which act as bridging ligands for the metals. The protein contains special Fe—S clusters called P clusters that have EPR resonances like those of no other Fe—S cluster. A soluble protein-fiee molybdenum and iron-containing cluster can be separated from the enzyme. This iron-molybdenum cofactor, or FeMo-co, was known to have approximately 1 Mo, 7-8 Fe, 4-6 S ", and one molecule of homocitrate ion. As for the P cluster, there was no agreement on the structure of FeMo-co for many years. In purified form FeMo-co does crystallize, and it can restore N2 reducing activity to samples of mutant N2ase that are inactive because they lack FeMo-co. On the other hand, no crystal structure of FeMo-co proved possible, and no synthetic model complex was found that could activate the mutant enzyme. 

Tolman W.B. (1995) Synthetic Modeling of the Interactions of Nitrogen Oxides with Copper Proteins Copper Nitrosyl Complexes Relevant to Putative Denitrification Intermediates, Adv. Chem. Ser., 246, 195. 

Because of the different kinds of enzyme, several synthetic models have been proposed which can be evaluated with regard to their composition and structure or to particular features of the enzymatic function. One of the most successful, proposed by Crabtree and co-workers some years ago,416 involves the ionic complex [NiL2]Cl2 (144 L = 2-HOC6H4CH = N-NHC(S)NH2). 

In a number of model studies it has been demonstrated that synthetic porphyrin complexes are able to be oxidized to oxo-metalloporphyrin complexes which will, in turn, oxidize organic substrates of the type mentioned previously. Thus, in an early investigation it was shown that a synthetic Fe(m) porphyrin will catalyze oxygenation of hydrocarbons using iodosylbenzene as the reduced-oxygen source (Groves, Nemo Myers, 1979). Following this report, a number of related cytochrome... 

Synthetic models of myoglobin and hemoglobin are complex molecules that mimic the stereochemical properties of the protein active center [24] and have oxygen affinities similar to those measured for the protein [25-27]. The first heme model that reversibly binds oxygen (i.e. the picket-fence-oxygen complex Fe(TpivPP)(l,2-Melm)(02), shown in Fig. 3.3) was obtained in the early nine-teen-seventies by Collman and coworkers (TpivPP = tetrapivalami-nophenyl porphyrin 2-meIm = 2-methylimidazole) [18]. Research on synthetic models of the protein has led to a deeper understand-... 

Chemical Reviews paper. We can only discuss a small number of these here, but some important categories are (1) synthetic Fe(II)-Cu(I) complexes and their reactions with O2, (2) oxidized heme-copper models (Fe(III)-X-Cu(II) complexes, where X equals 0x0- and hydroxo-bridged complexes, cyanide-bridged complexes, or other X-bridged complexes), (3) crosslinked histidine-tyrosine residues at the heme-copper center, and (4) Cua site synthetic models. 

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