System biomimetic

In addition to organometallic alkane conversion, a number of enzymes also carry out these reactions with air as oxidant. The key step is still under discussion but appears to be an H atom abstraction from the R-H bond by a metal oxo group, followed by a rebound step in which the resulting R radical abstracts OH from 

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Biomimetic chemistry of nickel was extensively reviewed.1847,1848 Elaborate complexes have been developed in order to model structural and spectroscopic properties as well as the catalytic function of the biological sites. Biomimetic systems for urease are described in Section 6.3.4.12.7, and model systems for [Ni,Fe]-hydrogenases are collected in Section 6.3.4.12.5. 

The cucurbit [n]uril family (CB[n]) of molecular containers possess remarkable binding affinities and selectivities (Ka values up to 1012M-1, Krei values up to 106) which renders them useful as a component of molecular machines, sensors, and biomimetic systems (123-125). Recently, Wagner and coworkers have reported (126) that CB[10] - with its spacious 870A3 cavity - is capable of acting as a host for free base and metalated tetra (Af-methylpyridinium)porphyrins 19a-d (Fig. 17). Despite the large ellipsoidal deformation of CB[10] upon complexation, the complexed porphyrins retain their fundamental UV/VIS, fluorescence, and electrochemical properties. The CB[ 10] porphyrin... 

According to X-ray and neutron diffraction structures [3, 4] the binding of CO to the heme leads to a bent FeCO unit. The Fe-C-O angle is, however, found to be linear in synthetic models of the protein (hiomimetic molecules). Because of this, it was originally thought that the FeCO distortion was responsible for the well known discrimination of the protein against CO - the affinity ratio C0/02 is lower in the protein than in biomimetic systems [1]. In... 

J. M. Valleton, Information processing in biomolecular-based biomimetic systems from macroscopic to nanoscopic scale, React. Polym., 12,109-131 (1990). 

The functions of phenylpropanoid derivatives are as diverse as their structural variations. Phenylpropanoids serve as phytoalexins, UV protectants, insect repellents, flower pigments, and signal molecules for plant-microbe interactions. They also function as polymeric constituents of support and surface structures such as lignins and suberins [1]. Therefore, biosynthesis of phenylpropanoids has received much interest in relation to these functions. In addition, the biosynthesis of these compounds has been intensively studied because they are often chiral, and naturally occurring samples of these compounds are usually optically active. Elucidation of these enantioselective mechanisms may contribute to the development of novel biomimetic systems for enantioselective organic synthesis. 

Both the face-to-face dimers and the T- and L-shaped dimers have relevance to biomimetic systems, especially in light of the work on the x-ray structure of the R. Viridis reaction center ill). As will be discussed in the following, the results obtained for the face-to-face dimer with an interplanar spacing of 5.35 A is prototypical of the results obtained for almost all the dimers investigated, and thus it provides a reasonable basis for illustrating a number of important features that arise from the calculations. However, it should be noted that the lack oP covalent links in our model dimers precludes the possibility of "through-bond" effects. 

These workers employed a more elaborate system than Wasielewski et al., which included the presence of a membrane to assist in maintaining the initial photo-induced charge separation. Although the complexity of the system did generate a number of technical problems, it probably comes closer to an actual, viable prototype of a workable biomimetic system. 

A redox stable, water-soluble iron porphyrin has been used as a model ligninase. The reactions of lignin model compounds catalyzed by this biomimetic system were found to be dependent on pH and the solvent being used. 

Biocatalysis and Biomimetic Systems, Artificial Photosynthetic Transformations Through (Willner and Willner). 

Interest in these studies arises from fundamental research where monolayers serve as models of biomimetic systems, as well as from important apphcations of such systems in molecular and bioelectronic devices, in sensors constructions, corrosion/inhibition phenomena, and synthesis of nanostructures... 

CHEMICAL SENSING BASED ON SUPRAMOLECULAR FUNCTIONS OF BIOMIMETIC SYSTEMS... 

In chemical terms the photoinduced electron transfer results in transfer of an electron across the photosynthetic membrane in a complex sequence that involves several donor-acceptor molecules. Finally, a quinone acceptor is reduced to a semiquinone and subsequently to a hydroquinone. This process is accompanied by the uptake of two protons from the cytoplasma. The hydroquinone then migrates to a cytochrome be complex, a proton pump, where the hydroquinone is reoxidized and a proton gradient is established via transmembrane proton translocation. Finally, an ATP synthase utilizes the proton gradient to generate chemical energy. Due to the function of tetrapyrrole-based pigments as electron donors and quinones as electron acceptors, most biomimetic systems utilize some... 

Biomimetic systems comprised of porphyrins and quinones have been studied extensively with regard to their electron transfer and charge transfer properties. Porphyrin-... 

The most common methods of attaching substituents to porphyrins utilize ortho-phenyl-substituted derivatives of 5,10,15,20-tetraphenyl-21 H,23//-por-phine (TPP). While (2-aminophenyl) TPP derivatives have been extensively used in the preparation of biomimetic systems,5 the corresponding (2-hy-droxyphenyl) TPP derivatives have only recently been utilized.6 One of the limitations to the use of hydroxy-substituted systems has been the synthesis of 5,10,15,20-tetrakis(2,6-dihydroxyphenyl)-21//,23//-porphine 3. 

The first reports on iron-catalyzed aziridinations date back to 1984, when Mansuy et al. reported that iron and manganese porphyrin catalysts were able to transfer a nitrene moiety on to alkenes [90]. They used iminoiodinanes PhIN=R (R = tosyl) as the nitrene source. However, yields remained low (up to 55% for styrene aziridination). It was suggested that the active intermediate formed during the reaction was an Fev=NTs complex and that this complex would transfer the NTs moiety to the alkene [91-93]. However, the catalytic performance was hampered by the rapid iron-catalyzed decomposition of PhI=NTs into iodobenzene and sulfonamide. Other reports on aziridination reactions with iron porphyrins or corroles and nitrene sources such as bromamine-T or chloramine-T have been published [94], An asymmetric variant was presented by Marchon and coworkers [95]. Biomimetic systems such as those mentioned above will be dealt with elsewhere. 

At least two systems can be cited as catalysts of peroxide oxidation the first are the iron (III) porphyrins (44) and the second are the Gif reagents (45,46), based on iron salt catalysis in a pyridine/acetic acid solvent with peroxide reagents and other oxidants. The author s opinion is that more than systems for stress testing these are tools useful for the synthesis of impurities, especially epoxides. From another point of view, they are often considered as potential biomimetic systems, predicting drug metabolism. Metabolites are sometimes also degradation impurities, but this is not a general rule, because enzymes and free radicals have different reactivity an example is the metabolic synthesis of arene oxides that never can be obtained by radical oxidation. 

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