Water biomimetic catalysis

At the present time, "interest in reversed micelles is intense for several reasons. The rates of several types of reactions in apolar solvents are strongly enhanced by certain amphiphiles, and this "micellar catalysis" has been regarded as a model for enzyme activity (. Aside from such "biomimetic" features, rate enhancement by these surfactants may be important for applications in synthetic chemistry. Lastly, the aqueous "pools" solubilized within reversed micelles may be spectrally probed to provide structural information on the otherwise elusive state of water in small clusters. 

A biomimetic artificial photylase model was described by Wiest et al. [18] The model recognised pyrimidine dimers in both organic solvents and water. Excitation of the complex with visible light led to cycloreversion of the pyrimidine dimer through photoinduced electron-transfer catalysis (Scheme 21). 

The controversy on the existence of in vivo Diels-Alder reactions cannot be put to rest here, but the numerous examples of natural products containing cyclohexene groups and the catalytic effectivity of biological surroundings support the idea of in vivo Diels-Alder reactions. Apart from cell-free extracts, RNA-based mixtures of metals also show catalytic activity and it was demonstrated that this catalyst system can be quite effective as an artificial Diels-Alder-ase . We will show that water, the prime solvent of biosynthesis, also catalyses [4 -+- 2]-cycloadditions. Considering that biosyntheses are often of exceptional selectivity, it is clear that understanding biomimetic transfonna-tions in water as the solvent is an important goal of modem chemistry. The possibilities offered by and the reasons for Diels-Alder catalysis in water will be the main topic of this chapter. 

Production of H2 fuel from water via solar energy is of exceedingly high interest.115 Catalysis may involve H2 complexes at least as intermediates, and H2 complexes had previously been implicated in solar energy conversion schemes based on photoreduction of water.116 Industrially important water-gas shift and related H2-producing reactions undoubtedly proceed via transient H2 complexes.117 Biomimetic H2 production, particularly solar driven (via photocatalysis), is also a challenge and may take a cue from models of the active site of hydrogenase coupled with models of nature s photosystems.91-93 Here, the formation of H-H bonds from protons and... 

Sulfur plays a central role in enzymatic catalysis and metal sulfido enzymes can activate various small molecules in biological conditions [20]. Much work has been devoted to the isolation and understanding of the intimate mechanisms of activation of biomimetic sulfur-containing complexes [21]. However, these systems do not present significant solubility in water under the studied conditions. We can nevertheless refer to the Sellman group s work in which the authors isolated sulfur-containing representative [FeMo] complexes of the cofactor of [FeMo] nitrogenases [22, 23], and were able to produce water-soluble complexes by introduction of... 

This is a biomimetic system and a model system for reaction kinetics, but also an actual medium for phase transfer catalysis. In this system, one of the solvents is usually water, and the other one is a low polar organic liquid (od), such as, for example, nitrobenzene or dichloroethane. 

As our intent in this review is to address questions of NO c reductions at heme active sites from a mechanistic inorganic perspective, the use of simple heme model compounds is quite useful and will be thoroughly described. Here similar issues of solubility and reactivity come into play, as biomimetic NO c catalysis requires either water-soluble porphyrins or surface-adsorbed catalysts. As might be expected, questions of intersite reactivity come into play both in the reactivity of transient intermediate species such as nitroxyl-adducts, and in multielectron reactivity required for assimilatory reductase activity. We will begin with brief descriptions of the known families of heme-based NOjc reductases and the all-too-few electrochemical investigations of these native enzymes. 

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