Electron transfer, activation control catalysis

It is becoming increasingly obvious that the activity of P450 BM3 is controlled both thermodynamically and by structural changes triggered by substrate (fatty acid or NADP(H)) or redox state of the enzyme. The availability of the crystal structures of substrate-free and bound forms allows investigation of the roles of various amino acids in the processes of substrate binding, electron transfer and oxidative catalysis. This can be achieved... 

A number of metal porphyrins have been examined as electrocatalysts for H20 reduction to H2. Cobalt complexes of water soluble masri-tetrakis(7V-methylpyridinium-4-yl)porphyrin chloride, meso-tetrakis(4-pyridyl)porphyrin, and mam-tetrakis(A,A,A-trimethylamlinium-4-yl)porphyrin chloride have been shown to catalyze H2 production via controlled potential electrolysis at relatively low overpotential (—0.95 V vs. SCE at Hg pool in 0.1 M in fluoroacetic acid), with nearly 100% current efficiency.12 Since the electrode kinetics appeared to be dominated by porphyrin adsorption at the electrode surface, H2-evolution catalysts have been examined at Co-porphyrin films on electrode surfaces.13,14 These catalytic systems appeared to be limited by slow electron transfer or poor stability.13 However, CoTPP incorporated into a Nafion membrane coated on a Pt electrode shows high activity for H2 production, and the catalysis takes place at the theoretical potential of H+/H2.14... 

A key aspect of metal oxides is that they possess multiple functional properties acid-base, electron transfer and transport, chemisorption by a and 7i-bonding of hydrocarbons, O-insertion and H-abstraction, etc. This multi-functionality allows them to catalyze complex selective multistep transformations of hydrocarbons, as well as other catalytic reactions (NO,c conversion, for example). The control of the catalyst multi-functionality requires the ability to control not only the nanostructure, e.g. the nano-scale environment around the active site, " but also the nano-architecture, e.g. the 3D spatial organization of nano-entities. The active site is not the only relevant aspect for catalysis. The local area around the active site orients or assists the coordination of the reactants, and may induce sterical constrains on the transition state, and influences short-range transport (nano-scale level). Therefore, it plays a critical role in determining the reactivity and selectivity in multiple pathways of transformation. In addition, there are indications pointing out that the dynamics of adsorbed species, e.g. their mobility during the catalytic processes which is also an important factor determining the catalytic performances in complex surface reaction, " is influenced by the nanoarchitecture. 

One of the important applications of mono- and multimetallic clusters is to be used as catalysts [186]. Their catalytic properties depend on the nature of metal atoms accessible to the reactants at the surface. The possible control through the radiolytic synthesis of the alloying of various metals, all present at the surface, is therefore particularly important for the catalysis of multistep reactions. The role of the size is twofold. It governs the kinetics by the number of active sites, which increase with the specific area. However, the most crucial role is played by the cluster potential, which depends on the nuclearity and controls the thermodynamics, possibly with a threshold. For example, in the catalysis of electron transfer (Fig. 14), the cluster is able to efficiently relay electrons from a donor to an acceptor, provided the potential value is intermediate between those of the reactants [49]. Below or above these two thresholds, the transfer to or from the cluster, respectively, is thermodynamically inhibited and the cluster is unable to act as a relay. The optimum range is adjustable by the size [63]. 

As demonstrated in this review, photoinduced electron transfer reactions are accelerated by appropriate third components acting as catalysts when the products of electron transfer form complexes with the catalysts. Such catalysis on electron transfer processes is particularly important to control the redox reactions in which the photoinduced electron transfer processes are involved as the rate-determining steps followed by facile follow-up steps involving cleavage and formation of chemical bonds. Once the thermodynamic properties of the complexation of adds and metal ions are obtained, we can predict the kinetic formulation on the catalytic activity. We have recently found that various metal ions, in particular rare-earth metal ions, act as very effident catalysts in electron transfer reactions of carbonyl compounds [216]. When one thinks about only two-electron reduction of a substrate (A), the reduction and protonation give 9 spedes at different oxidation and protonation states, as shown in Scheme 29. Each species can... 

Fig. 3. A hypothetical ribozyme that can catalyze electron transfer. Aptamers than can bind NAD+ (and, hence, NADH) are selected, and the binding domain is mapped. An oligonucleotide tail that can bind to an unpaired region near the NAD-binding domain is attached to FMN. The bound FMN-oligonucleotide will be adjacent to NADH when it is bound in the active site of the ribozyme. Electron transfer should occur owing to the proximity of the two substrates. The rate of the reaction can be controlled by varying the length of the oligonucleotide tail to vary the distance between NADH and FMN substrate. Although this catalyst is extremely simple (and employs the same principles of catalysis found in nonenzymatic template-directed ligation reactions), it would nevertheless demonstrate the ability of RNA to catalyze reactions other than phosphodiester bond transfers.

FIGURE 11. An escapement mechanism is sometimes used to control the direction of electron transfer within a redox cluster (small box). Here electron transfers from the substrate (close pair of circles filled with electrons) to a redox center on the left which is effectively insulated by distance from other members of a redox chain (further left) so only one electron can be transferred. The radical intermediate can transfer electrons to the chain on the right. The thermally activated escapement motion of the redox center then carries an electron to the chain at the left, and finally reassembles the cluster in preparation for the next catalysis. 

Important consequences result from the increase of the redox potential of metal clusters with their nuclearity. Indeed, independently of the metal, the smaller clusters are more sensitive to oxidation and can undergo corrosion even by mild oxidizing agents. Moreover, size-dependent redox properties explain the catalytic efficiency of colloidal particles during electron transfer processes. Their redox potentials control their role as electron relays the required potential being intermediate between the thresholds of the potentials of the donor (more negative) and of the acceptor (more positive). Catalytic properties of the nanoparticles are thus size-dependent. Haruta and co-workers reported that gold nanoparticles smaller than 5 nm have potential applications in catalysis as they are very active in... 

Actually, one of the most important applications of metal nanoparticles is in the field of catalysis. Catalysts should offer large specific area in order to accelerate the access of reactants to the active sites. Nanoparticles, such as those synthesized by radiolysis, are thus particularly efficient in a number of reactions. However, catalyzed reactions are controlled not only by the kinetics, but also by the thermodynamics. Thus, due to their redox properties, nanoparticles with small sizes and low polydispersities are able to play a role as intermediate electron relays in an overall electron transfer between a donor and an acceptor. 

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