Clusters redox processes

During oxidation of the MoFe protein the P clusters are the first to be oxidized at about -340 mV. This redox potential was first measured (40) using Mossbauer spectroscopy and exhibited a Nemst curve consistent with a two-electron oxidation process. It is possibly low enough for this redox process to be involved in enzyme turnover (see Section V). No additional EPR signal was observed from this oxidized form at this time. However, later a weak signal near g = 12 was detected and was finally confirmed, using parallel mode EPR... 

It is evident that the supported clusters have a strong affinity for hydride ligands provided by the support. The process by which the support delivers these ligands is referred to in the catalysis literature as reverse hydrogen spillover. The opposite process (spillover), well known for supported metals [36], is shown by the theoretical results to be a redox process in reverse spillover, the support hydroxyl groups oxidize the cluster. 

To successfully describe the structure and function of nitrogenase, it is important to understand the behavior of the metal-sulfur clusters that are a vital part of this complex enzyme. Metal-sulfur clusters are many, varied, and usually involved in redox processes carried out by the protein in which they constitute prosthetic centers. They may be characterized by the number of iron ions in the prosthetic center that is, rubredoxin (Rd) contains one Fe ion, ferredoxins (Fd) contain two or four Fe ions, and aconitase contains three Fe ions.7 In reference 18, Lippard and Berg present a more detailed description of iron-sulfur clusters only the [Fe4S4] cluster typical of that found in nitrogenase s Fe-protein is discussed in some detail here. The P-cluster and M center of MoFe-protein, which are more complex metal-sulfur complexes, are discussed in Sections 6.5.2. and 6.5.3. 

The rather complicated indium-phosphorus cluster framework 24 was obtained by the reaction of indium(III) chloride with PhP(SiMe3)2 in the presence of triethyl-phosphane [Eq. (12)] [29]. Apparently, a complex redox process occurred, in the course of which phosphorus atoms were oxidized by the formation of three P-P bonds, while six indium atoms were reduced from +3 to +2 accompanied by the formation of three In-In bonds. The In-In (average 274 pm) and P-P bond lengths (average 222 pm) represent localized single bonds. Other strategies for the... 

Metal-sulfur clusters probably constitute the best known class of clusters due to their considerable importance and common occurrence in the biological world, where they fulfil the role of electron carriers (see Chapter 12). This function is related to their high capacity to undergo a cascade of reversible redox processes. As an illustrative and introductive example, Figure 1 shows the redox ability of [Fe4(/r3-S)40/-C5H5)4].la... 

As expected on the basis of the presence of an Fe3S4 and an Fe4S4 cluster the response displays multiple redox processes. As a matter of fact there are three consecutive reduction processes, namely A, B and C, with characteristics of chemical reversibility. The processes A and B involve one-electron additions, whereas process C is a two-electron step. Processes A and C are assigned to the Fe3S4-centred reductions [3Fe-4S]+/0 and [3Fe-4S]0/2-, whereas the central system B is assigned to the Fe4S4-centred reduction [4Fe-4S]2+/+. The redox potentials (vs. NHE) for these processes are ... 

Recently, we demonstrated that the Zintl clusters [Geg]" react with chalcogen atoms (S, Se, and Te) in the presence of surfactant templates to form ordered mesoporous Ge-rich chalcogenides [74]. The mesostructured frameworks grow through a coupling reaction of (Ge9)-clusters with chalcogens in formamide/ethy-lenediamine mixture solution in an unusual reaction that seems to be a redox process (5). 

Actually, the kinetic study of the cluster redox potential by pulse radiolysis [31] (Section 20.3.2) somewhat mimics the process of the black-and-white photographic development, except that clusters are free in the solution (not fixed on AgBr crystals), and that they are produced by ionizing radiation (as in radiography and not by visible photons but the last choice had been incompatible with the time-resolved optical detection in the visible. Beyond the critical nuclearity, they receive electrons without delay from the developer already present (actually, the photographic development is achieved in a delayed step). 

Apart from the development in photography, most of nucleation and growth mechanisms based on a chemical reduction (Section 20.4.4) behave as development processes, and are likewise controlled by the nuclearity dependence of the cluster redox potential and by the potential of the electron donor. 

Manganese is an element that is essential for life. It is present at the active site of many en2ymes [4, 5]. Those en2ymes in which the metal center is involved in a redox process are manganese catalase [101], peroxidase [102], and SOD [103]. In addition, a cluster containing four Mn and one Ca atoms in the water-oxidizing center (WOC) of PSII is the site at which dioxygen is produced photosynthetically on Earth [3,104]. 

Fig.6 Reduction of the H-cluster analog and interconversion of redox processes (from Ref 24b).

The great recent development in electrochemical techniques will certainly be helpful for the study of redox processes of a metal which can occur in so many oxidation states. Multinuclear NMR spectrometers will allow increased use of 51V resonance as a routine method for the characterization of complexes in solution. Other recent developments are the study of polynuclear complexes, metal clusters (homo and hetero-nuclear) and mixed valence complexes, and it can be anticipated that these topics will soon become important areas of vanadium coordination chemistry, although the isolation of compounds with such complex... 

The [Fe4S4(SR)4]n and [Mo2Fe6S8(SR)9]n clusters undergo extensive redox processes predominantly localized in the M4S4 clusters. These types of reaction are also of bioinorganic interest.41,119,120... 

Some of the more interesting and valuable redox processes are multielectron in nature, suggesting the utility of coupling a two- or many-electron event into an excited state process. The study of the excited state photochemistry and photophysics of binuclear and polynuclear (cluster) molecules is thus becoming of importance, and two-electron reactions are being identified. 

The reduction state of the pterin was a point of uncertainty throughout these studies of molybopterin derivatives. The absence of fluorescence in anaerobic molybdopterin samples suggested a reduced pterin. Redox titration of XO and SO both indicated that the pterin could undergo a two-electron oxidation reaction (73, 74). Sulfite oxidase, for example, produced the fluorescence characteristic of an oxidized pterin after addition of 2 equiv of ferricyanide. However, titrating XO was problematic due to interfering redox processes of the iron-sulfur clusters. 

Another possible two-electron mechanism involves the direct transport of two electrons from a mononuclear transition metal complex to a substrate (S). Such a transport alters sharply the electrostatic states of the systems and obviously requires a substantial rearrangement of the nuclear configuration of ligands and polar solvent molecules. For instance, the estimation of the synchronization factor (asyn) for an octahedral complex, with Eq. 2.44 shows a very low value of asyn = 10 7to 10 8 and, therefore, a very low rate of reaction. The probability of two-electron processes, however, increases sharply if they take place in the coordination sphere of a transition metal, where the reverse compensating electronic shift from the substrate to metal occurs. Involvement of bi- and, especially, polynuclear transition metal complexes and clusters and synchronous proton transfer in the redox processes may essentially decrease the environment reorganization, and, therefore, provide a high rate for the two- electron reactions. 

The reactions catalyzed by laccases proceed by the monoelectronic oxidation of a suitable substrate molecule (phenols and aromatic or aliphatic amines) to the corresponding reactive radical (Riva, 2006). The redox process takes place with the assistance of a cluster of four copper atoms that form the catalytic core of the enzyme they also confer the typical blue color to these enzymes because of the intense electronic absorption of the Cu-Cu linkages (Piontek et al., 2002). The overall outcome of the catalytic cycle is the reduction of one molecule of oxygen to two molecules of water and the concomitant oxidation of four substrate molecules to produce four... 

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