Clusters redox potentials

R Langen, GM Jensen, U Jacob, PJ Stephens, A Warshel. Protein control of iron-sulfur cluster redox potentials. J Biol Chem 267 25625-25627, 1992. 

Stability means that clusters do not undergo coalescence nor corrosion by the medium, at least in the absence of oxygen. The quite negative value of ii°(MVM ) and the dependence of the cluster redox potential on the nuclearity have crucial consequences in the formation of early nuclei, their possible corrosion or their growth. As an example, the faster the coalescence, the lower is the probability of corrosion of the small clusters by the medium. The property of stability offers the means to apply to these clusters a larger amount of suitable characterization techniques than to transient oligomers. 

Figure 14 Mechanism of catalytic electron transfer involving metal clusters as relay. The thermodynamic conditions to be fulfilled are that the cluster redox potential be higher than the donor D and lower than the acceptor A potential, which implies that the cluster itself is a size range that offers the efficient redox potential. (From Ref. 63.)...

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. 

Langen, R., G. Jensen, U. Jacob, P. Stephens and A. Warshel. (1992). Protein Control of Iron-sulfur Cluster Redox Potentials. Journal of Biological Chemistry. 267 25625-25627. 

Figure 1. Principle of the determination of short-lived cluster redox potential by kinetics methods. The reference electron donor, S of a given potential and the metal atoms are generated by a single puke. During cluster coalescence, the redox potential of the couple E°(M -Mn) progressively increases, so that an effective transfer is observed after a critical time when the cluster potential becomes higher than that of the reference, constituting a threshold. Repeatedly, a new adsorption of excess cations, M, onto the reduced cluster, (n xkch (dlows another electron transfer from S with incrementation of nuclearity. The subcritical clusters Mn(n

Figure 10 shows the nucleaiity dependence of silver cluster redox potential in water together with the data just presented, the previously published values are reported forn = 1 (3), 2 (23), 5 (5), 10 (24), and 11 (25). The E° values for nuclearities n = 1 and n = 2 resulted from thermodynamic calculations. The value for n = 10 was obtained from electron transfer studies where the clusters were the donor and were corroded by HaO. As a function of the... 

The redox potentials of short-lived silver clusters have been determined through kinetics methods using reference systems. Depending on their nuclearity, the clusters change behavior from electron donor to electron acceptor, the threshold being controlled by the reference system potential. Bielectronic systems are often used as electron donors in chemistry. When the process is controlled by critical conditions as for clusters, the successive steps of monoelectronic transfer (and not the overall potential), of which only one determines the threshold of autocatalytical electron transfer (or of development) must be separately considered. The present results provide the nuclearity dependence of the silver cluster redox potential in solution close to the transition between the mesoscopic phase and the bulk metal-like phase. A comparison with other literature data allows emphasis on the influence of strong interaction of the environment (surfactant, ligand, or support) on the cluster redox potential and kinetics. Rela-... 

Figure 5. Principle of the determination of short-lived cluster redox potential by kinetics method. The reference electron donor S of given potential and the metal atoms are generated by the same single pulse. Earing the cluster...

Actually, the kinetics study of the cluster redox potential ( 4.2) mimics the process of the photographic development, except clusters are free in the solution (not fixed on AgBr crystals) and, beyond the critical nuclearity, they... 

Figure 1. Radiolytic reduction of metal ions. After their formation, the atoms coalesce and also become associated with excess metal ions. The clusters can be stabilized by ligands, polymers or supports. The cluster redox potential increases with the nuclearity n. The smallest oligomers can undergo corrosion by...

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