Redox and electron transfer

The chlorophyll molecule (309) is involved in initiating photosynthesis in green plants and contains magnesium coordinated to a partially reduced porphyrin (namely, a chlorin derivative). Life relies ultimately on the unique redox and electron transfer abilities of the chlorophylls which are necessary for the conversion of light to chemical energy. Chlorophyll mainly absorbs light from the far red region of the spectrum... 

Redox-active cofactors are important species in biological systems, playing vital roles in redox and electron-transfer processes. Among the structurally and functionally diverse redox enzymes, flavoproteins containing the flavin cofactors flavin adenine dinucleotide (FAD) or flavin mononucleotide (FMN) are involved in many different biochemical processes serving as a highly versatile redox... 

Of special significance are the catalytic properties of small metal clusters. At their surface such clusters have a large number of atoms with a low coordination number to which substrates bind. Catalytic reactions are being studied in hydrogenation, hydrosilylation, hydration, and the Heck reaction. Metal clusters are also of importance with regard to redox and electron transfer processes such as the photochemical decomposition of water (fuel cells) and photocatalytic hydrogenation. 

The physical biochemistry of these two classes of 4Fe-4S proteins has been extensively studied and reviewed, especially with regard to redox and electron transfer chemistry (2-22), whereas the magnetic properties of the cluster have been probed by electron paramagnetic resonance (EPR) (23-34), nuclear magnetic resonance (NMR) (24, 25, 35-54), and susceptibility measurements (56-61). Mossbauer spec-... 

Peroxidases are heme-iron proteins involved in oxidative stress control. The mechanisms of this class of enzymes involve rather complex redox and electron transfer processes. These include alterations of spin state and ligands to the active-site heme as well as a novel role for Ca ions in the process as discussed by Moura and colleagues (Chapter 6). The topology and mechanism of the electron transfer process can be studied in detail using dynamic NMR data and molecular modeUng tools as illustrated by Pettigrew and co-workers (Chapter 7). The action of these types of peroxidases can be considered of crucial importance for the redox state regulation of the cell. 

For redox and electron-transfer properties of [Os(bipy)j] + and [Os(phen)3] + see p. 539. Osmium(IIJ). Oxidation of [Os(bipy)3] + with chlorine yields the violet [Os(bipy)3] ion," "" while the red [Os(phen)3] is similarly prepared by oxidation of [Os(phen)3] +.Resolution of [Os(phen3)f + has been achieved by oxidation of Ae corresponding [Os(phen)3] species. Spectroscopic properties. The electronic absorption spectra of [Os(bipy)3] " and of [Os(phen)3] " " have been measured, as have their circular dichroism spectra " "" and also Ae spectra of [Os(4,4 -Me2bipy)3] " The lifetimes of the charge transfer (MLCT) excited states of... 

His research interests include catalysis by cobalt, iron and manganese, complexes (biomimetic dioxygen activation, catalytic oxidation and carbonylation), as well as kinetics and mechanisms of inorganic reactions in solution (fast redox and electron transfer). 

Dehydrogenation, Ammoxidation, and Other Heterogeneous Catalysts. Cerium has minor uses in other commercial catalysts (41) where the element s role is probably related to Ce(III)/Ce(IV) chemistry. Styrene is made from ethylbenzene by an alkah-promoted iron oxide-based catalyst. The addition of a few percent of cerium oxide improves this catalyst s activity for styrene formation presumably because of a beneficial interaction between the Fe(II)/Fe(III) and Ce(III)/Ce(IV) redox couples. The ammoxidation of propjiene to produce acrylonitrile is carried out over catalyticaHy active complex molybdates. Cerium, a component of several patented compositions (42), functions as an oxygen and electron transfer through its redox couple. 

In biochemical systems, acid-base and redox reactions are essential. Electron transfer plays an obvious, crucial role in photosynthesis, and redox reactions are central to the response to oxidative stress, and to the innate immune system and inflammatory response. Acid-base and proton transfer reactions are a part of most enzyme mechanisms, and are also closely linked to protein folding and stability. Proton and electron transfer are often coupled, as in almost all the steps of the mitochondrial respiratory chain. 

Fe-protein, the unique, highly specific electron donor to MoFe-protein, mediates coupling between ATP hydrolysis and electron transfer to MoFe-protein and also participates in the biosynthesis and insertion of FeMoco into MoFe-protein. Fe-protein contains one ferredoxin-like [Fe4S 4 2 /1+ cluster as its redox center. There is now evidence for an [Fe4S4]° super-reduced state in which four high-spin iron(II) (S= 2) sites are postulated. These were previously discussed in Section 6.3 and illustrated in Table 6.1.16 The [Fe4S4] cluster in this state bridges a dimer of... 

In a new twist on this subject, electrochemical activation parameters have been obtained for two series of redox couples that undergo coupled spin-state change and electron transfer (28). One series is [M(tacn)2]3+/2+ where M = Fe, Co, Ni, and Ru, and tacn = 1,4,7-triazacyclononane. The other is [Fe(pzb)2]+/0, where pzb-= hydrotris(pyrazol-l-yl)borate... 

Although the initial steps of Schemes IA, IIA, and IIIA are strongly supported by the experimental data, the subsequent reactions and electron-transfer steps are based solely on the electrochemical measurements of Figures 1-3, 6 and 7. Intermediates have not been detected or isolated, but there is self consistency in the redox thermodynamics between the M/ OH systems and the M+/02 systems. The cyclic voltammograms also indicate the presence of common intermediates between the two systems. 

In host-guest systems based on electron donor/ acceptor interactions, association/dissociation can be driven by redox processes so that it is possible to design electrochemical switches than can be used to control energy- and electron-transfer processes. 

Most PET fluorescent sensors for cations are based on the principle displayed in Figure 10.7, but other photoinduced electron transfer mechanisms can take place with transition metal ions (Fabbrizzi et al., 1996 Bergonzi et al., 1998). In fact, 3d metals exhibit redox activity and electron transfer can occur from the fluorophore... 

Before proceeding it is necessary to clarify the terminology of two fairly similar processes, at least as far as their nomenclature is concerned redox catalysis and electron-transfer chain catalysis. 

The electronic properties of CNTs, and especially their band structure, in terms of DOS, is very important for the interfacial electron transfer between a redox system in solution and the carbon electrode. There should be a correlation between the density of electronic states and electron-transfer reactivity. As expected, the electron-transfer kinetics is faster when there is a high density of electronic states with energy values in the range of donor and acceptor levels in the redox system [2]. Conventional metals (Pt, Au, etc.) have a large DOS in the electrochemical potential... 

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