Linked redox centres

SINGLE ELECTRON-TRANSFER REACTIONS, ELECTRON-TRANSFER SERIES AND LINKED REDOX CENTRES... 

Linkage isomerization reactions, 465 Linked redox centres, 493 Liquid-liquid extraction, 538-549 Lowry, Thomas Martin, 16 Luteocobalt—see Cobalt, hexaammine-Luteocobaltic chloride —see Cobalt, hexaammine-, chloride... 

These results show that significant anodic shifts in the ferrocene oxidation wave result if cations are added to the conjugated receptor systems where the 7r-electron system links the heteroatoms of the ionophore to the redox centre. 

A range of redox centres have been found in the membranes from cells grown on glycerol and fumarate including menaquinone, ubiquinone, the iron-sulfur centres of fumarate reductase, cytochromes d, b and a, other iron-sulfur centres and molybdenum. Some of these centres may be assignable to specific enzymes, such as molybdenum to formate dehydrogenase. Other dehydrogenases (for NADH and lactate) are also present, and are linked to the cytochromes by menaquinone. However, there have been relatively few studies on the function of these components. 

The chemistry of cluster complexes, e.g. of the sort [FeitSi, (SR) i,] 2, is of particular interest since such complexes are known to be close representations or synthetic analogues of the redox centres present in various iron-sulphur proteins. It is important to know whether the valence electrons are localized or delocalized in such complexes - in fact several studies by e.s.r., n.m.r., and, more recently, resonance Raman spectroscopy have shown that such clusters are delocalized rather than trapped-valence species. This result is linked with the most important biophysical property of iron-sulphur proteins, viz. that of electron transfer. Rapid electron transfer is possible if any consequential geometric rearrangements around the metal atom sites are small, as implied by many resonance Raman results on such cluster complexes (cf. the small-displacement approximation, which provides a basis for enhancement to fundamental but not to overtone bands) (22). Initial studies of [MSi,]2- ions (M = Mo or W) (23,24) have since been supplemented by studies of dinuclear species e.g. [(PhS)2FeS2MS2]2 (25) and cluster species... 

Fig. 3.7. Cubic model of a redox-linlced proton pump. OX and RED denote a redox centre in the oxidised and reduced state. The bar marked M or C next to OX and RED indicates an acidic group, the function of which is linked to the redox centre. M and C mean that the group is connected protonically either with the aqueous matrix or cytoplasmic phases, respectively. When the group is protonated the bar is supplemented with H. Left and right faces of the cube separate states in electronic and protonic contact with the input and output sides of the transducer, respectively. Allowed transitions between these are indicated by thick arrows. Dotted lines denote forbidden transitions. If the latter gain significant probability relative to allowed transitions proton transport becomes decoupled from electron transfer (so-called slipping ). (From Ref. 8.)...

When a redox-active transition metal is used as the signalling imit of a receptor, anion binding is coupled to electron transfer, i.e. anion binding changes the redox potential (couple) of the transition metal. This electrochemical shift can be represented as A, the difference in redox potentials between the receptor anion complex and the receptor alone. Concomitantly, electron transfer at the redox centre also changes the affinity of the receptor for the guest species. These coupled processes are linked thermodynamically by Eq. 1, where Kred and Kox are the stability constants of the reduced and oxidised forms of the receptor anion complex respectively [7]. 

The above examples demonstrate that metal centres can conveniently be used to link redox active fragments in order to generate multi-electron redox active systems. If a stable system is desired, a metal ion that forms inert complexes should be chosen. This is the case of the cis-[Pt (Fcpy)2Cl2] system. On the contrary, the [Fe (Fcsal2-)3]3- species possesses a labile metal centre and the redox active ligand can be removed, for instance on addition of a strong add. It should be noted that the two considered metal centres (Pt, Fel ) do not present individual redox activity in the coordinative environments examined and play a purely 2irchitectviral role. 

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