Redox shuttles

Onnerud, H. Zhang, L. Gellerstedt, G. Henriksson, G. Polymerization of monolignols by redox shuttle-mediated enzymatic oxidation a new model in lignin biosynthesis I. Plant Cell 2002, 14, 1953-1962. 

The half-order of the rate with respect to [02] and the two-term rate law were taken as evidence for a chain mechanism which involves one-electron transfer steps and proceeds via two different reaction paths. The formation of the dimer f(RS)2Cu(p-O2)Cu(RS)2] complex in the initiation phase is the core of the model, as asymmetric dissociation of this species produces two chain carriers. Earlier literature results were contested by rejecting the feasibility of a free-radical mechanism which would imply a redox shuttle between Cu(II) and Cu(I). It was assumed that the substrate remains bonded to the metal center throughout the whole process and the free thiyl radical, RS, does not form during the reaction. It was argued that if free RS radicals formed they would certainly be involved in an almost diffusion-controlled reaction with dioxygen, and the intermediate peroxo species would open alternative reaction paths to generate products other than cystine. This would clearly contradict the noted high selectivity of the autoxidation reaction. 

During the past two decades, the redox shuttle mechanism has been influencing researchers as the most promising solution to the challenge of cathode overcharge, and among the limited number of publications, most of the additives were selected based on their redox potentials. 

The effect of these ferrocene-based additives on overcharge protection is shown in Figure 44, where AA cells based on lithium, LhMn02, and electrolytes with or without additives were overcharged. In the absence of these redox shuttles (A), the cell voltage continues to rise, indicating the occurrence of major irreversible decompositions within the cell whereas the presence of shuttle agents (B—E) locks the cell potential in the vicinity of their redox potentials... 

Redox shuttles based on aromatic species were also tested. Halpert et al. reported the use of tetracyano-ethylene and tetramethylphenylenediamine as shuttle additives to prevent overcharge in TiS2-based lithium cells and stated that the concept of these built-in overcharge prevention mechanisms was feasible. Richardson and Ross investigated a series of substituted aromatic or heterocyclic compounds as redox shuttle additives (Table 11) for polymer electrolytes that operated on a Li2Mn40g cathode at elevated temperatures (85 The redox potentials of these... 

Overcharge tests were carried out in LiCo02 cathode half-cells that contained these additives, and a redox shuttle effect was observed between 4.20 and 4.30 V, close to the redox potentials of these additives. The same shuttling effect was observed even after 2 months of storage for these cells, indicating the stability and redox reversibility of these additives. A closer examination of the capacity retention revealed that 4-bromo-l,2-dimethoxybenzene seemed to have the best shuttle-voltage performance for the 4.0 V lithium cell used." The stability of these additives against reductive decomposition was also tested by the authors on metallic lithium as well as on carbonaceous anodes, and no deterioration was detected. 

Following Adachi et al., aromatic compounds with similar functionalities were proposed for polymer electrolytes as redox shuttle additives, which included bipyridyl and biphenyl carbonates and di-fluoroanisoles. All these additives could protect the cathode from overcharging in the vicinity of 4.1... 

In order to carry out all of these different functions, peroxisomes are equipped with a unique set of enzyme proteins, catalysing the different reactions involved. In addition, the peroxisomal membrane contains specific transporters in order to take up substrates from the cytosol and release the end products of peroxisomal metabolism. Since peroxisomes lack a citric acid cycle as well as a respiratory chain, the end products of peroxisomal metabolism, such as acetyl-CoA, propionyl-CoA and a range of other acyl-Co A esters predominantly derived from fatty acid beta-oxidation, are exported from the peroxisomal interior and shuttled to mitochondria for full oxidation to C02 and H20. The same applies to the NADH produced during beta-oxidation, which is reoxidised via redox-shuttles so that the NADH generated in peroxisomes is ultimately reoxidised in the mitochondrial respiratory chain at the expense of molecular oxygen. 

Figure 2-14. Oxidation of monolignols via a manganese oxalate redox shuttle as proposed by Onnerud et al. (2004).

Figure 7 Diagram of a dual photocatalysts system (z scheme) employing a redox shuttle (Abe et al., 2005).

Five-membered heterocyclic ring systems can also act as redox shuttles. An example using an uncharged form of the reduced heterocycle is shown in equation (42). - The yv-alkylated derivatives of (95), which provide positively charged ions on hydride loss, seem to be somewhat more reactive. ... 

Wolfbauer, G. (1999). The electrochemistry of dye sensitized solar cells, their sensitizers and their redox shuttles. Ph. D. Thesis, Monash University, Melbourne. 

In wines, traces of iron, which are picked up, perhaps, from processing and/or storage, or copper, which are picked up from mildew sprays, such as Bordeaux mixture, affect the oxidative stability of wines by acting as the redox shuttles as they transfer between oxidation states. Winemakers discovered that adding ferricyanide to wine, in a process known as blue fining, precipitates copper and iron and thereby reduces their concentrations below 1 ppm, which is considered to be acceptable. Critical control of ferricyanide addition is necessary, as cyanide is also a contaminant that must be measured. Where vineyards have replaced cherry and apple orchards, low concentrations of arsenic have started to appear but they are present at very low concentrations in high quality wines. The arsenic appears from arsenical compounds such as lead and calcium arsenates that were used for many decades as pesticides on apples and cherry orchards. 

Biology has solved this problem by introducing multi-cofactor enzymes in which the overall distance between two redox sites is divided into a number of shorter distances (multiple cofactors with different redox potentials instead of just one cofactor) or by introducing small redox shuttle proteins such as cytrochromes in the respiratory chain. Thus, ET cascades have been proven to be very efficient and useful. This principle was borrowed from nature not only for direct ET-based biosensors but also for mediated ET-based biosensors. Note that the overall efficiency of ET cascades depends on the entire architecture of a biosensor. A striking... 

Figure 1. Potential energy diagrams for photochemical water splitting at pH = 0 (A) single semiconductor system (B) with an electron acceptor (C) with an electron donor (D) dual semiconductor system (z scheme) employing a redox shuttle. Modified from ref 108.

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