Long-range redox

B. Long-Range Redox Chemistry Mediated by the DNA Base Stack... 

We have begun to synthesize more conq>lex molecules such as tetrads and pentads where free energy gradients will be exploited to achieve long-range redox separation by sequential electron transfer. An example is Bzq-Lys(Anq)-Lys(Ru b2m) -Lys(PTZ )-Lys )-OMe where the teminal sites are a ben uinone derivative (Bzq) and a fotocene Privative (Fee). Following visible excitation and intrastrand electron transfer, 1.1 eV will be stored in a redox-separated state in which there are three spacers between die final oxidative and reductive equivalents. 

Patolsky F, Weizmarm Y, WiUner, I. 2004. Long-range electrical contacting of redox enzymes by SWCNT connectors. Angew Chem Int Ed 43 2113-2117. 

Chi Q, Farver O, Ulstrup J (2005) Long-range protein electron transfer observed at the singlemolecule level in situ mapping of redox-gated tunneling resonance. Proc Natl Acad Sci USA 102 16203-16208... 

Transient behavior of the catalyst at high oxidation state. The significance of the redox type reactions between reaction medium and catalyst is depicted in Figure 7, where the conversion of CO to CO2 has been stimulated under shift conditions. After a long range pretreatment of the catalyst in a mixture of t O/ (0,30 bar 1 0) a periodic operation has been established consisting of a testing phase of 1 min with a mixture of CO, H 0, N ... 

Electron mediators successfully used with oxidases include 2,6-dichlorophenolindophol, hexacyanoferrate-(III), tetrathiafulvalene, tetracyano-p-quinodimethane, various quinones and ferrocene derivatices. From Marcus theory it is evident that for long-range electron transfer the reorganization energies of the redox compound have to be low. Additionally, the redox potential of the mediator should be about 0 to 100 mV vs. standard calomel electrode (SCE) for a flavoprotein (formal potential of glucose oxidase is about -450 mV vs SCE) in order to attain rapid vectrial electron transfer from the active site of the enzyme to the oxidized form of the redox species. 

Concept The rates of long-range electron transfer (ET) and excitation energy transfer (EET) processes between a pair of chromo-phores (redox couple) may be strongly facilitated by the presence of an intervening non-conjugated medium, such as saturated hydrocarbon bridges, solvent molecules and n-stacks, e.g.,... 

Electron transfer processes induce variations in the occupancy and/or the nature of orbitals which are essentially localized at the redox centers. However, these centers are embedded in a complex dielectric medium whose geometry and polarization depend on the redox state of the system. In addition, a finite delocalization of the centers orbitals through the medium is essential to-promote long-range electron transfers. The electron transfer process must therefore be viewed as a transition between two states of the whole system. The expression of the probability per unit time of this transition may be calculated by the general formahsm of Quantum Mechanics. 

The possibility that there might be long-range electron transfer between redox-active centers in enzymes was first suspected by biochemists working on the mechanism of action of metalloenzymes such as xanthine oxidase which contain more than one metal-based redox center. In these enzymes electron transfer frequently proceeds rapidly but early spectroscopic measurements, notably those by electron paramagnetic resonance, failed to provide any indication that these centers were close to one another. 

When electrons can transfer either by contact approach of the redox pair or long-range transfer, the appropriate equation for a scavenger (electron acceptor) of charge z is... 

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