Exchange mediated

Table 8.24 lists the main redox exchanges mediated by biological activity and their equilibrium constants at pH = 7. 

A similar bromine-lithium exchange mediated isomerization of 2-lithiopyridine 219 (Scheme 106) was used in a synthesis of atpenin B (see below). ... 

The chloride-bicarbonate exchanger mediates antiport of the anions CF and... 

EXCHANGE-MEDIATED ENERGY TRANSFER IN VISCOUS MEDIA... 

From the data given in equations (24)-(28), we may conclude that super-exchange-mediated ET through the DNA duplex is significant and that (3 for DNA may be comparable to the upper limit of j3 observed for conjugated hydrocarbon bridges. However, it now seems clear that DNA does not behave as a molecular wire in ET reactions, as has been posited.178... 

Intermolecular metal transfer between MT forms or between MT and other metalloproteins has also been observed. Interestingly, in vitro studies have shown that the intermolecular zinc exchange mediated by Zny-MT is orders of magnitude faster than the direct exchange between free zinc and a number of other zinc proteins. Moreover, Zny-MT has been shown to be able to transfer zinc to apoproteins possessing a much lower affinity for this metal ion, for example, alkaline phosphatase or carboxypeptidase. In this case, however, only one Zfr ion out of the seven in Zny-MT was transferred, implying the presence of one weakly bound metal ion. ... 

Figure 27. Representation of the competition between direct electron exchange (at around the conduction band edge at the surface) and exchange mediated by surface electron levels. Surface states may exchange electrons with the redox system in the energy interval at around the electrochemical potential. Surface states exchange electrons with the conduction band by electron capture and thermal electron excitation (vertical arrows).

In the following paragraphs we will briefly discuss the mechanisms of electronic excitation transfer most frequently encountered in molecular systems, and outline the formalism most commonly used to interpret energy-transfer data. More attention will be devoted to the exchange-mediated excitation transfer processes because of their particularly close relationship to charge transfer. 

Figure 2. Schematic representation oF electron, hole, and triplet energy transfer between the localized HOMO and LUMO orbitals of the donor and the acceptor. Note that the above diagram also applies to the exchange-mediated singlet excitation transfer if the spin of the electron in the LUMO orbital is reversed.

Aronson PS. Ion exchangers mediating NaCI transport in the renal proximal tubule. Cell Biochem.Biophys. 2002 36 147-53. 

The static contribution to the electron exchange mediated triplet energy transfer was determined by measuring the triplet carotenoid rise time for each compound in a polystyrene glass at ambient temperatures, where large-scale intramolecular motions are precluded. As expected for the similar covalent... 

Electron transfer effects in porphyrins are of biological relevance for a number of reasons, not least in the understanding of mechanisms involved in photodynamic therapy. Studies reported include fluorescence quenching of porphyrins by oxidants such as p-benzoquinone , photoinduced electron transfer of porphyrin-acceptor molecules in solid state °, and ps experiments on quinone substituted monometallic porphyrin dimer which show evidence for super-exchange mediated electron transfer in photosynthetic systems . 

FIGURE 3.5 Halogen-metal exchange-mediated benzyne Diels-Alder reaction with cyclopentadiene. 

Exchange-mediated A variant of the cross-channel mechanism. Instead of directly displacing an atom, the diffusing atom pushes down into the surface, causing one of its neighbors to pop up. This leads to an apparent shift in the diffusing atom s position. ... 

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