Electron transfer bridging groups

Wendoloski et carried out molecular dynamics studies of the complex between cyt 65 and Cc, and reported that complex formation altered the position of the side chain on Cc residue Phe82 to a new position between the two heme groups. This led to the proposal that the aromatic ring on Phe82 may act as an electron-transfer bridge between the heme groups of cyt bi and Cc. However, Wilhe et al. found that substitution of Phe 82 with Tyr, Gly, Leu, or He did not change the rate... 

In each case, it is the carbon atom that has the negative formal charge that makes it the "electron-rich" end of the structure. Moreover, there are antibonding orbitals in both CO and CN that can accept electron density transferred from the nonbonding d orbitals on the metal (as described in Section 16.10). Because of the nature of these ligands, it is not uncommon for them to function also as bridging groups in which they are bonded to two metal atoms or ions simultaneously. 

In these cases, it is found that X is transferred quantitatively from the Co3+ complex to the Cr2 + complex as electron transfer is achieved. Therefore, it is likely that electron transfer occurs through a bridging ligand that is simultaneously part of the coordination sphere of each metal ion and that the bridging group remains as part of the coordination sphere of the inert complex produced. The electron is thus "conducted" through that ligand. Rates of electron transfer are found to depend on the nature of X, and the rate varies in the order... 

Synthetic chemistry enables us to mimic the energy- and electron-transfer processes by linking together donor and acceptor groups by means of covalent bonds or bridging groups, rather than using the protein matrix found in natural systems. 

Figure 4. Schematic of electron transfer processes for 2,6-disubstituted phenol. The ligand groups are indicated as Am and the intermediate polymer chain segments as straight lines, (a) Hydroxo-bridged catalyst (b) chloro-bridged catalyst.

In this equation g(r) is the equilibrium radial distribution function for a pair of reactants (14), g(r)4irr2dr is the probability that the centers of the pair of reactants are separated by a distance between r and r + dr, and (r) is the (first-order) rate constant for electron transfer at the separation distance r. Intramolecular electron transfer reactions involving "floppy" bridging groups can, of course, also occur over a range of separation distances in this case a different normalizing factor is used. 

Similar experimental studies were carried out on molecules in which the bridge is constituted by conjugated groups [83, 84]. General references on photoinduced intramolecular electron transfers may also be found in two recent reviews [85, 86]. 

This is called a chemical, radical or stepwise mechanism. Or was it (ii) by the action of the bridging group to increase the probability of electron transfer by tunneling, termed resonance transfer 56,9i... 

Chromium(II) is a very effective and important reducing agent that has played a significant and historical role in the development of redox mechanisms (Chap. 5). It has a facile ability to take part in inner-sphere redox reactions (Prob. 9). The coordinated water of Cr(II) is easily replaced by the potential bridging group of the oxidant, and after intramolecular electron transfer, the Cr(III) carries the bridging group away with it and as it is an inert product, it can be easily identified. There have been many studies of the interaction of Cr(II) with Co(III) complexes (Tables 2.6 and 5.7) and with Cr(III) complexes (Table 5.8). Only a few reductions by Cr(II) are outer-sphere (Table 5.7). By contrast, Cr(edta) Ref. 69 and Cr(bpy)3 are very effective outer-sphere reductants (Table 5.7). 

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