Hydrophobic metal complexes, electron transfer

In nature these complexes are connected with proteins and often they are situated inside membranes. By using the silver-porphyrin complexes as a simple model for the more complex systems occurring in nature we can compare their electron transfer rate constants in aqueous solutions with those in solutions containing micelles which incorporate our model complexes. In this way we are trying to get more information about the effect of hydrophobic barriers on electron transfer. Well understood oxidation reduction reactions of transition metal complexes which are reacting with an outer sphere mechanism are measured under equal conditions to separate pure electrostatic effects from other influences. 

The influence of DNA on the photo-electron transfer process between a variety of donor-acceptor couples has been examined during the last ten years. For all the systems studied, the metal complex interacts with the DNA and plays the role of electron acceptor or donor in the hydrophobic DNA microenvironment, whereas the other partner of the process, i.e. the reducing or oxidising agent in the ground state, is localised either on the DNA double helix, or does not interact with the nucleic acid and remains in the aqueous phase. Thus three... 

As a first step toward this purpose, we have studied the chelation effect of tetrapeptides of sequences Cys-X-Y-Cys, by preparation of metal complexes of mainly the first transition series. The hydrophobic effect of the peptides was also studied by utilizing the side chain bulkiness of the amino acid residues interposed between the two cysteine residues. A special effect of aromatic side chains of tyrosine, phenylalanine, and tryptophan has also been examined in order to assess their ability to ease electron transfer to and from the nearby iron core. 

We have prepared and studied a number of surfactant, hydrophobic and water soluble luminescent metal complexes. These can serve as excited substrates in light-induced electron transfer reactions. Both the quenching processes and subsequent reactions can be strongly affected by incorporation of the substrate and/or quencher in an organized assembly. This paper focuses mainly on studies in micelles. 

Interfacial electron-transfer reactions between polymer-bonded metal complexes and the substrates in solution phase were studied to show colloid aspects of polymer catalysis. A polymer-bonded metal complex often shows a specifically catalytic behavior, because the electron-transfer reactivity is strongly affected by the pol)rmer matrix that surrounds the complex. The electron-transfer reaction of the amphiphilic block copol)rmer-bonded Cu(II) complex with Fe(II)(phenanthroline)3 proceeded due to a favorable entropic contribution, which indicated hydrophobic environmental effect of the copolymer. An electrochemical study of the electron-transfer reaction between a poly(xylylviologen) coated electrode and Fe(III) ion gave the diffusion constants of mass-transfer and electron-exchange and the rate constant of electron-transfer in the macromolecular domain. 

The X-ray crystal structure of plastocyanin has recently been established (10), which indicated that the core of the molecule is hydrophobic and notably aromatic because six of the seven phenylalanine residues are clustered there. Polar side chains are distributed on the exterior of plastocyanin molecule. Many hypotheses have been proposed to explain the electron-transfer pathway to and from the metal center of plastocyanin, such as a tunnelling mechanism along hydrophobic channels (11). High reactivity and entropic favorability have been reported for the electron-transfer reaction of plastocyanin with Fe(II) complex (12). The Cu complex bound to the amphiphilic block copolymer is interesting as a metal compound of plastocyanin, because both polymer and apoprotein environments are considered to produce a hydrophobic environment and a large effect on the electron-transfer reaction through its entropic contribution. 

The kinetics of the oxidations of Chromatium vinosum HIPIP by several ferrocenium derivatives show no inhibitions by charged redox-inactive metal complexes, and display a pH dependence (pK = 6.90) in which protonation reduces the HIPIP reactivity by a factor of two. Electron transfer at an uncharged hydrophobic patch near Cys-46 (4 A from the Fe4S4 core to the surface), enhanced by deprotonation of His-42, is inferred from the data. The self-exchange rate constant for the HIPIPo/HIPIPr couple is estimated to be 5 x 10 s from... 

These processes are quite rapid with an apparent rate constant which exceeds lO" cm s" [5,6] The only example of electron transfer reaction which has been observed was between the hydrophobic ferro-cinium - ferrocene redox couple in nitrobenzene and the hydrophilic hexacyanoferrate redox couple in water [9]. A more complex mechanism is involved in the case of ion transfer facilitated by an iono-phore[10]. This is the case, for example in the transfer of the alkali and alkaline earth metal cations across a water/nitrobenzene interface facilitated by synthetic neutral cyclic or acyclic iono-phores derived from 3,6-dioxaoctanedicarboxylic acid [11]. 

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