Redox bridging molecules

Figure 1.1 Stepwise production of metal-particle multilayer arrays. The attachment ofthe Au or Ag nanoparticles onto ITO-modified glass was achieved by using silanes that have an amine terminus group. This modification step allows for further modification with nanoparticles onto the surface of the ITO. After the nanoparticle attachment a redox-active bridging molecule was...

The main idea demonstrated by Willner and coworkers [20] is the ability to construct multilayered nanoparticle electrodes, which are porous. In a related study Patolsky et al. extended this idea further using biocatalysts to detect H2O2 [18]. In this example, the construction of the electrode is similar to the one described above but the redox-active bridging molecule was replaced with microperoxidase-11 (MP-11). 

One may try to overcome these difficulties by embedding the bridging molecules into bilayer membranes. If their redox active terminal groups are able to undergo reactions with the electron donors and acceptors from aqueous phases, an efficient transmembrane PET will be possible. 

In a recent development novel nano-cluster based devices enabled to switch the conductivity of a non-junction by changing the oxidation state of a bridging molecule. Some redox-active molecules contain a molecular center where reduction or oxidation can be achieved more or less reversibly supporting quite large currents. A fundamental prerequisite is the overlapping of the electron energy bands of the molecule with those of... 

Figure 1.9 Examples of functionally important intrinsic metal atoms in proteins, (a) The di-iron center of the enzyme ribonucleotide reductase. Two iron atoms form a redox center that produces a free radical in a nearby tyrosine side chain. The iron atoms are bridged by a glutamic acid residue and a negatively charged oxygen atom called a p-oxo bridge. The coordination of the iron atoms is completed by histidine, aspartic acid, and glutamic acid side chains as well as water molecules, (b) The catalytically active zinc atom in the enzyme alcohol dehydrogenase. The zinc atom is coordinated to the protein by one histidine and two cysteine side chains. During catalysis zinc binds an alcohol molecule in a suitable position for hydride transfer to the coenzyme moiety, a nicotinamide, [(a) Adapted from P. Nordlund et al., Nature 345 593-598, 1990.)...

The thioredoxin domain (see Figure 2.7) has a central (3 sheet surrounded by a helices. The active part of the molecule is a Pa(3 unit comprising p strands 2 and 3 joined by a helix 2. The redox-active disulfide bridge is at the amino end of this a helix and is formed by a Cys-X-X-Cys motif where X is any residue in DsbA, in thioredoxin, and in other members of this family of redox-active proteins. The a-helical domain of DsbA is positioned so that this disulfide bridge is at the center of a relatively extensive hydrophobic protein surface. Since disulfide bonds in proteins are usually buried in a hydrophobic environment, this hydrophobic surface in DsbA could provide an interaction area for exposed hydrophobic patches on partially folded protein substrates. 

Metallothioneins (MT) are unique 7-kDa proteins containing 20 cysteine molecules bounded to seven zinc atoms, which form two clusters with bridging or terminal cysteine thiolates. A main function of MT is to serve as a source for the distribution of zinc in cells, and this function is connected with the MT redox activity, which is responsible for the regulation of binding and release of zinc. It has been shown that the release of zinc is stimulated by MT oxidation in the reaction with glutathione disulfide or other biological disulfides [334]. MT redox properties led to a suggestion that MT may possesses antioxidant activity. The mechanism of MT antioxidant activity is of a special interest in connection with the possible antioxidant effects of zinc. (Zinc can be substituted in MT by some other metals such as copper or cadmium, but Ca MT and Cu MT exhibit manly prooxidant activity.)... 

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.,... 

The crystal structure of the sodium salt of 30 (NAMI) is shown in Fig. 9, where Na(I) bridges two molecules of 30 via oxygens of S-bound DMSO and water. This complex may be readily reduced in vivo (E1/2, -0.001 V) (166), whereas the bis-imidazole complex 28 has a lower redox potential and is more difficult to reduce. The reduction potential of 28 is strongly pH dependent (AE = —118 mV/pH unit near pH 7), reduction being more favorable at acidic pH values (167). This complex hydrolyses at a similar rate to cisplatin (ty ca. 3 h at 310 K) and, like cis-platin, aquation appears to be necessary for DNA binding (168). 

Since an increased number of disulfide bridges in relatively short polypeptide chains leads to compact globular structures with the disulfides mainly buried in the nonpolar core, such excised protein fragments should represent, even in the precursor molecules, stable subdomains. Therefore, sufficient structural information can be retained for a correct refolding at least to some extent, if appropriate experimental conditions are applied in terms of peptide concentration, redox reagents, temperature and/or reaction buffers. A great deal of... 

The binuclear complex (9) may be produced from monomeric [V(nhet)] by two paths (i) by combination of two molecules of [V(nhet)(OH)]- or [V(nhet)(OH)]- and [V(nhet)(H20)],336 or (ii) by a cross-redox reaction between [Viv0(nhet)] and [Vn(nhet)] (see also Section 33.5.9.3).337,338 The unusually intense spectral features of [VnVIV0(nhet)2]2- originate in oxo bridging between V11 and V1 7 in the cross reaction.336-338 This intermediate has a short lifetime (—25 ms), but it is unusually long by comparison with other inner-sphere systems. 

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