Redox response

Macrocycles attached to redox responsive groups such as ferrocene (78) can give selective transition metal ion receptors. The X-ray structure reveals a five-coordinate zinc with distorted square-pyramidal geometry bound to the four macrocycle nitrogens and an iodide. In the solid state the two ferrocenyl groups are positioned on the same side of the ligand with distances to the metal center of 5.347(7) and 6.120(8) A and these distances can be related to the redox behavior.689... 

MPA-bridged SOD-electrode complex could be formed via a variety of interactions between MPA and the SODs, such as electrostatic, hydrophobic, and/or hydrogen bonding interactions, which is believed to be responsible for the observed direct electron transfer properties of the SODs. Besides, such interactions substantially enable the SODs to be stably confined at the MPA-modilied Au electrode, which can be further evident from the re-observation of the redox responses of SODs in a pure electrolyte solution containing no SOD with the MPA-modified electrode previously used in SOD solutions. 

Figure 6.7 illustrates the voltammetric response of the third-generation SOD-based 02 biosensors with Cu, Zn-SOD confined onto cystein-modified Au electrode as an example. The presence of 02" in solution essentially increases both the cathodic and anodic peak currents of the SOD compared with its absence [150], Such a redox response was not observed at the bare Au or cysteine-modified Au electrodes in the presence of 02". The observed increase in the anodic and cathodic current response of the Cu, Zn-SOD/cysteine-modified Au electrode in the presence of 02 can be considered to result from the oxidation and reduction of 02, respectively, which are effectively mediated by the SOD confined on the electrode as shown in Scheme 3. Such a bi-directional electromediation (electrocatalysis) by the SOD/cysteine-modified Au electrode is essentially based on the inherent specificity of SOD for the dismutation of 02", i.e. SOD catalyzes both the reduction of 02 to H202 and the oxidation to 02 via a redox cycle of its Cu (II/I) complex moiety as well as the direct electron transfer of SOD realized at the cysteine-modified Au electrode. Thus, this coupling between the electrode and enzyme reactions of SOD could facilitate the development of the third-generation biosensor for 02". ...

Hamilton, S.M. Hattori, K.H. 2008. Spontaneous potential and redox responses over a forest ring. Geophysics, 73, B67-B75. 

In 1990 we reported the synthesis of new redox-responsive crown ether molecules that contain a conjugated link between the crown ether unit and a ferrocene redox-active centre (Beer et al., 1990a). Examples of some of the species synthesized are shown in Fig. 5. The electrochemical behaviour of these species was investigated and also the electrochemical behaviour of their analogues with a saturated link between the ferrocene unit and the crown ether. The changes in the CVs of [2a] upon addition of magnesium cations are shown in Fig. 6. The metal cation-induced anodic shifts of [2a], [2b] and also their saturated analogue [3] and vinyl derivatives [4a], [4b] are shown in Table 1. 

Fluorescent redox switches based on compounds with electron acceptors and fluorophores have been also reported. For instance, by making use of the quinone/ hydroquinone redox couple a redox-responsive fluorescence switch can be established with molecule 19 containing a ruthenium tris(bpy) (bpy = 2,2 -bipyridine) complex.29 Within molecule 19, the excited state of the ruthenium center, that is, the triplet metal-to-ligand charge transfer (MLCT) state, is effectively quenched by electron transfer to the quinone group. When the quinone is reduced to the hydroquinone either chemically or electrochemically, luminescence is emitted from the ruthenium center in molecule 19. Similarly, molecule 20, a ruthenium (II) complex withhydroquinone-functionalized 2,2 6, 2"-terpyridine (tpy) and (4 -phenylethynyl-2,2 6, 2"- terpyridine) as ligands, also works as a redox fluorescence switch.30... 

Nicotinamide is an important redox moiety in biological system. The nicotin-amide-perylene diimide dyad 23 can work as a redox-responsive fluorescence switch.33 Dyad 23, in which nicotinamide is on the oxidation state, exhibits strong fluorescence. However, it becomes nonfluorescent when nicotinamide is reduced due to the electron transfer from the reduced nicotinamide to the photoexcited perylene diimide. The fluorescence of dyad 23 can be reversibly switched off and on chemically by successive reduction with NaBH3CN and oxidation with tetrachlorobenzoquinone and switched electrochemically over 10 cycles without significant degradation. 

Similarly, the luminescence of complexes 38,39,40,41, and 42 can be modulated by changing the redox states of the respective metal ions. Complexes 38 and 39 show emission in the Ni(II) state, whereas the emission is quenched in the Ni(III) state generated after oxidation.46 The fluorescence due to the naphthalene unit in complex 40 is observed in the Ni(II) state after reduction to the corresponding Ni(I) state the naphthalene fluorescence is distinctly reduced.47 Water-soluble complexes 41 and 42 also works as redox-responsive fluorescence switches in a similar way 48... 

Ouchane, S. Kaplan, S. Topological analysis of the membrane-localized redox-responsive sensor kinase PrrB from Rhodobacter sphaeroides... 

We have recently prepared a new chromophoric and redox-responsive ionophore (4) containing a tricyanovinyl redox-active moiety (35) (Scheme 2). Electronic absorption spectra of (4) exhibit hypochromic shifts on binding Group IA and IIA metal cations and cyclic voltamme-tric electrochemical investigations reveal that (4) electrochemically recognizes Na+ and K+ guest cations, resulting in one-wave CV shifts of the tricyanovinyl reduction wave (80 and 20 mV, respectively) to more anodic potentials. 

During the last few years we have been preparing a variety of metallocene macrocyclic molecules designed to be not only redox responsive... 

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