Redox-driven translocation

Metal Translocation Based on the Fe "/Fe" Couple The first example of redox-driven translocation of a metal center was based on the Fein/Fen couple and took place in ditopic ligands containing (i) a tris-hydroxamate compartment and (ii) a tris-(2,2 -bipyridine) compartment.5... 

Figure 2.8 Redox-driven translocation of a copper center, based on the Cu"/Cu change. The Cu11 ion stays in the tetramine compartment of the ditopic ligand 10, whereas the Cu1 ion prefers to occupy the bis-(2,2 -bipyridine) compartment. The translocation of the copper center between the two compartments is fast and reversible when carried out through the Cun-to-Cu1 reduction with ascorbic acid and Cu -to-Cu" oxidation with H202, in a MeCN solution.

Figure 2.9 Redox-driven translocation of the anion X (e.g., chloride), based on the Nin/Nim change. The nickel center acts both as an engine and as areceptor for the X anion (when in the Ni111 state). Occurrence of the reversible X translocation is afforded by the following sequence of anion affinity Nim > Cu11 > Ni11.

Fig. 18. The model for the redox-driven translocation of a metal ion between a two compartments ligand. The metal center exists in two oxidation states of comparable stability, connected by a fast and reversible one-electron redox change. The two compartments display different coordinating tendencies, e.g., compartment A is hard, compartment B is soft. In these circumstances, the oxidized metal (the smaller ball) will prefer to stay in compartment A, whereas the reduced form (larger ball) will occupy compartment B. Therefore, it is expected that, following consecutive oxidation/reduction, the metal center will translocate sequentially between A and B...

Fig. 19. The redox-driven translocation of an iron center within a heteroditopic ligand containing a hard compartment [the tris(hydroxamate) donor set, lower level preferred by Fe(III)] and a soft compartment [tris(2, 2 -bipyridine) donor set, upper level chosen by the Fe(II) center). Chemical reduction (with ascorbic acid) and oxidation (with peroxydisulfate) make the iron center translocate from one level to the other...

In the most general situation, a redox-active metal ion is translocated from a given site to another site of the same molecular system, following a chemical (a redox reaction) or an electrochemical input. The redox-driven reversible translocation of a metal ion in a two-component molecular system is schematically sketched in Fig. 2.2. 

Figure 3.52 Redox-driven chloride translocation in a mixed Cu/Ni complex of 3.74.

Fabbrizzi, L., Gatti, F., Pallavicini, P., Zambarbieri, E., Redox-driven intramolecular anion translocation between transition metal centres. Chem. Eur. J. 1999, 5, 682-690. 

Deppenmeier U. Redox-driven proton translocation in methano-genic Archaea. Cell. Mol. Life Sci 2002 59 1513-1533. 

Redox-Driven Anion Translocation Between Metal Centers. 108... 

Consecutive oxidation and reduction processes would make the metal center M shuttle back and forth, between A and B, along a determined route. The rate of the translocation process should depend on the nature of the coordinative interactions between M and receptors A and B, whether labile or inert, and on the feasibility of the stereochemical rearrangement which may accompany the metal displacement. Examples of redox-driven metal ion translocation within a two-component system have been recently investigated, and refer to the Fe(III)/Fe(II) and Cu(II)/Cu(I) couples. 

The CyD resides preferentially on the trans-azobenzene component and photoisomerization to the cis-azobenzene state causes translocation of the Fc-y -CyD to the alkyl chain component of the assembly. This light-driven translocation is reversible and proceeds by isomerization of azobenzene between the trans and cis states. The chronoamperometric response of the redox-active ferrocene group associated with a CyD unit reflects a change of position on the molecular array. 

Proton gradients can be built up in various ways. A very unusual type is represented by bacteriorhodopsin (1), a light-driven proton pump that various bacteria use to produce energy. As with rhodopsin in the eye, the light-sensitive component used here is covalently bound retinal (see p. 358). In photosynthesis (see p. 130), reduced plastoquinone (QH2) transports protons, as well as electrons, through the membrane (Q cycle, 2). The formation of the proton gradient by the respiratory chain is also coupled to redox processes (see p. 140). In complex III, a Q,cycle is responsible for proton translocation (not shown). In cytochrome c oxidase (complex IV, 3), trans-... 

Molecular Motions Driven by Transition Metal Redox Couples Ion Translocation and Assembling-Disassembling of Dinuclear Double-Strand Helicates... 

Figure 2.3 Translocation of an iron center within a two-compartment ligand, driven by the FeIn/Fen redox couple. Fe111 prefers the inner compartment, which provides six oxygen donor atoms and retains a triply negative charge Fe11 chooses the peripheral compartment consisting of three bpy subunits. Consecutive chemical reduction and oxidation makes the metal move back and forth between the two compartments.

Figure 2.6 A square scheme illustrating the pendular motion of an iron center, driven hy the Fe Fe1" redox couple. As judged from voltammetric experiment carried out at varying potential scan rate, the lifetime for both translocation processes is <10 ms.

Figure 2.7 The hypothesized translocation of a heteroditopic cylindrical ligand driven by the Cu VCu1 redox couple.

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