Electron transfer between complexes

Reconstitution Experiments Demonstrate the Need for Carriers to Mediate Electron Transfer between Complexes Oxidative Phosphorylation... 

We consider first reactions in aqueous solution which can be described in terms of electron transfer between complex ions of the d block elements, e.g. ... 

Because of differences in the protein component of the cytochromes and small differences in the heme structure, each heme has a different reduction potential. The cytochromes of the b-Ci complex have a higher energy level than those of cytochrome oxidase (a and a ). Thus, energy is released by electron transfer between complexes 111 and IV. The iron atoms in the cytochromes are in the Fe state. As they accept an electron, they are reduced to Fe. As they are reoxidized to Fe, the electrons pass to the next component of the electron transport chain. 

The expression for the perturbation operator leading to reaction was discussed in connection with the nonorthogonality of the electronic wave functions of the initial and final states. For the first time, this problem was investigated in a two-level approximation in 1964 by Kuznetsov (see also Refs. 31, 35, and 36). The final results for the probability of electron transfer between complex ions obtained in Ref. 53 represent a limiting case of the expressions derived in Refs. 49 and 57. Deviation of some general relations in Ref. 53 from those derived earlier (see, e.g.. Ref. 49) is due to a certain mathematical inaccuracy in Ref. 53 (see Ref. 58). 

Enalapril maleate modified the activities of NADH oxidase and NADH cytochrome c reductase, probably inhibiting electron transfer between complex 1 and complex III in rat kidney mitochondria (Basso et al. 1991). 

Thenoyltrifluoroacetone and carboxin and its derivatives specifically block Complex II, the succinate-UQ reductase. Antimycin, an antibiotic produced by Streptomyees griseus inhibits the UQ-cytochrome c reductase by blocking electron transfer between bn and coenzyme Q in the Q site. Myxothiazol inhibits the same complex by acting at the site. 

Figure 9-6. The consequences of a self-exchange electron transfer between a ground state cobalt(ii) and a ground state cobalt(iii) complex.

Macrocyclic receptors made up of two, four or six zinc porphyrins covalently connected have been used as hosts for di- and tetrapyridyl porphyrins, and the association constants are in the range 105-106 M-1, reflecting the cooperative multipoint interactions (84-86). These host-guest complexes have well-defined structures, like Lindsey s wheel and spoke architecture (70, Fig. 27a), and have been used to study energy and electron transfer between the chromophores. A similar host-guest complex (71, Fig. 27b) was reported by Slone and Hupp (87), but in this case the host was itself a supramolecular structure. Four 5,15-dipyridyl zinc porphyrins coordinated to four rhenium complexes form the walls of a macrocyclic molecular square. This host binds meso-tetrapyridyl and 5,15-dipyridyl porphyrins with association constants of 4 x 107 M-1 and 3 x 106 M-1 respectively. 

Advanced EPR techniques such as CW and pulsed ENDOR, electron spin-echo envelope modulation (ESEEM), and two-dimensional (2D)-hyperfine sublevel correlation spectroscopy (HYSCORE) have been successfully used to examine complexation and electron transfer between carotenoids and the surrounding media in which the carotenoid is located. 

Similar to those observed with the cysteine-modified electrode in Cu, Zn-SOD solution [98], CVs obtained at the MPA-modified Au electrode in phosphate buffer containing Fe-SOD or Mn-SOD at different potential scan rates (v) clearly show that the peak currents obtained for each SOD are linear with v (not v 1/2) over the potential scan range from 10 to 1000 mVs-1. This observation reveals that the electron transfer of the SODs is a surface-confined process and not a diffusion-controlled one. The previously observed cysteine-promoted surface-confined electron transfer process of Cu, Zn-SOD has been primarily elucidated based on the formation of a cysteine-bridged SOD-electrode complex oriented at an electrode-solution interface, which is expected to sufficiently facilitate a direct electron transfer between the metal active site in SOD and Au electrodes. Such a model appears to be also suitable for the SODs (i.e. Cu, Zn-SOD, Fe-SOD, and Mn-SOD) with MPA promoter. The so-called... 

Electron transfer between metal ions contained in complexes can occur in two different ways, depending on the nature of the metal complexes that are present. If the complexes are inert, electron transfer occurring faster than the substitution processes must occur without breaking the bond between the metal and ligand. Such electron transfers are said to take place by an outer sphere mechanism. Thus, each metal ion remains attached to its original ligands and the electron is transferred through the coordination spheres of the metal ions. 

Mizutani et al. [16] have demonstrated that ferrocene derivatives, attached by means of covalent bonds to the surface of bovine serum albumin, have been able to mediate the electron transfer between the glucose oxidase and the electrode through the osemium complex. 

However, because of the mostly very slow electron transfer rate between the redox active protein and the anode, mediators have to be introduced to shuttle the electrons between the enzyme and the electrode effectively (indirect electrochemical procedure). As published in many papers, the direct electron transfer between the protein and an electrode can be accelerated by the application of promoters which are adsorbed at the electrode surface [27], However, this type of electrode modification, which is quite useful for analytical studies of the enzymes or for sensor applications is in most cases not stable and effective enough for long-term synthetic application. Therefore, soluble redox mediators such as ferrocene derivatives, quinoid compounds or other transition metal complexes are more appropriate for this purpose. 

Figure 2. Deuterium isotope effect for electron transfer between ammine complexes as a function of the reduced electronic energy gap AE/EC where Er is the total reorganization energy E, = Eg + Ec. Key for parameters — —, hci)H/kBT — 2.0 and Es/Ec = 0 ------------------, Es/Ec = 1 and-------, Es/Ec = 2.

Braun M, Atalick S, Guldi Dirk M, Lanig H, Brettreich M, Burghardt S, Hatzimarinaki M, Ravanelli E, Prato M, Van Eldik R, Hirsch A (2003) Electrostatic Complexation and Photoinduced Electron Transfer between Zn-Cytochrome c and Polyanionic Fullerene Dendrimers. Chem. Eur. J. 9 3867-3875. 

---