Redox spectral properties

There is no easy understanding of the spectral properties of these compounds in general, which may or may not have a built-in chromophoric system responsible for a long-wavelength absorption like 7,8-dihydropteridin-4-one or a blue-shifted excitation like its 5,6-dihydro isomer. More important than the simple dihydropteridine model substances are the dihydropterins and dihydrolumazines, which are naturally occurring pteridine derivatives and reactive intermediates in redox reactions. 

Spectroelectrochemistry may be used to probe the effect of changing the redox state of components within an ISA on the spectral properties of the assembly. It may also provide information about the rates of electron transfer at the electrode/electrolyte interface. As illustrated in Figure 3.18, spectroelectrochemistry employs a two- or... 

Very little is known about the nature of the weak interactions of CAs in solutions where a vast majority of their chemical reactions has been studied. Particularly, the study of donor-acceptor complexes of CAs by modern physical-chemical methods is still of great interest. Besides, complexation of CAs with donors or acceptors of electron density is a useful tool for modifying the stability, reactivity and spectral properties of CAs. Systematic investigations of the redox properties of CAs are needed in order to elucidate the role of electron transfer in the transformations of CAs. 

A. Spectral Properties of the Redox-Active Metal Sites. 358... 

The formation of a blue (neutral) semiquinone in high yield upon irradiation of thioredoxin reductase in the presence of a large excess of EDTA is shown in Fig. 3a. The semiquinone is further reduced to FADH at an even slower rate with maximal semiquinone formation at 4 hr. In contrast to this very slow semiquinone production, enzyme reduced by NADPH in the dark and subsequently exposed to light is rapidly converted to the semiquinone. The rate depends on the amount of NADPH used in the reduction with 0.5 mole NADPH per FAD the half-time is less than 0.5 min, with 2.0 moles NADPH per FAD the half-time is about 2 min. The rate of free radical production (EPR) exactly parallels the rate of increase in absorbance at 580 nm. The exact spectral characteristics of the semiquinone depend on the state of oxidation of the disulfide-dithiol. In the dithiol form the maximum is at 578 nm while in the disulfide form the maximum is at 588 nm. That the spectral properties are determined by the redox state of the disulfide is indicated by three findings. If semiquinone is produced by irradiation following reduction of the enzyme by 0.5 mole/FAD, the maximum is at 588 nm, while if the semiquinone is formed following reduction by 2.0 moles/FAD, the maximum is at 578 nm. Oxidation of enzyme irradiated in the presence of excess EDTA for various lengths of time requires ferricyanide stoichiometric with the ob-... 

Structures and spectral Properties of the Redox-active Metal Sites... 

Chloroplast cytochromes can readily be characterized quantitatively by optical spectroscopy, as they have characteristic absorption spectra as well as distinct absorption-difference spectra. The thylakoids in higher plants (e.g., spinach) contain three cytochromes Cyt b559, in either of two forms associated with PS II, and Cytb and Cyt/in the Cyt b complex. Although the a-absorption bands of all three cytochromes lie close to one another in the 550-560 nm region, it is possible to spectrally isolate the individual cytochromes by imposing carefully chosen redox potentials, as the redox-potential values of the different types of chromophores are sufficiently different from one another. Some of the optical and redox properties of the three spinach cytochromes are shown in the table accompanying the spectral properties in Fig. 8. 

Very recently the synthesis of a covalently bound chlorophyll-a-dimer analogue has been reported703). It was shown that bis-(chlorophyllide-a)ethylene glycol diester with porphyrin rings held together via nucleophilic hydrogen bridging closely resembles in its spectral properties, photochemical activity and redox potential to Chl-aj and therefore can serve as an in vitro model for the native Chl-ai-dimer-protein complex. 

In the sulfate-reducing bacteria there is another cytochrome which resembles cytochrome c3 in the spectral properties and redox potential but differs from this cytochrome in molecular mass this is cytochrome c3 (26 kDa) which has eight heme C molecules in the molecule. The cytochrome molecule is composed of two polypeptides of 13 kDa (Loufti et al., 1989). On the basis of the amino acid sequence, however, the 13 kDa polypeptide differs from cytochrome c3 (Guer-lesquin et al., 1982 LeGall and Peck, 1987 Loufti et al., 1989). Desulfovibrio gigas cytochrome c3 (26 kDa) molecule is composed of two 13 kDa molecules bound to each other by an S-S bond (Bruschi et al., 1996). It is claimed that cytochrome c3 (26 kDa) is very effective as the electron donor for thiosulfate reductase (Hat-chikian et al., 1972). 

Shen J, El Ojaimi M, Chkounda M, Gros CP, Barbe JM, Shao J, Guilard R, Kadish KM (2008) Solvent, anion, and structural effects on the redox potentials and UV-visible spectral properties of mononuclear manganese corroles. Inorg Chem 47 7717-7727... 

Cytochrome c oxidase is the terminal member of the respiratory chain in all animals and plants, aerobic yeasts, and some bacteria." " This enzyme is always found associated with a membrane the inner mitochondrial membrane in higher organisms or the cell membrane in bacteria. It is a large, complex, multisubunit enzyme whose characterization has been complicated by its size, by the fact that it is membrane-bound, and by the diversity of the four redox metal sites, i.e., two copper ions and two heme iron units, each of which is found in a different type of environment within the protein. Because of the complexity of this system and the absence of detailed structural information, spectroscopic studies of this enzyme and comparisons of spectral properties with 02-binding proteins (see Chapter 4) and with model iron-porphyrin and copper complexes have been invaluable in its characterization. 

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