Electron Transfer Sequence

There has been some controversy about the need for N2 in the formation of HD. Burgess et al. [29] reported that N2 was not required. They used argon as their diluent gas and took the word of the supplier that it was free of N2. Not only is commercial argon seldom free of N2, but it is difficult to remove the last traces of N2, and very little N2 is required to support HD formation. To settle this difference in experimental observations, Li and Burris [30] made it a point to rid their diluent gas of contaminating N2. One can absorb N2 on molecular sieve at liquid N2 temperature the problem is that argon liquefies and freezes before you get down to die temperature of liquid N2. So Li used neon as his inert gas and captured any contaminating N2 on molecular sieve in a liquid N2 bath. When the atmosphere above the nitrogen-ase system was carefully freed of N2 there was no formation of HD. 

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In the original rapid-freezing work on xanthine oxidase (53) it was found that in experiments employing about 1 mole of xanthine per mole of enzyme and an excess of oxygen, the time sequence of appearance of the various EPR signals was molybdenum (V), followed by flavin semi-quinone radical (FADH), followed by iron. This suggested that the electron transfer sequence might be ... 

Palmer, G., Bray, R.C., and Beinert, H. 1964. Direct studies on the electron transfer sequence in xanthine oxidase by electron paramagentic resonance spectroscopy. The Journal of Biological Chemistry 239 2657-2666. 

The large number of cytochromes identified contain a variety of porphyrin ring systems. The classification of the cytochromes is complicated because they differ from one organism to the next the redox potential of a given cytochrome is tailored to the specific needs of the electron transfer sequences of the particular system. The cytochromes are one-electron carriers and the electron flow passes from one cytochrome type to another. The terminal member of the chain, cytochrome c oxidase, has the property of reacting directly with oxygen such that, on electron capture, water is formed ... 

Two one-electron transfers with different extents of reversibility. In the case where not all the processes of a consecutive electron transfer sequence are reversible, the irreversibility of a particular step becomes evident by the absence of the reverse peak in its pertinent response. For all other aspects the preceding considerations remain valid. 

Let us now illustrate some significant examples of the redox propensity of related (but less widely studied61) quinone diimines. As happens for 1,2-dioxolenes, such ligands also exhibit the reversible electron transfer sequence o-benzoquinone diiminejo-benzosemiquinone diimine monoanion/ o-phenylendiamine dianion illustrated in Scheme 4. 

As the brain NO-synthase is highly homologous with cytochrome P450 reductase, it appears that the enzyme carries out its own reductase function to activate oxygen, and the electron transfer sequence might be expected to similar in the two enzymes [142, 147]. 

The simplest level of mechanistic information for the nitrogenases is the electron-transfer sequence shown in Figure 11. This scheme has been discussed in several previous reviews [71,72], and herein we shall outline only the basic pattern as a foundation for the later discussion. 

Thus, 26 evidently mimics the BPh to QA to Q electron transfer sequence that occurs in the natural reaction center. One might wonder, however, why the lifetime of the final P+-Q-Q 7 state is so short, given the relatively long methylene chains joining the moieties and the much longer lifetimes found for the D-D-A triads. A likely possibility is that the flexible methylene chains allow the molecule to fold back on itself, so that the cation and anion are rather close together and charge... 

Fig. 16. Electron-transfer sequences between paramagnetic and diamagnetic nickel species. These reactions cannot be found by the program as presently designed, but were proposed by Hegedus (127b). At this stage, the chemist has to help TAMREAC.

Controversial results have been deduced from the researches of different groups on the sensitization behavior of porphyrin arrays as mentioned above. Some of them revealed that porphyrin arrays have advantages over porphyrin monomers in the sensitization of wide band gap semiconductor while the rest showed the opposite results. The porphyrin dimer composed of a metal free porphyrin and a zinc porphyrin seems the most successful porphyrin array sensitizer so far. Programing the photoinduced energy and electron-transfer sequence in a porphyrin array is the key step toward a good porphyrin array sensitizer. 

But one may go further and say that these are the primary steps, the only ones which are true photochemistry and which are, therefore, unique to photosynthesis. The pigment-protein complexes responsible for light harvesting and for the electron transfer sequences within the reaction centres of Photosy stem 1 (PS1) and Photosy stem 2 (PS2) are the heart of the photosynthetic process. 

Figure 1. Nitrate reductase from Neurospora crassa —composition and presumed electron transfer sequence (16, 18)...

Multiple electron transfer — When redox moieties can accept (or donate) more than one electron the electrochemical response can show a series of waves or peaks each associated with a single electron transfer or single waves or peaks associated with concerted multiple electron transfers (see -> concerted electron transfers). These multiple electron transfer sequences can be depicted as... 

From the above data a rough picture of the electron transfer sequence of Complex I may be constructed (Fig. 3.14). 

As indicated in Figure I, wild-type bacterial reaction centers also contain a carotenoid polyene. This polyene is not involved as a donor or acceptor in the normal electron transfer sequence, although carotenoid radical cations have been observed spectroscopically in photosynthetic preparations under certain conditions [18,19]. In many of the artificial photosynthetic systems which will be discussed below, the carotenoid is used as a convenient secondary electron donor. Carotenoids do perform two important functions in photosynthesis. They provide photoprotection from singlet oxygen damage, and act as light-gathering antennas for the special pair (see Sections III and IV). 

The present view is that cytochrome a is the acceptor of electrons from cytochrome c, but that a simple linear electron-transfer sequence from cytochrome a to Cua and then to the cytochrome 03/Cub centre is unlikely. Instead the sequence shown in equation (63) holds, where cytochrome a is in rapid equilibrium with Cua. These views depend largely upon pre-steady-state kinetics of the redox half reactions of the enzyme with its two substrates, ferrocytochrome c and O2. However, these conclusions are not in accord with kinetic studies under conditions when both substrates are bound to the enzyme, and which show maximal rates of electron transfer from cytochrome c to O2. In particular some of the cytochrome c is oxidized at a faster rate than a metal centre in the oxidase. In contrast, at high ionic strength conditions, where the cytochrome c and the cytochrome oxidase are mainly dissociated, oxidation of cytochrome c occurs only slowly following the complete oxidation of the oxidase. These results for the fast oxidation of cytochrome c have been interpreted in terms of direct electron transfer from cytochrome c to the bridged peroxo intermediate involving 03 and Cub, or to a two-electron transfer to O2 from cytochromes a and 03 during the initial phase of the reaction. 

The EPR signals produced in Rb. sphaeroides reaction centers exposed to a series of 3 flashes, as reported by Butler, Calvo, Fredkin, Issacson, Okamura and Feher, are shown in Fig. 4, bottom. Flash excitation was performed with the reaction-center preparation at pH 8 and 23 °C, while the EPR measurements were carried out at 2.1 K. The proposed electron-transfer sequence is shown in the upper part of Fig. 4. The result obtained by applying the first flash is shown in Fig. 4 (A). A g=1.8 EPR signal with a linewidth of440 G representing the Fe Qe complex appeared. With the reduction of the oxidized... 

Fig. 10. (A) and (B) Two models for the electron-transfer sequence in bacterial reaction centers. (C) Population densities of various intermediary states as a function of time calculated according to the model shown in (B). See text for discussion. Figure source (A) and (8) Holzapfel, Finkele, Kaiser, Oesterheldt, Scheer, Stilz and Zinth (1989) Observation of a bacteriochlorophyll anion radical during the primary charge separation in reaction center. Chem Phys Lett 160 5 (C) S Schmidt, T Arit, P Hamm, H Huber, T NSggle, J WachtveitI, M Meyer, H Scheer and W Zinth (1994) Energetics of the primary electron transfer reaction reveaied by ultrafast spectroscopy on modified bacterial reaction centers. Chem Phys Lett 223 118.

The primary photochemical reaction in the PS-I reaction center occurs with the following electron-transfer sequence ... 

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