Pathways for electron flow

We need to answer the questions What properties distinguish a good electron source, a good electron acceptor, and a good pathway for electron flow There are relatively few pathways through which the common electron sources and sinks react. 

Although there are thousands of different organic reactions, they can be explained by mechanisms that are a combination of relatively few pathways for electron flow. The following are twelve of the most common generic electron flow pathways. These pathways should become a very important part of your mechanistic vocabulary. You will need to have an excellent command of these dozen pathways to be able to combine them to postulate a reasonable mechanism for almost any reaction. 

For simplification, a much smaller 7x7 matrix (Table 8.1) will be used that contains representatives of the more common sources and sinks. Within each cell of the matrix is the subsection in Sections 8.2 to 8.8 that covers the appropriate pathways for electron flow for that particular combination of source and sink. Any important combinations not included in the simplified matrix are covered in Sections 8.9 to 8.12. 

All electrochemical cells consist of at least two electrodes, an anode where oxidation reactions occur and a cathode for reduction reactions, with a conductive electrolytic solution between the anode and cathode. To maintain an overall charge balance, the electrons produced at the anode are consumed at the cathode an external wire connecting the electrodes provides the pathway for electron flow. The electrical circuit is completed by current flow through the electrolyte solution. 

Earlier experiments with the cytochrome reductase system showed that antimycin A causes a crossover (see problem 4 in the text) between cytochrome b and the c cytochromes. This work and other experiments led to the proposed pathway for electron flow in cytochrome reductase ... 

Corrosion of metals is defined as their spontaneous deterioration by chemical interaction with the surrounding environment. It is a two-component system involving the interaction of the metal or alloy with a medium or environment. In the absence of an environment (e.g., vacuum), corrosion will not occur. Most corrosion reactions are electrochemical in natme, and for electrochemical corrosion to occur, a cell consisting of an anode, a cathode, an electrolyte, and a pathway for electron flow between the anode and the cathode is needed. 

Under +1 V of forward bias (Fig. lid), there is no pathway for current flow. At +2 V, however, the orbitals have adjusted to give a downhill path from MA to acceptor to donor to MD, and Aviram-Ratner current flows. On the reverse bias side, however, two pathways exist for current flow at —1 V (Fig. 1 lb) as well as —2 V (Fig. 11a). These pathways (Fig. 11a, b) are asymmetric rectification via HOMO and via LUMO, and they are in the anti-Aviram-Ratner direction, i.e., from donor to acceptor. This could allow for anti-Aviram-Ratner rectification under moderate biases. Note, however, that the electrons in Fig. 11a, b must tunnel over longer distances than those in Fig. lie, because there is only one way-station, instead of two. The Aviram-Ratner current flow under the higher bias of Fig. lie could therefore be much more intense than the reverse flow of Fig. 1 lb or 1 la. 

The electron transfer reaction from copper to heme within the ternary protein complex was also studied in solution by stopped-flow spectroscopy. Analysis by Marcus theory of the temperature dependence of the limiting first-order rate constant for the redox reaction (Davidson and Jones, 1996) yielded values for the of 1.1 eV and H b of 0.3 cm , and predicted an electron transfer distance between redox centers which was consistent with the distance seen in the crystal structure. Thus, the electron transfer event is rate-limiting for this redox reaction. Experiments are in progress to determine the validity of the predicted pathways for electron transfer shown in Figure 7. 

Q These containers are constructed and arranged so that zinc is oxidized on one side whiie copper ions are reduced on the other. A wire connected between the zinc and copper strips provides a pathway for the flow of electrons. 

What do you think would happen if you separated the oxidahon half-reaction from the reduchon half-reachon Can redox occur Consider Figure 21-la in which a zinc strip is immersed in a soluhon of zinc sulfate and a copper strip is immersed in a solution of copper(II) sulfate. Two problems prohibit a redox reac-hon in this situahon. First, with this setup there is no way for zinc atoms to transfer electrons to copper(II) ions. This problem can be solved by connechng a metal wire between the zinc and copper strips, as shown in Figure 21-lb. The wire serves as a pathway for electrons to flow from the zinc strip to the copper strip. 

A FIGURE 8-20 Schematic depiction of the cytochrome c oxidase complex showing the pathway of electron flow from reduced cytochrome c to O2. Heme groups are denoted by red diamonds. Blue arrows indicate electron flow. Four electrons, sequentially released from four molecules of reduced cytochrome c, together with four protons from the matrix, combine with one O2 molecule to form two water molecules. Additionally, for each electron transferred from cytochrome c to oxygen, one proton is transported from the matrix to the intermembrane space, or a total of four for each O2 molecule reduced to two H2O molecules. 

Figure 20.1 These containers are constructed and arranged so that zinc will be oxidized on one side, while copper ions will be reduced on the other. In a, zinc metal Is Immersed In 1M zinc sulfate solution, and copper metal In 1M copper sulfate. In b, a wire joining the zinc and copper strips provides a pathway for the flow of electrons, but the pathway Is not complete. Electron transfer Is still not possible. 

Tn summary, it has been established that the reaction center of PS II is relatively stable against trypsin attack, but that one or more components on the reducing side of PS II are accessible to trypsin and are modified by the enzyme in a time-dependent manner, resulting in loss of electron transport activity to such artificial electron acceptors as DCPIP and methyl viologen (19). In addition, trypsin treatment causes herbicide-insensitive electron flow that results from the creation of a new artificial pathway for electron transfer to certain added electron acceptors, such as ferricyanide. This new electron transfer probably arises directly from the primary acceptor Q (j, 27). The... 

The component at the right-hand side of each arrow is the oxidant for the reduced form of the component at the left-hand side. According to this formulation cytochrome b is not on the main pathway of electron flow. 

Fig. 10 Aviram-Ratner rectification via HOMO and LUMO. (a) A D-o-A molecule is sandwiched between two metal electrodes. MD is the electrode proximal to the donor, MA is the electrode proximal to the acceptor, is the electrode metal work function, IPD is the ionization potential of the donor, EAa is the electron affinity of the acceptor, (b) No pathway for current exists when a voltage is applied in the reverse bias direction, (c) Under a comparable voltage to (b) but in the forward bias direction, rectification results from electrons flowing from MA to LUMO to HOMO to MD...

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