Electron transfer through-space

Some of the first protein systems where pulse radiolysis was used to help determine mechanism were those of blue copper proteins. These are proteins that are blue in solution and contain what are known as type (I) and type (2) copper centers. Two of the most well-known and well-characterized examples of these are azurin and cytochrome c. It was the studies of these systems that opened up the field of long-distance electron transfer in proteins and, by using the protein structure as a framework for electron transfer through space and through bonds, allowed for the development of a broad theoretical basis and many fascinating experiments on long-range electron transfer. Here, I will limit the discussion to electron transfer studies in azurin as illuminated by pulse radiolysis studies. ... 

In Figures 8.17 and 8.18 the results of a READS-TCT calculation are given. The three-dimensional spin densities of the negative ions of the cyclic Cg and C10 are shown in Figure 8.17, while those for the cyclic Cg(-) and linear C5(-) are shown in Figure 8.18. The experimental electron affinity of Cio is 2.30 eV, whereas that for the cyclic Cg has not been reported. The CURES-EC and density functional values are 2.47 eV and 2.59 eV for the cyclic Cg. The READS TCT calculation shows electron transfer from C10 (q = 0.36) to Cg (q = 0.63). This indicates the Ea of Cg > 2.30 eV. The electron affinity of the cyclic Cg is about the same as that for the linear C5, as are the q values. The experimental Ea for the linear C5 is 2.57 eV. This is consistent with an Ea of cyclic Cg = 2.5 0.1 eV. The spin densities reflect the charge distribution and illustrate electron transfer through space. 

Electron transfer through complex I. The reduction of FMN to FMNH2 by NADH requires the uptake of one proton from the matrix. FMNH2 subsequently transfers electrons to a series of iron-sulfur centers and releases protons to the solution in the intermembrane space. When the non-sulfur centers reduce UQ to UQH2, two more protons are taken up from the matrix. 

Finally, it is also important to mention that electron transfer by proteins is a process that has been studied in great detail recently. The prevailing hypothesis at present is that electron transport "through bonds" in the protein is more efficient than electron transport "through space". Regardless of the precise mechanism, however, electrons can be transported at reasonable rates over relatively long distances (up to 15 to 20 A), and thus from one protein cofactor to another. 

The acceptor for electrons transferred through the cytochrome bci complex is a soluble cytochrome, a one-electron carrier, in the periplasmic space, which is reduced from the Fe to the Fe state. The reduced cytochrome (analogous to cytochrome c in mitochondria) then diffuses to a reaction center, where it releases its electron to a positively charged chlorophyll, returning the chlorophyll to the ground state and the cytochrome to the Fe state. This cyci/c electron flow generates no oxygen and no reduced coenzymes. 

Electron transfer through the liquid/vapor interface can be studied by various methods which differ only in detail. A diode test cell is used which is filled with liquid to some level between the electrodes (see Figure 14). Electrons are injected into the liquid at the cathode either by the photoelectric effect or from the ionization tracks produced by radioactive sources deposited at the electrode surface. Irradiation of liquid and vapor space by a burst of high energy X-rays can also be used. Under an applied electric field the electrons move toward the liquid/vapor interface. At the interface, they encounter a barrier where they will be trapped for a certain time x. 

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