Quinones second electron transfer

After the second electron transfer from semiquinone to heme 6l (step 4), the interaction between the Rieske cluster and the resulting quinone is weakened so that the reduced Rieske protein can now occupy the preferred ci positional state (E), which allows rapid electron transfer from the Rieske cluster to heme Ci (step 5). 

Fig. 3, and therefore reduce the quantum yield of C-P+-QT. However, this does not necessarily mean that the yield of the final C+-P-QT state will be similarly reduced. Indeed, it was found that addition of a second methylene spacer as in 5 increased the quantum yield of the final state by a factor of 1.44 [56], With 3 and 4 methylene groups (6 and 7) the yield decreased to 0.65 and 0.56 that of 4, respectively. This complicated distance dependence for the yield of the final state is in part a consequence of the fact that increasing the porphyrin-quinone separation not only reduces the rate of step 2, but also that of charge recombination (step 3). The rate of the second forward electron transfer step 4 is essentially unaffected, since this step does not involve porphyrin-quinone electron transfer. Thus, increasing the separation will decrease the quantum yield of step 2, but increase the ratio kjk3, which determines the efficiency of the second electron transfer step. With 5, the loss in quantum yield of step 2 is more than compensated for by the increase of efficiency of step 4, and the overall quantum yield increases. In 6 and 7, any increase in the efficiency of step 4 evidently cannot compensate for the decrease in quantum yield for step 2, and the overall quantum yield decreases. 

The second electron transfer step was designed into the molecule by making the second quinone a better electron acceptor than the first. Evidence for this step is provided by the lifetime of the charge separated state, which was about 300 ps in benzene. A similar molecule lacking the second quinone moiety had a lifetime of only about 130 ps. 

In the electroreduction of aromatic hydrocarbons, nitro compounds, and quinones in aptotic solvents, the first step is the transfer of an electron from the electrode to form a radical anion. Once the radical anion is formed, electron repulsion will decrease the facility with which a second electron transfer occurs. But solvation and ion pairing diminish the effect of electron repulsion and tend to shift the reduction potential for the addition of the second electron to more... 

A similar ECE two-electron reduction was also observed for a ubiquinone-thiourea model system, where the complexing component not only serves as a proton source, but also enables specific recognition, which in turn provides direct control of the switching between one- and two-electron reactions. In these host-guest complexes, hydrogen bonding to the quinone allows direct two-electron reduction to occur via a facilitated proton transfer. In this system, however, it appears to be difficult to determine whether proton transfer follows or precedes the second electron transfer. Preliminary studies suggest a pathway between a pre- or post-reduction proton transfer. 

The absorption of light in the reaction center (RC) of photosynthetic bacteria induces electron transfer from the special bacteriochlorophyll pair (P) through a series of one-electron acceptors (bacteriopheophytin, and a primary quinone, Q ) to a two-electron acceptor quinone, Qg [1], In RCs from sphaeroides, both and Qg are ubiquinone-10. It is generally believed that the doubly reduced secondary quinone (hydroquinone dianion) will form quinol (hydroquinone) by taking up two protons before being released from the RC and replaced by another quinone from the quinone-pool. The rate of quinol formation can be limi ted by either of these processes the second electron transfer from Qb to Q/vQb the... 

In all mutants (except MAV2 and pH>10) as well as in Wt, the first electron transfer shows only a very weak pH-dependence. This is in line with the idea, that protonation events are not rate limiting for the stabilization of the first charge on Qb. It is well established that the transfer of the second electron and of the first proton to Qb are closely correlated. The second proton is not correlated to electron transfer but is essential for the replacement of the doubly reduced quinone by an oxidized one and thus for the observation of binary oscillations of the semiquinone state in a sequence of flashes. Therefore the loss of a residue which is essential in proton donation is expected either to strongly decrease the rate of the second electron transfer or to block the binary oscillations after the second flash. 

The electron on the bj heme facing the cytosolic side of the membrane is now passed to the bfj evcie on the matrix side of the membrane. This electron transfer occurs against a membrane potential of 0.15 V and is driven by the loss of redox potential as the electron moves from bj = — O.IOOV) to bn = +0.050V). The electron is then passed from bn to a molecule of UQ at a second quinone-binding site, Q , converting this UQ to UQ . The result-... 

Of more intrinsic interest are processes involving two electrons, since these constitute the great bulk of organic reductive processes, especially in protonic solvents. The reason for this is that the first electron transfer will generate an unstable radical species, whereas the second can regenerate a stable closed-shell product. One example that we have already encountered is C02 reduction but a case that has been well studied is that of quinone reduction as ... 

The second type of biological electron transfer involves a variety of small molecules, both organic and inorganic. Examples of these are (a) nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP) as two electron carriers and (b) quinones and flavin mononucleotide (FMN), which may transfer one or two electrons. The structure of NAD and its reduced counterpart NADH are shown in Figure 1.12. 

A semiquinone can be readily oxidized to the parent compormd by molecular oxygen and can then re-enter the reductase-catalyzed reaction. The enzymatic reduction and autoxidation of quinones rmder aerobic conditions generates superoxide anion radicals, and this process is known as redox cycling (Figure 2). Flydroquinones are less prone to transfer electrons to oxygen, because the second-electron potential is often too high. 

Quinone Reduction This is a reversible, one-electron transfer reaction to the semi-quinone radical, followed by a second, reversible electron transfer that results in the formation of hydroquinone, as shown in Fig. 13.2. 

Hydroquinone oxidation to quinone occurs via two linked one-electron transfer stages. The first step (typically metal catalysed) yields the semiquinone radical intermediate, which is resonance stabilised. The second step involves electron transfer to molecular oxygen to generate superoxide and quinone [70]. This reaction mechanism is common to all hydroquinones, catechols, resorcinols, and so forth. 

The basic forms of phenols (phenolate anions) are easily oxidized to semiquinone radicals through electron transfer. These radicals can then react with another radical to form an adduct through radical coupling or, in the case of o-diphenols, undergo a second oxidation step yielding o-quinones that are electrophiles as well as oxidants. Oxidation reactions are very slow in wine, due to the low proportion of phenolate ions at wine pH values, but take place extremely rapidly when oxidative enzymes are involved (see Section 5.5.2.2). 

On the planet Earth, the most important photoreaction occurs in green plants or in green or purple organisms. Their photochemical reaction centers contain a special pair of chlorins (cf. the purple bacterium Rhodobacter sphaeroides. Fig. 6.2). Solar photons cause electron transfer and generate a radical ion pair. Within two picoseconds, the negative charge is transferred to a second chlorin, and from it to a quinone. ... 

Figure 9-28. The ring-opening oxidation of catechol by dioxygen in the presence of copper salts. The first step presumably involves an electron transfer type of process to generate the quinone, followed by the oxygen atom transfer in the second step.

Comparison of the time resolved data in several solvents for 28 with that for a similar molecule in which one of the quinones was replaced by a dimethoxyphenyl group showed an increase in the lifetime of the fluorescence decay, as would be expected from the removal of a second acceptor, as discussed above. However, the data are complicated by the fact that the decays of the dimethoxyphenyl analog were decidedly biexponential. This result was interpreted in terms of an orbital symmetry effect on the electron transfer reaction. Interesting solvent and temperature dependencies were also observed with 28. 

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