Quinones first electron transfer

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 two one-electron transfer events exhibit the two basic roles for proton uptake and redistribution—weakly coupled charge stabilization for the first electron (m w r s l), and strongly coupled bond formation for the second (net uptake of 2H+ per QH2). However, it is now evident that the two are functionally related, and part of the charge-compensating H+ uptake, e.g., on the first electron transfer, is destined for delivery to the quinone head group in the second reduction step. 

Although the acid cluster plays a distinct and identifiable role in the relaxation response of the protein on the first electron transfer, electrostatic calculations indicate that the full energetic contribution is quite widely distributed, with small ionization state changes of a number of residues. On the other hand, proton delivery to the quinone head group during the second reduction step is a targeted function, with discrete termini at the quinol oxygens. 

It seems reasonable to think that the H-bonded connection between Qb-, SerL22S, and AspHL21s, established after the first electron transfer, provides the terminal path for delivery of the first proton to the quinone, ahead of the second electron. From the X-ray structures, a water molecule is positioned between AspL21s and AspM17 and a complete pathway could comprise... 

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. 

We propose that the first step in the formation of quinones, as shown in Scheme 3 for BP, involves an electron transfer from the hydrocarbon to the activated cytochrome P-450-iron-oxygen complex. The generate nucleophilic oxygen atom of this complex would react at C-6 of BP in which the positive charge is appreciably localized. The 6-oxy-BP radical formed would then dissociate to leave the iron of cytochrome P-450 in the normal ferric state. Autoxidation of the 6-oxy-BP radical in which the spin density is localized mainly on the oxygen, C-l, C-3 and C-12 (19,20) would produce the three BP diones. 

J.R. Bolton If the electron reaches the quinone via the linkage, then the transfer must involve the molecular orbitals of the linking structure and thus the solvent will have only a secondary effect. If the electron transfer occurs through the solvent, then the solvent should have a first-order effect on the rate. 

The first of these new, electron transferring components was coenzyme Q (CoQ). Festenstein in R.A. Morton s laboratory in Liverpool had isolated crude preparations from intestinal mucosa in 1955. Purer material was obtained the next year from rat liver by Morton. The material was lipid soluble, widely distributed, and had the properties of a quinone and so was initially called ubiquinone. Its function was unclear. At the same time Crane, Hatefi and Lester in Wisconsin were trying to identify the substances in the electron transport chain acting between NADH and cytochrome b. Using lipid extractants they isolated a new quininoid coenzyme which showed redox changes in respiration. They called it coenzyme Q (CoQ). CoQ was later shown to be identical to ubiquinone. 

Each ion-radical reaction involves steps of electron transfer and further conversion of ion-radicals. Ion-radicals may either be consnmed within the solvent cage or pass into the solvent pool. If they pass into the solvent pool, the method of inhibitors will determine whether the ion-radicals are prodnced on the main pathway of the reaction, that is, whether these ion-radicals are necessary to obtain the hnal prodnct. Depending on its nature, the inhibitor may oxidize the anion-radical or reduce the cation-radical. Thns, quinones are such oxidizers whereas hydroquinones are reducers. Because both anion and cation-radicals are often formed at the first steps of many ion-radical reactions, qninohydrones— mixtures of quinones and hydroquinones—turn out to be very effective inhibitors. Linares and Nudehnan (2003) successfully used these inhibitors in studies on the mechanism of reactions between carbon monoxide and lithiated aromatic heterocycles. 

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 simplest covalently linked systems consist of porphyrin linked to electron acceptor or donor moiety with appropriate redox properties as outlined in Figure 1. Most of these studies have employed free base, zinc and magnesium tetrapyrroles because the first excited singlet state is relatively long-lived (typically 1-10 ns), so that electron transfer can compete with other decay pathways. Additionally, these pigments have relatively high fluorescence quantum yields. These tetrapyrroles are typically linked to electron acceptors such as quinones, perylenes , fullerenes , acetylenic fragments (14, 15) and aromatic spacers and other tetrapyrroles (e.g. boxes and arrays). 

The Primary Donor Triplet State iP7a0 If in the charge separation process electron-transfer in PS I beyond the first acceptor A0 is blocked by treatment with sodium dithionite at high pH and illumination, which reduces the iron-sulfur centres (F) and the quinone (A, the triplet state of the donor, 3P7ao, is obtained via radical-pair recombination from the triplet RP according to ... 

In summary, it would appear that the oxidation of a catecholamine probably first involves the formation of a semi-quinone radical (this can be brought about by an one-electron transfer, e.g. from Cu++ ions,14 or by photoactivation 1) which rapidly undergoes further oxidation (e.g. with atmospheric oxygen) to an intermediate open-chain quinone (such as adrenaline-quinone) and then cyclizes by an oxidative nucleophilic intramolecular substitution to the amino-chrome molecule. Whilst the initial formation of a leucoaminochrome by non-oxidative cyclization of the intermediate open-chain quinone in some cases cannot be entirely excluded at the moment (cf. Raper s original scheme for aminochrome formation72), the... 

Figure 5.7 Electron transfer processes in the first stages of photosynthesis. The energy of light E absorbed by the antenna chlorophylls is transferred to the special pair (BChl)FC is the ferrocytochrome, BPh the bacteriopheophytin and QFe, Q are quinones...

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.

The vast majority of the dyad models for photosynthetic electron transfer have consisted of synthetic porphyrins covalently linked to quinones. The first such models were reported in the late 1970 s. Kong and Loach prepared the ester-linked dyad 2 in 1978 [38], and the amide 3 was reported by Tabushi and coworkers in 1979 [39]. A large number of these P-Q systems have now appeared in the literature. The reader is referred to several reviews [13, 34, 40], including the recent review by Connolly and Bolton [41] for a complete compilation of these results. 

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. 

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