Second electron transfer, proton coupling

The voltammetric response of curcumin and carthamin must, in principle, be dominated by the oxidation of the phenol and/or methoxyphenol groups (see Scheme 2.2). The electrochemistry of methoxyphenols has claimed considerable attention because of their applications in organic synthesis [159-163]. As studied by Quideau et al., in aprotic media, 2-methoxyphenols are oxidized in two successive steps into cyclohexadienone derivatives [163], whereas a-(2)- and a-(4-methoxyphenoxy) alkanoic acids undergo electrochemically induced spirolac-tonization to develop synthetically useful orthoquinone bis- and monoketals. In the presence of methanol, the electrochemical pathway involves an initial one-electron loss, followed by proton loss, to form a monoketal radical. This undergoes a subsequent electron and proton loss coupled with the addition of alcohol to form an orthoquinone monoketal. The formal electrode potential for the second electron transfer... 

Figure 10 shows the proposed ubiquinol oxidation and electron bifurcation mechanism at Qp site. (A) In the absence of the ubiquinone, the side chain of Glu-271 is connected to the solvent in the mitochondrial intermembrane space via a water chain. (B) As a reduced ubiquinol molecule binds to the site, the side chain of Glu-271 flips to form a hydrogen bond to the bound ubiquinone. (C) Now, the ISP, which is moving around the intermediate position by thermal motion is trapped at the b" position by a hydrogen bond to the bound ubiquinone. (D,E) Coupled to deprotonation, the first electron transfer occurs. Since the Rieske FeS cluster has a much higher redox potential (ca. +300 mV) than heme bl (ca. 0 mV), the first electron is favorably transferred to ISP. This yields ubisemiquinone, (F,G). After ubisemiquinone formation, the hydrogen bond to the His-161 of ISP is destabilized. The ISP moves to the c position, where the electron is transferred from the Rieske FeS cluster to heme c. Now unstable ubisemiquinone is left in the Qp pocket. The redox potential of the deprotonated ubisemiquinone is assumed to be several hundred millivolts. Now the electron transfer to the heme bl is a downhill reaction. (H) Coupled to the second electron transfer, the second proton is transferred to Glu-271 and subsequently to the mitochondrial intermembrane space. The fully oxidized ubiquinone is released to the membrane. 

Coenzyme Q is a quinone derivative with a long isoprenoid tail. The number of five-carbon isoprene units in coenzyme Q depends on the species. The most common form in mammals contains 10 isoprene units (coenzyme Qio) For simplicity, the subscript will be omitted from this abbreviation because all varieties function in an identical manner. Quinones can exist in three oxidation states (Figure 18.10). In the fully oxidized state (Q), coenzyme Q has two keto groups. The addition of one electron and one proton results in the semiquinone form (QH ). The semiquinone form is relatively easily deprotonated to form a semiquinone radical anion (Q ). The addition of a second electron and proton generates ubiquinol (QH2), the fiilly reduced form of coenzyme Q, which holds its protons more tightly. Thus, ybr quinones, electron-transfer reactions are coupled to proton binding and release, a property that is key to transmembrane proton transport. 

The ISC mechanism is not normally considered to be plausible. Both the RR and the RS mechanism may be further classified according to the sequence of microscopic steps (electron transfer, coupling, protonation and/or cyclization) and with reference to which of the steps is rate determining. For the RS mechanism, the second electron transfer may take place by a reaction in solution (as indicated in Scheme 2) or at the electrode if the coupling step is very fast. 

Several amino acid residues of H form electrostatically coupled clusters ofionizable residues near Qb (Lancaster et al., 1996). In the Rb. sphaeroides RC, Glu H173 is a member of a cluster which also includes Asp L213, a residue required for proton uptake by Qb (see below). Mutation of Glu HI 73 to Gin retards both the kinetics of the first electron transfer and of the proton-linked second electron transfer (Takahashi and Wraight, 1996). Another characteristic for the region proximal to and Qb are clusters of firmly bound water molecules (Abresch etal., 1998 Fritzsch etal., 1998). Several side chains... 

Reoxidation of the cosubstrate at an appropriate electrode surface will lead to the generation of a current that is proportional to the concentration of the substrate, hence the coenzyme can be used as a kind of mediator. The formal potential of the NADH/NAD couple is - 560 mV vs. SCE (KCl-saturated calomel electrode) at pH 7, but for the oxidation of reduced nicotinamide adenine dinucleotide (NADH) at unmodified platinum electrodes potentials >750 mV vs. SCE have to be applied [142] and on carbon electrodes potentials of 550-700 mV vs. SCE [143]. Under these conditions the oxidation proceeds via radical intermediates facilitating dimerization of the coenzyme and forming side-products. In the anodic oxidation of NADH the initial step is an irreversible heterogeneous electron transfer. The resulting cation radical NADH + looses a proton in a first-order reaction to form the neutral radical NAD, which may participate in a second electron transfer (ECE mechanism) or may react with NADH (disproportionation) to yield NAD [144]. The irreversibility of the first electron transfer seems to be the reason for the high overpotential required in comparison with the enzymatically determined oxidation potential. 

Ru Ru step and a self-exchange rate of 2xlO" M s for the c -[Ru 0)2(L)] " /cw [Ru (0)2(L)]+ couple has been estimated a mechanism involving a pre-equilibrium protonation of ci5-[Ru (0)2(L)]+ followed by outer-sphere electron transfer is proposed for the Ru Ru step. For reduction by [Fe(H20)6] +, an outer-sphere mechanism is proposed for the first step and an inner-sphere mechanism is proposed for the second step. ... 

The observation that so many compounds reduce in such a narrow potential range is curious. We hypothesize that one reason is that many of these reactions are catalyzed via the TAA+/TAA-mercury couple. The mediated reactions then include reduction of compounds, like benzene, whose E° for single electron transfer is more negative than —3.1 V(SCE). A second reason for the possibility of reducing many compounds in this narrow potential range is that the reduction rates often depend on proton availability, which can be adjusted to make the process feasible. 

Cpe at the plateau of the first wave was reported to give the coupling product (the pinacol). At higher pH values (2-9) the pH-dependent first wave moves towards more cathodic potentials, finally merging with the pH-independent second wave. Cpe at the plateau of the combined two-electron wave gives the alcohol as product according to the mechanism outlined in Eqs. (67), (68) and (69). The protonated anion radical is now easier to reduce than the ketone, resulting in the two-electron transfer observed. 

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