Gated Electron Transfer Reactions

Fig. 13. Representative Trumpet Plots for the [3Fe-4S]+/0 couple in native and D15N mutant forms of Azotobacter vinelandii ferredoxin I adsorbed on a PGE electrode. The plots for D15N also show the fits based on k0 t = 2.5 s-1. Note the intermediate region of the plot (pH 5.50) in which an oxidation peak is not observed because ET is gated. Data points shown in red are for the pH values indicated whereas data points shown in blue are for the uncoupled electron-transfer reaction occurring at pH > pffoiuater- Reproduced from Ref. (33) by permission of the Royal Society of Chemistry.

The mechanism of proton translocation in complexes I and IV is not yet understood. Here, the electron-transfer reactions may cause protein conformational changes that open gates for proton movement first on one side of the membrane and then on the other. 

Pogozelski WK, Tullius TD (1998) Oxidative strand scission of nucleic acids Routes initiated by hydrogen abstraction from the sugar moiety. Chem Rev 98 1089-1107 Poole JS, Eladad CM, Platz MJ, Fredin ZP, Pickard L, Guerrero EL, Kesser M, Chowdhury G, Kotande-niya D, Gates KS (2002) Photochemical electron transfer reactions of tirapazamine. Photochem Photobiol 75 339-345... 

Grove TZ, Kostic NM. Metalloprotein association, self-association, and dynamics governed by hydrophobic interactions simultaneous occurrence of gated and true electron-transfer reactions between cytochrome and cytochrome c6 from Chlamydomonas reinhardii. J Am Chem Soc 2003 125 10598-607. 

Bishop, G. R., and Davidson, V. L., 1997, Catalytic role of monovalent cations in the mechanism of proton transfer which gates an interprotein electron transfer reaction. Biochemistry 36 3586nl3592. 

Graige, M. S., Feher, G., and Okamura, M. Y., 1998, Conformational gating of the electron transfer reaction Qa Qb QaQb bacterial reaction centers of Rhodobacter sphaeroides determined by a driving force assay. Proc. Natl. Acad. Sci. USA, 95 11679911684. 

Hammes-Schiffer S. Theoretical perspectives on proton-coupled electron transfer reactions. Acc. Chem. Res. 2001 34 273-281. Khoshtariya DE, Wei J, Liu H, Yue H, Waldeck DH. Charge-transfer mechanism for cytochrome c adsorbed on nanometer thick films, distinguishing frictional control from conformational gating. J. Am. Chem. Soc. 2003 125 7704-7714. 

Figure 6.13 A framework of a covalently bound CjeHssSH monolayer (green blocks) is interrupted by thinner, also covalently bound template molecules (black holes), e.g. a quinone- or steroid-thiol. These template molecules serve as gates for electron transfer reactions and can be closed by signal molecules (grey blocks) which bind to those templates, e.g. carbohydrates which react with the C-17 substituent of steroid 11. The current of redox-active ions from the bulk water volume stops.

Conformational gating does not appear to be involved in electron transfer reactions in ruthenated systems, but it may influence the rates of the back reactions in RCs. 

The redox coupling of the two hemes shows different degrees of complexity in these enzymes. Pseudomonas nautica (PnNiR) shows ideal behavior and values for heme c and heme d are -1-234 mV and +199 mV, respectively. " In contrast, PpNiR shows strong heme-heme interaction resulting in hysteretic behavior attributed to kinetically gated, conformationally dependent coopera-tit vity in two-electron transfer reactions. ... 

For several other ligands, for example, L , the electron transfer reactions were shown to proceed via the gated mechanism, that is, via the formation of imstable transients with the coordination configuration of the reactant ill) (Scheme 2). 

The long-range electron transfer reactions in ruthenium-modified myoglobin, in which the labile heme unit has been replaced by various metalloporphyrins, have been reviewed. The reductions of the [Ru(NH3)5] moiety, attached at His-48, by Pd- and Pt-substituted hemes in myoglobin proceed at rates of 9 1 x 1(P and 1.2x lO s", respectively. The difference in rates for electron transfer between Fe (heme) and Mg or Zn(porphyrin) centers in [a(Fe(II)P),j3(M T)] hemoglobin hybrids indicates a direct process as opposed to the involvement of a conformational gate. Using [Co(NH3)5Cl] to quench the Zn state, a rate constant of 2.4 x 10 s has been measured for back electron transfer within [a(Zn- -P)i8(Fe(III)CN)]. ... 

The PSII complex contains two distinct plastoquiaones that act ia series. The first is the mentioned above the second, Qg, is reversibly associated with a 30—34 kDa polypeptide ia the PSII cote. This secondary quiaone acceptor polypeptide is the most rapidly tumed-over proteia ia thylakoid membranes (41,46). It serves as a two-electron gate and connects the single-electron transfer events of the reaction center with the pool of free... 

Current research aims at high efficiency PHB materials with both the high speed recording and high recording density that are required for future memory appHcations. To achieve this aim, donor—acceptor electron transfer (DA-ET) as the hole formation reaction is adopted (177). Novel PHB materials have been developed in which spectral holes can be burnt on sub- or nanosecond time scales in some D-A combinations (178). The type of hole formation can be controlled and changed between the one-photon type and the photon-gated two-photon type (179). 

The standard formalisms for describing ET processes assume that in reactions such as Eqs. (1) and (2) there is but a single stable conformational form for each of the precursor and successor electron-transfer states. However, for a system that displays two (or more) alternative stable conformations with different ET rates, dynamic conformational equilibrium can modulate the ET rates. Major protein conformational changes can occur at rates that are competitive with observed rates of ET [9], and such gating [10] may occur in non-rigid complexes such as that between zinc cytochrome c peroxidase (ZnCcP) and cytochrome c (see below) or even within cytochrome c [5]. 

The fact that ET and conformational reactions thus are sequential (Scheme III), and not concerted, is an important factor in efforts to disentangle eonforma-tional and electron-transfer influences, because standard detection methods monitor only the ET event, and not conformational changes within one electronic state. In many, if not most, instances the measured time course of a single gated ET reaction is likely to be indistinguishable from a reaction without gating. 

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