Vectorial electron transfer

Hwang K C and Mauzerall D C 1992 Vectorial electron transfer from an interfacial photoexcited porphyrin to ground-state Cgg and C g and from ascorbate to triplet Cgg and C g in a lipid bilayer J. Am. Chem. Soc. 114 9705-6... 

The dynamics of electron-transfer at the interfaces of Ti02/Dye 2/electrolyte are summarized in Fig. 20.7. Two forward electron-transfer steps are much faster than the corresponding reverse electron-transfer (charge recombination) on the order of 103 to 106. The results well explain the high rjei value due to the efficient and vectorial electron-transfer in Dye 2-sensitized DSC. 

Tennakone et al.%S) used triphenylmethane type (metallochromic) organic dye (Dye 21, 22), both of which show a large bathochromic shift on complexing with metal ions. The molecular orbital calculation of these dyes in chelating condition with the Tilv ion revealed that the LUMOs of these dyes are localized on the TiIV ion, but the HOMOs are delocalized in the whole dyes. Such MO distribution similar to the LMCT transition in transition metal complexes should contribute to the vectorial electron transfer (high rj ) from the excited dye to Ti02. 

A chromophore such as the quinone, ruthenium complex, C(,o. or viologen is covalently introduced at the terminal of the heme-propionate side chain(s) (94-97). For example, Hamachi et al. (98) appended Ru2+(bpy)3 (bpy = 2,2 -bipyridine) at one of the terminals of the heme-propionate (Fig. 26) and monitored the photoinduced electron transfer from the photoexcited ruthenium complex to the heme-iron in the protein. The reduction of the heme-iron was monitored by the formation of oxyferrous species under aerobic conditions, while the Ru(III) complex was reductively quenched by EDTA as a sacrificial reagent. In addition, when [Co(NH3)5Cl]2+ was added to the system instead of EDTA, the photoexcited ruthenium complex was oxidatively quenched by the cobalt complex, and then one electron is abstracted from the heme-iron(III) to reduce the ruthenium complex (99). As a result, the oxoferryl species was detected due to the deprotonation of the hydroxyiron(III)-porphyrin cation radical species. An extension of this work was the assembly of the Ru2+(bpy)3 complex with a catenane moiety including the cyclic bis(viologen)(100). In the supramolecular system, vectorial electron transfer was achieved with a long-lived charge separation species (f > 2 ms). 

A surface also provides a communicable interface, through which the adsorbate can be addressed directly. In doing so, it provides a powerful means of directing processes within the assembly. For example, if the surface is conducting it may be used to induce vectorial electron transfer. 

For this purpose an electron transfer across the bilayer boundary must be accomplished (14). The schematic of our system is presented in Figure 3. In this system an amphiphilic Ru-complex is incorporated Into the membrane wall. An electron donor, EDTA, is entrapped in the inner compartment of the vesicle, and heptylviolo-gen (Hv2+) as electron acceptor is Introduced into the outer phase. Upon illumination an electron transfer process across the vesicle walls is initiated and the reduced acceptor (HVf) is produced. The different steps involved in this overall reaction are presented in Figure 3. The excited sensitizer transfers an electron to HV2+ in the primary event. The oxidized sensitizer thus produced oxidizes a Ru located at the inner surface of the vesicle and thereby the separation of the intermediate photoproducts is assisted (14). The further oxidation of EDTA regenerates the sensitizer and consequently the separation of the reduced species, HVi, from the oxidized product is achieved. In this system the basic principle of a vectorial electron transfer across a membrane is demonstrated. However, the quantum yield for the reaction is rather low (0 4 X 10 ). 

T. Ito, 1. Yamazaki, N. Ohta, External Electric Field Effect on Interlayer Vectorial Electron Transfer from Photoexcited Oxacarbocyanine to Viologen in Langmuir-Blodgett Films , Chem. Phys. Lett., 277, 125 (1997)... 

Recently, de novo-synthesized four-helix polypeptides were applied to mimic functions of cytochrome b and to tailor layered cross-linked electrocatalytic electrodes. A four-helix bundle de novo protein (14728 Da) that includes four histidine units in the respective A -helices was assembled on Au electrodes (Figure 22A). Two units of Fe(III)-protoporphyrin IX were reconstituted into the assembly to yield a vectorial electron-transfer cascade [157]. The de novo-synthesized protein assembly forms affinity complexes with the cytochrome-dependent nitrate reductase (NR) and with Co(II) protoporphyrin IX-reconstituted myoglobin [158]. The resulting layered complex of Fe(III) de novo protein-NR or Fe(lll)-de novo protein-Co(II)-reconstituted myoglobin was cross-linked with glutaric dialdehyde to yield electrically contacted electrocatalytic electrodes. The Fe(lll)-de novo protein-NR electrode assembly was applied for the electrocatalyzed reduction NO3 to NOt" and acted as an amperometric sensor (Figure 22B). The Fe(III)-de novo... 

Figure 25. (A) Vectorial electron transfer in the two-heme-reconstituted de novo protein molecules organized as a monolayer at an electrode surface. (B) Transient current recorded with the two-heme reconstituted de novo protein monolayer during the double-potential step chronoamperometric experiment. The potential steps from -0.2 to -0.5 V (vs. SCE) to reduce the hemes in the protein, and after 70 ms the potential steps back, from —0.5 to —0.2 V, to oxidize the reduced hemes. The experiment was performed in 0.1 M phosphate buffer, pH 7.0, under argon.

Vectorial electron transfer switchable electron transfer amplified electron transfer... 

Early examples in the 1980s were aimed at the design of composite systems for photoelectrolytic generation of H2. Thus, Nafion and Si02 were used as supports for coprecipitated ZnS and CdS for photoassisted HER from aqueous sulfide media [407]. Subsequent work has addressed the mechanistic role of the support in the photoassisted HER [408]. Vectorial electron transfer was demonstrated in bipolar Ti02-Pt or CdSe-CoS photoelectrode panels arranged in series arrays for the photodecomposition of water to H2 and O2 [409, 410]. 

Photosensitized vectorial electron transfer from the (morpholine)ethenesulfonate anion (MES ) to 1,5-anthraquinone disulfonate (AQDS ) mediated by dibutyl (BTDB) or diethyl (BTDE) esters of 2,l,3-benzothiadiazole-4,7-dicarboxylic acid solubilized in CTAB micelles has also been reported [73], The proposed mechanism of BTD-mediated electron transfer is shown in Figure 7, where reductive quenching of photoexcited BTD by interfacially adsorbed MES is followed by electron transfer from reduced BTD to the AQDS , recycling the sensitizer. 

Figure 3. Fiincvional organisation oFpholosyslem I (in protein complexes contained in the thylakoid membrane. Excitation energy is harvested by chlorophyll (Chi) and carotenoids (Car) molecules and transfered to the special pair (Chlj). Vectorial electron transfer across the membrane takes place front excited Chi to plastoqutnone (pQ) via phcopliitin (Ph) and quinone (Q) electron mediators.

An important aspect of the function of photosynthetic complexes is their asymmetric arrangement in respect to the membrane and to the external and internal phases of the cellular compartments. This arrangement allows the catalysis of vectorial electron transfer and the performance of electrical work by promoting charge separation across the membrane dielectric barrier. It allows also in some cases the net translocation of protons across the membrane. These two processes are at the basis of the mechanism of energy conservation in photosynthesis coupled to the formation of ATP, which is added, in oxygenic photosynthesis, to the conservation of redox energy in the form of reduced pyridine nucleotide coenzymes. 

Nakashima, N., Nakanishi, X., Nakatani, A., Deguchi, Y., Murakanu, H., Sagara, X., and Irie, M. Photoswitching of a vectorial electron transfer reaction at a diarylethene mothfied electrode. Chem. Lett. 1997, 591-592. 

A further system providing photoswitchable redox-activated properties with amplification features via a secondary electrocatalytic vectorial electron transfer reaction has been exemplified by diarylethene (45) molecules incorporated into a long-chain thiol monolayer adsorbed on a Au electrode due to hydrophobic interactions [85]. In the closed isomeric state (45a), the monolayer demonstrates well-defined reversible cyclic voltammetry, whereas the open (45b)-state is completely redox-inactive. The electrochemically active 45a-state provides electrocatalytic reduction of Fe(CN)g-, thus enabling a vectorial electron cascade that amplifies the photonic input. 

Many other supermolecules containing carotenoids have been synthesized and their photochemistry characterized (Osuka et al., 1990 Moore et al 1994 Cardoso et al, 1996), but not yet investigated with EMR. One of these, a carotenoid- porphyrin-quinone triad has been used to demonstrate light-induced vectorial electron transfer across an artificial photosynthetic hposome membrane with subsequent production of ATP catalyzed by FoF -ATPase (Steinberg-Yfrachetal., 1998). 

Most part of the studies of red-ox processes involving a series of vectorial electron-transfer reactions between cofactors iimnobilized in membrane protein often make use of substances soluble in solution, which are able to either mediate the electron transfer to an electrode or undergo selective electron transfer with one of the components of the chain in order to isolate single steps of the series. The choice of the more suitable mediators to use in the different systems is based on several considerations the red-ox potential, the capabihty of a fast equilibration with the protein and (evenmally) with the electrode, the apdmde to difluse both in the aqueous phase or in the protein envitomnent and not to chemically react with the biological red-ox component. ... 

In this paper we briefly review the protonation associated with the singly reduced quinones (3-5) to be followed by recent work on the protonation of the doubly reduced secondary quinone, Qb". The proto-nation associated with Qa and Qi is important for vectorial electron transfer from the primary to the secondary quinone it affects the energy difference between QaQb md Qa91 The main contribu-... 

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