Artificial Reaction Centres

Whilst these approaches are extremely interesting scienhfically, to be of any real commercial value in solar energy utilisation the stability of the systems must advance by orders of magnitude. 

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The assembly of molecular components for light harvesting and charge separation in artificial photosynthetic systems is of current interest. To mimic the multistep electron transfer in natural systems, Imahori et al. prepared a mixed SAM that combines an artificial antenna system (pyrene) with an artificial reaction centre (porphyrin), to examine the possibihty of photoin-duced energy transfer in two-dimensional assemblies (Fig. 29, 20) [27,147]. The ratio of porphyrin pyrene in the mixed SAM estimated from the absorption spectra on the gold surface is significantly lower than that of the solution from which the SAM was prepared. The strong tz-tz interaction of... 

G. Steinberg-Yfrach, P. A. Liddell, S. C. Hung, A. L Moore, D. Gust, T. A. Moore, Conversion of Light Energy to Proton Potential in Liposomes by Artificial Photosynthetic Reaction Centres , Nature 1997,385,239-241. 

However, the duality of substituent constants and the attempt to deal with crossconjugation by selecting a+, a or o in any given case is somewhat artificial. The contribution of the resonance effect of a substituent relative to its inductive effect must in principle vary continuously as the electron-demanding quality of the reaction centre is varied, i.e. the extent to which it is electron-rich or electron-poor. A sliding scale of substituent constants would be expected for each substituent having a resonance effect and not just a pair of discrete values a+ and a for —R, or a and a for +R substituents83. 

The study of artificial photosynthesis has been the subject of ongoing attention for many years now due to the need for sustainable energy resources. In natural photosynthesis a lightharvesting antenna system with a large optical cross-section (for example the LH2 complex) absorbs a photon that is funneled by energy transfer (ET) to the reaction centre [1-3]. Excellent candidates to mimic the natural antenna system are molecules that efficiently absorb light and are able to transfer the captured energy to other parts of the molecule. Molecules based on Zn and free-base porphyrins are examples of compounds that can be used as models for the LID complex [4]. 

Moore, T.A., Moore, A.L. and Gust, D. 2002. The design and synthesis of artificial photosynthetic antennas, reaction centres and membranes. Phil. Trans. R. soc. Lond. B 357, 1481-1498. 

Non-Forster fluorescence quenching of trans-etiochlorin by magnesium oc-taethylporphine in phosphatidylcholine vesicles gives evidence for a statistical pair energy trap. Energy transfer also occurs in the excited singlet manifold of chlorophyll. " The photophysics of bis(chlorophyll)-cyclophanes, models of photosynthetic reaction centres, have been explored for use in artificial photosynthesis.Picosecond time-resolved energy transfer in phycobilosomes have also been studied with a tunable laser. The effect of pH on photoreaction cycles of bacteriorhodopsin, " the fluorescence polarization spectra of cells, chromatophores, and chromatophore fractions of Rhodospirillum rubrum, and a brief review of the mechanism and application of artifical photosynthesis are all relevant to the subject of this Chapter. 

Figure 25. (A) Structure of an artificial photosynthetic reaction center, the molecular triad C-P-Q, and the proton-shuttling quinone, Qsl (B) Schematic diagram showing orientation of the triad In the liposome and the sequence of events after photoexcitation (see table at right and text for details) (C) Fluorescence excitation spectra of the pH-indicator dye pyraninetrisulphonate as a measure of the concentration of the protonated form of the indicator dye (D) Fluorescence excitation-band intensity as a function of irradiation time in the absence and in the presence of FCCP. Figures adapted from Steinberg-Yfrach, Liddeii, Hung, (AL) Moore, Gust and (TA) Moore (1997) Conversion of light energy to proton potential in liposomes by artificial photosynthetic reaction centres. Natu re 385 239-241.

Steinberg-Yfrach, G., Liddell, P. A., Hung, S.-C., Moore, A. L., Gust, D., Moore, T. A. (1997). Conversion of light energy to proton potential in liposomes by artificial photosynthetic reaction centres. Nature, 385 239. 

SYNTHESIS, RECONSTITUTION AND EPR SPECTROSCOPY OF ARTIFICIAL QUINONE ELECTRON ACCEPTORS IN REACTION CENTRES FROM PURPLE BACTERIA... 

Secondary photosynthetic electron transfer was detected after addition of artificial electron acceptors and donors, including a quinone dependent activit y on adding decylplastoquinone. An equivalent activity was obtained on addition of the much more hydrophobic plastoquinone-9 molecule but only when a reconstitution procedure was adopted in which a diacyl glycerolipid extract of thylakoids was used. Thermoluminescence measurements showed that reconstitution with plasto-quinone-9 and lipid involved the binding of the quinone to some of the reaction centres in a preparation. Thus a limited reconstitution of quinone-reaction centre interactions could be achieved without proteins other than those already present in the isolated reaction centre. 

Electron flow beyond the pheophytin has also been demonstrated in the presence of the artificial electron acceptor, silicomolybdate [SiMo] (6,8). More recently, we have reported that in the presence of SiMo, P680 and the radical of the monomeric chlorophyll can be detected at cryogenic temperatures by esr (9). In a small proportion of centres, electron transfer from the tyrosine radical to P680" also occurred (9). Here, we report upon the reconstitution of the reaction centre complex with the exogenous quinones, DPQ and DBMIB. 

Transient absorption spectroscopy has been used to study isolated Photosystem 2 (PS2) reaction centres stabilised by the use of anaerobic conditions. In the absence of added artificial electron donors and acceptors, the light induced electron transfer properties of the reaction centre are restricted to the formation of the radical pair P680+Pheophytin and charge recombination pathways from this state [1]. This charge recombination has been observed to produce a 23% yield of a chlorophyll triplet state [1]. Attempts to reconstitute these particles with quinone have until now been limited to the observation of a steady state, quinone-mediated photoreduction of the cytochrome b-559 [2]. 

The photosystem 11 reaction centre complex isolated in Triton X-100 (1,2) does not retain the intrinsic quinone acceptors, and Q, but does show electron transfer to the artificial electron acceptor, silicomolybdate (3). In the presence of this acceptor the light minus dark difference spectrum, which we attribute to oxidation of P680, includes a distinct shoulder at about 670 nm (4). Reaction centres isolated in Triton X-100 and investigated under aerobic conditions have been found to be very susceptible to photodamage (4,5). Here we have used maltoside-exchanged reaction centres and anaerobic conditions to investigate in more detail the photoaccumulation of oxidised chlorophyll donors and their absorption characteristics. 

Photosystem II (PSIl) membranes which are reasonably pure and stable can be isolated from chloroplasts and cyanobacteria (2,3). These PSIl membranes contain the antenna pigments, reaction centre chlorophylls, O2 evolution complex and the two polypeptides D2 and Di, which bind the primary quinone acceptors Qa and Qb respectively. They are, therefore, capable of light-induced O2 evolution in the presence of artificial PSIl acceptors such as dimethyl benzoquinone (DMBQ) or dichlorobenzoquinone (DCBQ). The PSIl membranes can be deposited on TI02 without appreciable loss of photosynthetic activity. 

In Chapter 2 we saw that hydrogenases of the three basic types are made by organisms that have existed over billions of years. In Chapter 6, the strnctnres of the proteins were laid ont in three dimensions. In Chapter 7 we saw that the metal centres of the protein could exist in particnlar chemical states. We can now begin to understand how the hydrogenases catalyse their reactions with snch extraordinary efficiency. Furthermore we ask, can similar catalysts be constrncted artificially ... 

Fig. 3.3. Tentative mechanism of reduction of dioxygen. The scheme shows some of the more significant reaction steps at the haem iron-Cug centre of cytochrome oxidase. The reaction may be initiated by delivery of dioxygen to the reduced enzyme (in anaerobiosis top of figure). An initially formed oxy intermediate is normally extremely short-lived, but can be stabilised and identified in artificial conditions (see Refs. 92, 99,129, 134). Concerted transfer of two electrons from Fe and Cu to bound dioxygen yields a peroxy intermediate. This, or its electronic analogue, is stabilised in the absence of electron donors (ferrocytochrome a and/or reduced Cu ), and has been termed Compound C [129,130,132). It may also be observed at room temperature, and is then probably generated from the oxidised state by partial oxidation of water in the active site, in an energy-linked reversed electron transfer reaction [29] (see also Refs. 92, 99). Also the ferryl intermediate [92,99,100] has been tentatively observed in such conditions [29]. In aerobic steady states the reaction is thought to involve the cycle of intermediates in the centre of the figure (dark frames). The irreversible step is probably the conversion of g = 6 (see Refs. 98, 133) to peroxy .

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