Reaction center mimics

Sherman B, Vaughn M, Bergkamp J, Gust D, Moore A, Moore T (2014) Evolution of reaction center mimics to systems capable of generating solar fuel. Photosynth Res 120(l-2) 59-70. doi 10.1007/sl 1120-013-9795-4... 

El-Khouly, M. E., Ju, D. K., Kay, K., D Souza, F. andFukuzumi, S. Supramolecular tetrad of subphthalo-cyanine-triphenylamine-zinc porphyrin coordinated to fullerene as an antenna-reaction-center mimic Formation of a long-lived charge-separated state in nonpolar solvent. Chem. Eur. J. 16, 6193-6202, 2010. 

Most of the interest in mimicing aspects of photosynthesis has centered on a wide variety of model systems for electron transfer. Among the early studies were experiments involving photoinduced electron transfer in solution from chlorophyll a to p-benzoquinone (21, 22) which has been shown to occur via the excited triplet state of chlorophyll a. However, these solution studies are not very good models of the in vivo reaction center because the in vivo reaction occurs from the excited singlet state and the donor and acceptor are held at a fixed relationship to each other in the reaction-center protein. 

This review highlights recent studies of synthetic, covalently linked multicomponent molecular devices which mimic aspects of photosynthetic electron transfer. After an introduction to the topic, some of the salient features of natural bacterial photosynthetic reaction centers are described. Elementary electron transfer theory is briefly discussed in order to provide a framework for the discussion which follows. Early work with covalently linked photosynthetic models is then mentioned, with references to recent reviews. The bulk of the discussion concerns current progress with various triad (three-part) molecules. Finally, some even more complex multicomponent molecules are examined. The discussion will endeavor to point out aspects of photoinitiated electron transfer which are unique to the multicomponent species, and some of the considerations important to the design, synthesis and photochemical study of such molecules. 

These two molecules were the antecedents of a variety of oligomeric porphyrin and chlorophyll derivatives which were constructed as mimics of the reaction center special pair. Many of these systems exhibit interesting optical properties which allow modelling of the electronic interactions within the special pair and/or the antenna function of chlorophyll, which involves singlet-singlet energy transfer... 

Thus, 26 evidently mimics the BPh to QA to Q electron transfer sequence that occurs in the natural reaction center. One might wonder, however, why the lifetime of the final P+-Q-Q 7 state is so short, given the relatively long methylene chains joining the moieties and the much longer lifetimes found for the D-D-A triads. A likely possibility is that the flexible methylene chains allow the molecule to fold back on itself, so that the cation and anion are rather close together and charge... 

To take up the questions raised in this book, one would need to find papers with titles such as Twelve Intermediate Steps Leading to the Bacterial Photosynthetic Reaction Center, A Proto-Cilium Could Generate a Power Stroke Sufficient to Turn a Cell by Ten Degrees, Intermediates in Adenosine Biosynthesis Effectively Mimic Adenosine Itself in RNA Function, and A Primitive Clot Made of Randomly Aligned Fibers Would Block Circulation in Veins Smaller Than 0.3 Millimeters. But the papers are missing. Nothing remotely like this has been published. 

Nevertheless, other chromophores have been investigated and they have provided interesting insights, particularly porphyrin and Cso groups, since these serve as useful mimics of the cofactors present in the photosynthetic reaction center (Figure 37). Electron transfer involving porphyrins and fullerenes will be presented in more detail elsewhere in this Handbook Volume III, Part 2, Chapter 2 and Volume II, Part 1, Chapter 5 respectively), and so only a brief discussion is presented here. An excellent overview of photoinduced ET processes in Cso-based multichromophoric systems has been produced previously [116]. 

The fact that the pKa of the various oxidation states of quinones differ dramatically has been exploited in a recent design of artificial systems that mimic the light-driven transmembrane proton transport characteristic of natural photosynthesis [218]. Triad artificial reaction centers structurally related to 41 were vectorially inserted into the phospholipid bilayer of a liposome (vesicle) such that the majority of the quinone moieties are near the external surface of the membrane, and the majority of the carotenoids extend inward, toward the interior surface. The membrane... 

Viologens have been used as covalently linked quenchers for Cu(I) bipyridine chromophores, leading to fast (<10 ns) charge separation and remarkably slow (30 ns to 2 ps, depending on solvent) charge recombination [174]. More complex systems, similar to (5) but with two viologens on the same bypyridine of the Ru(II) chromophore, have been designed to mimic the presence of two ET in the reaction center of natural photosynthesis [175]. 

In spite of this success, dyads like 1 suffer from a major limitation as mimics of the natural electron transfer process. The very structural and electronic features which ensure rapid photoinitiated electron transfer, and consequently a high quantum yield, in these molecules also favor rapid charge recombination (step 3 in Figure 2). Thus, the P -Q " state lives at most a few hundred picoseconds in solution. The P-Q systems, and indeed other dyad-type artificial photosynthetic molecules, are unable to reproduce the long-lived charge separation characteristic of the reaction center. The stored energy is quickly lost as heat. 

Chemists are studying the structure and kinetics of the photosynthetic reaction center both to understand the fundamentals of this important natural process and to design new materials that mimic nature s ability to harvest light energy at such high efficiency. Artificial photosynthesis may lead to carbon-based materials that will replace the silicon collectors in solar cells in the 21st century. This will help reduce human dependence on stored fossil fuels as energy sources in the future. 

Figure 18. In [2]catenane 20 + [86], upon excitation of its [Rutbpy), p moiety, a very fast electron transfer process to a bipyridinium unit occurs. Owing to the catenane structure, the two bipyridinium units do not possess the same reduction potential (half-wave potential values versus SCE for the inside and outside units are indicated) such a redox asymmetry could mimic that of the cofactors in the bacterial photosynthetic reaction center.

In 1970, Dan Reed, Tom Chaney and this author used a cytochrome-free, reaction-center complex from Rb. sphaeroides R-26 and tried to reconstitute it with mammalian cytochrome c in an attempt to mimic in vivo electron transfer. Although P870 can undergo rapid oxidation by a light flash, its rereduction is very slow, as expected, in the absence of efficient secondary donors, as shown in Fig. 12, upper row. However, P870 may be reduced very rapidly by an externally added redox mediator such as reduced PMS. For example, in the presence of 0.1 mM reduced PMS, P870 can be re-reduced in 36 /js. 

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