Photoinduced electron transfer photosynthesis

The area of photoinduced electron transfer in LB films has been estabUshed (75). The abiUty to place electron donor and electron acceptor moieties in precise distances allowed the detailed studies of electron-transfer mechanism and provided experimental support for theories (76). This research has been driven by the goal of understanding the elemental processes of photosynthesis. Electron transfer is, however, an elementary process in appHcations such as photoconductivity (77—79), molecular rectification (79—84), etc. 

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. 

Ganguly, T. Pal, S. K. 2006. Photoinduced electron transfer research to build model compounds of artificial photosynthesis and solar energy conversion. J. Chinese Chem. Soc. 53 219-226. 

Organized molecular assemblies containing redox chromophores show specific and useful photoresponses which cannot be achieved in randomly dispersed systems. Ideal examples of such highly functional molecular assemblies can be found in nature as photosynthesis and vision. Recently the very precise and elegant molecular arrangements of the reaction center of photosynthetic bacteria was revealed by the X-ray crystallography [1]. The first step, the photoinduced electron transfer from photoreaction center chlorophyll dimer (a special pair) to pheophytin (a chlorophyll monomer without... 

Photoinduced electron transfer (PET) is often responsible for fluorescence quenching. This process is involved in many organic photochemical reactions. It plays a major role in photosynthesis and in artificial systems for the conversion of solar energy based on photoinduced charge separation. Fluorescence quenching experiments provide a useful insight into the electron transfer processes occurring in these systems. 

The amide functionality plays an important role in the physical and chemical properties of proteins and peptides, especially in their ability to be involved in the photoinduced electron transfer process. Polyamides and proteins are known to take part in the biological electron transport mechanism for oxidation-reduction and photosynthesis processes. Therefore studies of the photochemistry of proteins or peptides are very important. Irradiation (at 254 nm) of the simplest dipeptide, glycylglycine, in aqueous solution affords carbon dioxide, ammonia and acetamide in relatively high yields and quantum yield (0.44)202 (equation 147). The reaction mechanism is thought to involve an electron transfer process. The isolation of intermediates such as IV-hydroxymethylacetamide and 7V-glycylglycyl-methyl acetamide confirmed the electron-transfer initiated free radical processes203 (equation 148). 

Wasielewski MR (1992) Photoinduced electron-transfer in supramolecular systems for artificial photosynthesis. Chem Rev 92 435... 

Green plant photosynthesis, which feeds the world, runs on photoinduced electron transfer (PET). 121 This principle was developed in chemical contexts by Albert Weller over three decades ago, 131 and became adapted for use in fluorescent switching contexts in the late 1970s and early 1980s. 14-211 A general design principle emerged soon afterwards. 221... 

Kurreck, H, Eiger, G., Von Gersdorff, J., Wiehe, A., and Moebius, K. (1998) EPR studies on photoinduced electron transfer in triad model compounds of photosynthesis Applied Magnetic Resonance 14, 203-215. 

Wasielewski, M.R. (2002) High time resolution Q-band EPR study of sequential electron transfer in a triad oriented in a liquid crystal, J. Phys. Chem. A 106,1933-1937 Wasielewski, M.R. (1992) Photoinduced electron transfer in supramolecular systems of artificial photosynthesis, Chem. Rev. 92, 435-461. 

Wasielewski s research interests comprise photoinduced electron transfer and charge transport in organic molecules and materials, artificial and natural photosynthesis, self-assembly of nanoscale materials, spin dynamics of multispin organic molecules, materials for molecule-based optoelectronics and spintronics, and time-resolved optical and magnetic resonance spectroscopy. His research has resulted in over 300 publications. Dr. Wasielewski was elected a fellow of the American Association for the Advancement of Science in 1995, and has held numerous distinguished lectureships and fellowships. Among Wasielewski s recent awards are the 2004 Photochemistry Research Award of the Inter-American Photochemical Society and the 2006 James Flack Norris Award in Physical Organic Chemistry of the American Chemical Society. 

Photoinduced electron transfer, using the presently described molecular systems, remains an interesting and promising topic—particularly in relation to charge separation and ultimately artificial photosynthesis. Nevertheless, a new area has recently emerged, which is that of multicomponent molecular sets undergoing controlled motions, under the action of an external signal. There is no doubt that this... 

Figure 29. Z-scheme of the photoinduced electron-transfer and dark enzymatic reactions operating in the photosynthesis of green plants. Mn = Mn-containing enzyme complex catalyzing water oxidation and O2 evolution Chi a and Chi b = photoactivated primary electron acceptors in photosystems I and II, respectively A and I = primary electron donors in photosystems I and II, respectively ADP = adenosine diphosphate ATP = adenosine triphosphate.

Recently a number of covalently linked porphyrin-quinone systems such as IS (Malaga et al., 1984) or 16 (Joran et al., 1984) have been synthesized in order to investigate the dependence of electron-transfer reactions on the separation and mutual orientation of donor and acceptor. These systems are also models of the electron transfer between chlorophyll a and a quinone molecule, which is the essential charge separation step in photosynthesis in green plants. (Cf. Section 7.6.1.) Photoinduced electron transfer in supra-molecular systems for artificial photosynthesis has recently been summarized (Wasielewski, 1992). 

As for selective electron transport in LUVs, many assays have been developed early on in studies directed toward artificial photosynthesis [37]. An elegant recent example applies the HPTS assay to active transport [17]. Namely, photoinduced electron transfer is detected as internal pH increase due to proton consumption during the reduction of a water-soluble quinone trapped together with HPTS within LUVs. 

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