Photoinduced electron transfer, photosynthetic reaction center

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... 

In the study of the ultrafast dynamics of photosynthetic bacterial reaction centers, we are concerned with the photoinduced electron transfer [72]... 

In making rotaxanes usable as parts of molecular devices and with the purpose of studying long range election transfer processes within large molecular systems of well controlled geometries, the introduction of photoactive and electroactive compounds has been a valuable development. Photoinduced electron transfer between porphyrin species has a particular relevance to the primary events occurring in bacterial photosynthetic reaction center complexes, and so is a well studied phenomenon. 

Photoinduced Electron Transfer in Electron Donor-Acceptor Linked Molecules Mimicking the Photosynthetic Reaction Center... 

In the natural photosynthetic reaction center, ubiquinones (QA and QB), which are organized in the protein matrix, are used as electron acceptors. Thus, covalently and non-covalently linked porphyrin-quinone dyads constitute one of the most extensively investigated photosynthetic models, in which the fast photoinduced electron transfer from the porphyrin singlet excited state to the quinone occurs to produce the CS state, mimicking well the photo synthetic electron transfer [45-47]. However, the CR rates of the CS state of porphyrin-quinone dyads are also fast and the CS lifetimes are mostly of the order of picoseconds or subnanoseconds in solution [45-47]. A three-dimensional it-compound, C60, is super-... 

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]. 

Photoinduced electron transfer reactions have been studied in the dyads 44(6), 45(9), and 46(11). The 6-bond porphyrin-quinone dyad 44(6) serves as a model for aspects of ET in the photosynthetic reaction center, since congeners of these chromophores are present in the center as cofactors (Figure 37). Photoinduced ET in... 

Photoinduced intramolecular electron transfer reactions in triad assemblies have been extensively examined with regard to mimicking the photosynthetic center. Novel bridging ligands L25a and L25b were prepared, in which the naphthalene bis(dicarboximide) unit is linked by two bidentate (2-pyridyl)benzimidazole (Ll).55 Under photoirradiation, photoinduced electron transfer from the Ru(bpy)2 moiety to the diimide site takes place. 

Among the systems proposed as models for the photosynthetic reaction center, supramolecular assemblies in which Ru(II)-polypyridine complexes and 4,4 -bipyridinium units are held together noncovalently in threaded and interlocked structures have been extensively studied [43, 82-88]. In such assemblies, connections between the molecular components rely on charge transfer interactions between the electron acceptor bipyridinium units and aromatic electron donor groups (Fig. 3). For instance, in the various pseudorotaxanes formed in acetonitrile solution at 298 K by the threading of cyclophane 4 + by the dioxybenzene-containing tethers of 192+ (Fig. 17) [84], an efficient photoinduced electron... 

Fig. 15. Plot ofthe extent of absorbance change due to P7007P430" recombination measured at 695 nm vs. the redox potential of 14 quinones and 7 non-quinone carbonyl compounds, the fluorenones (individual compounds are identified below the plot). The solid curve is the theoretical, one-electron Nernst curve centered near the redox potential of FeS-X in vivo. Data adapted from Itoh and Iwaki (1992) Exchange ofthe acceptor phylloquinone by artificial quinones and fluorenones in green plant photosystem I photosynthetic reaction center. In N Malaga, T Okada and H Masuhara (eds) Dynamics and Mechanism of Photoinduced Transfer and Related Phenomena. p.533. Elsevier,...

Synthetic carotenoids incorporated into multicomponent sutructures such as triad 8 have been used as spectroscopic labels to follow the flow of triplet energy and the spin dynamics in model photosynthetic reaction centers. Excitation of the porphyrin moiety in 8 yields C- P-C,.q which decays by photoinduced electron transfer to yield - "- , . This state rapidly evolves into the final charge separated state, C -P-(Liddell et al., 1997). The fact that both electron transfer steps in 8 occur even in a glassy matrix at 77K has made it possible to observe some unusual... 

Photoinduced electron and energy transfer are fundamental processes m nature In photosynthetic organisms, photoinduced electron transfer that induces conversion of light into chemical energy begins when photonic excitation reaches the so-called reaction center (RC) Usually. 

The simplest supramolecular species capable of performing such type of process are covalently-linked three-component systems ("triads"). Two possible schemes for charge separating triads are shown in Fig. 5. Although the scheme in Fig. 5b is reminiscent of the natural photosynthetic reaction center, that of Fig. 5a seems to be more popular in the field of artificial triad systems. The functioning principles are shown in an orbital-type energy diagram in the lower part of Fig. 5. In both cases, excitation of a chromophoric component (1) is followed by a primary photoinduced electron transfer to a primary acceptor (2). This is followed by a secondary thermal electron transfer process (3) electron transifer from a donor component to the oxidized chromophoric component (case a), or electron transfer from the primary acceptor to a secondary acceptor component (case b). The primary process competes with excited-state... 

Figure 19.10. Electron Chain in the Photosynthetic Bacterial Reaction Center. The absorption of light by the special pair (P960) results in the rapid transfer of an electron from this site to a bacteriopheophytin (BPh), creating a photoinduced charge separation (steps 1 and 2). (The asterisk on P960 stands for excited state.) The possible return of the electron from the pheophytin to the oxidized special pair is suppressed by the "hole" in the special pair being refilled with an electron from the cytochrome subunit and the electron from the pheophytin being transferred to a quinone (Q ) that is farther away from the special pair (steps 3 and 4). The reduction of a quinone (Qg) on the periplasmic side of the membrane results in the uptake of two protons from the periplasmic space (steps 5 and 6). The reduced quinone can move into the quinone pool in the membrane (step 7).

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