Reaction center artificial

These aspects of protein adsorption and assembly at layered materials are essential prerequisites for the construction of assemblies consisting of protein-pigment complexes or artificial reaction centers housed in protein assemblies. 

From a functional point of view, this important property can be readily built into low molecular weight chromophore assemblies acting as artificial reaction centers (coordination compounds, the population of CT states is directly related to the concept of light-induced charge separation in photosynthesis. Whenever such CT states are photoreactive and lead to the formation of the same kind of permanent redox products as observed in photosynthesis, the most essential features of the primary light reactions have been successfully duplicated. In a more strict sense, this is of course only true, if actinic red or NIR-light of comparable wavelength is absorbed by both the natural and artificial photosynthetic systems. 

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... 

Artificial photosynthesis was initiated by flash excitation of liposomes containing the triad and Qs, the quinone shuttle, using 5-ns, 430-nm flashes centered on the Soret band ofthe porphyrin. The sequence of events taking place upon photoexcitation is shown in Fig. 25 (B) right, where step involves photoexcitation of the artificial reaction center, P, the porphyrin, followed by charge separation and electron transfer, forming Q -P-C, which can be detected by the absorbance inaease due to the radical cation C. The yield ofQ -P-C was 0.1 and its lifetime was -110 in the absence ofQs and -60 ns in liposomes containing Qs. 

Although carotenoid triplet states are not formed in appreciable yield by the usual intersystem crossing pathway, they can be produced via triplet-triplet energy transfer from other species or by radical pair recombination processes in artificial reaction centers (Gust et al., 1 2 Liddell et al 1997 Carbonera et al., 1998). Thus, their transient absorption characteristics are well known (Mathis and Kleo, 1973 Bensasson et al., 1976). Typically, they have strong absorption maxima at about 540 nm (Cr - 1 = 10 ... 

The excited state difference spectrum of the artificial reaction center (pheophytin o-pyrochlorophyll a dimer) is shown in Fig. 19. The salient features of this spectrum are a peak at 800 nm, bleaching at both 700 and 680 nm and a broadband absorbance between 650 and 450 nm. To interpret this spectrum one must compare it to spectra of the excited singlet states of the dimer and pheophytin, and the cation of the dimer and the anion of pheophytin (see Figs. 20 and 21). ... 

Triads and tetrads involving Cgo, porphyrins, and Fc have been synthesized, investigated, and reviewed by Imahori et Among these systems representing an artificial reaction center, the... 

Luo, C., Guldi, D. M., Imahori, H., Tamaki, K. and Sakata, Y. Sequential energy and electron transfer in an artificial reaction center Formation of a long-lived charge-separated state. J. Am. Chem. Soc. 122, 6535-6551, 2000. 

However, the duality of substituent constants and the attempt to deal with crossconjugation by selecting cr+, a or a 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 center is varied, i.e. whether 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 o and a for + R substituents55. 

Selection of an active-site model almost always leads to truncations of the hydrogen-bond network. Upon optimization of the active-site structure, this may lead to the formation of artificial hydrogen bonds that disrupt the structure. Freezing selected coordinates in the active-site model can prevent some of these hydrogen bonds to form. Another remedy could be to include more residues around the metal center, but larger QM models are much more expensive and there will probably still be truncated hydrogen bonds, although further away from the reaction center. 

Fig.4.30 Immobilization ofthe bacterial photosynthetic reaction center on tailored three-dimensional wormlike mesoporous W03-Ti02 films for artificial photosynthetic systems (A) procedure of film coating (B) proposed scheme of photoelectric conversion. Reprinted with permission from [229], Y. Lu et at., Langmuir 2005, 21, 4071. 2005, American Chemical Society.

Figure 1. Schematic representation of the artificial photosynthetic reaction center by a monolayer assembly by A-S-D triad and antenna molecules for light harvesting (H), lateral energy migration and energy transfer, and charge separation across the membrane via multistep electron transfer (a) Side view of mono-layer assembly, (b) top view of a triad surrounded by H molecules, and (c) energy diagram for photo-electric conversion in a monolayer assembly.

In the present review, first we will describe how to fabricate artificial photosynthetic reaction center in nanometer scales by making use of phase separation in mixed monolayers of hydrocarbon (HC) and fluorocarbon (FC) amphiphiles [2,5,20-26] as shown in Fig. 2b [3]. The phase separated structures were studied by SPMs such as AFM, SSPM, and scanning near-field optical/atomic force microscopy (SNOAM) [27-33] as well as a conventional local surface analysis by SIMS [3,5], The model anionic and cationic HC amphiphilic... 

Figure 4. Detection of the change in photo-induced surface dipole moments in highly oriented A-S-D triads in artificial photosynthetic reaction centers as the local surface potential change in nano-domains measured by SSPM.

Reaction of 217 with Cjq leads to the amino-protected porphyrin-fulleropyrroli-dine, which can easily be deprotected to the corresponding amine [229, 277]. By further functionalization via amide coupling an easy access to extended donor-acceptor systems is possible. A carotene-porphyrin-fullerene triad was prepared by reaction of the amine with the appropriate carotene acid chloride. The motivation for the synthesis of all these donor-acceptor systems is the attempt to understand and imitate the photosynthetic process. On that score, a model for an artificial photosynthetic antenna-reaction center complex has been achieved by attaching five porphyrin cores in a dendrimer-like fashion to the fullerene [242]. 

Imahori H, Guldi DM, Tamaki K et al (2001) Charge separation in a novel artificial photosynthetic reaction center lives 380 ms. J Am Chem Soc 123 6617-6628... 

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