Natural reaction center

Energy transfer studies of 11 and related molecules demonstrated that the carotenoid moiety is active in antenna function (singlet-singlet energy transfer) and photoprotection (triplet-triplet energy transfer) in these molecules, just as it is in natural reaction centers [51, 62]. 

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... 

As mentioned above, the energy levels in Fig. 8 have been estimated from cyclic voltammetric studies and may be slightly in error in that they do not explicitly correct for coulombic stabilization of intermediates such as C-PA-PB -Q 7. If such stabilization were to drop the energy of C-PA-P -Qr below that of C-PA -Pb-Q t, then the formation of the final C+-PA-PB-Q 7 state might be thought of as a single step from C-PA-PB -Q7 in which the porphyrin PA facilitates the transfer via superexchange [8, 26]. As was noted earlier, the accessory bac-teriochlorophyll in the natural reaction center may play a similar role. 

It is clear, then, that a more sophisticated artificial photosynthetic system must include some ordering principle which imposes constraints upon the spatial relationships of the small organic molecules, and perhaps modifies their environment in other ways as well. In the natural reaction center, the membrane protein provides this function. In artificial systems, covalent linkages can perform a similar service. 

C-P-Q Triad Molecules. As discussed above, the natural reaction center has solved the problem of energy loss due to rapid charge recombination by employing a multistep electron transfer strategy. The same strategy may be applied to the porphyrin-quinone type systems. As we pointed out in 1982 [28], this requires the addition of a secondary electron donor or acceptor moiety. This strategy came to fruition in 1983 when we reported the synthesis of carotenoid-porphyrin-quinone (C-P-Q) triad 2 [29, 30]. This molecule features porphyrin and quinone moieties similar to those found in 1, but a... 

In the early 1980s we designed an artificial photosynthetic reaction center that overcomes this problem by using a multistep electron transfer strategy such as that found in natural reaction centers (Moore et al, 1984). More recently, molecular triad 9 which consists of a porphyrin chromophore (P) bound covalently to a naphthoquinone derivative (NQ) and a carotenoid electron donor (C) was designed both to undergo multistep electron transfer and to organize... 

The remarkable efficiency of reaction-center photochemistry has encouraged the design and the study of synthetic models. Most research on artificial photosynthesis has been directed toward mimicry of the natural reaction center (RC). The center functions as a molecular-scale solar photovoltaic device that converts light energy into chemical energy that can be transported and stored for maintenance, growth, and... 

Collman et al. prepared many FeP/Cu model compounds modeling the enzyme active site. The most recently obtained supramolecular systems are remarkably similar to the natural reaction center, reproducing the Fe to Cu distance and their first coordination sphere (Figure A series of studies... 

The extension from triads to larger systems, tetrads and pentads, for photoinduced charge separation is currently the object of extensive research.In some cases, performances close to those of natural reaction centers have been reached. 

Deisenhofer, J., et al. Structure of the protein subunits in the photosynthetic reaction center of Rhodopseudomonas viridis at 3 A resolution. Nature 318 618-624, 1985. 

What molecular architecture couples the absorption of light energy to rapid electron-transfer events, in turn coupling these e transfers to proton translocations so that ATP synthesis is possible Part of the answer to this question lies in the membrane-associated nature of the photosystems. Membrane proteins have been difficult to study due to their insolubility in the usual aqueous solvents employed in protein biochemistry. A major breakthrough occurred in 1984 when Johann Deisenhofer, Hartmut Michel, and Robert Huber reported the first X-ray crystallographic analysis of a membrane protein. To the great benefit of photosynthesis research, this protein was the reaction center from the photosynthetic purple bacterium Rhodopseudomonas viridis. This research earned these three scientists the 1984 Nobel Prize in chemistry. 

Pedersen, P.L. Amzel, L.M. (1993). ATP syntheses. Structure reaction center, mechanism and regulation of one of nature s most unique machines. J. Biol. Chem. 268,9937-9940. 

In inert systems such as technetium and rhenium, ligand substitution reactions-including solvolysis-proceed under virtually irreversible conditions. Thus, the nature of the reaction center, the nature of the leaving group, and the nature and position of the other ligands in the complex affect the rates and activation parameters in a complicated manner. Most substitution reactions take place via interchange mechanisms. This is not too surprising when the solvent is water - or water-like - and where, in order to compete with the solvent,... 

AEba = —45 kJ mol 1 for the HO + SiH4 reaction and AEba = —43 kJ mol-1 in the reaction of hydrogen atom with water. The repulsion of the electron orbitals of the atoms forming the reaction center AER plays an important role in all the radical abstraction reactions. In the interaction of radicals with molecules the contribution of this repulsion ranges from 25 to 46 kJ mol-1. In reactions of molecules with hydrogen atoms the contribution is naturally smaller, varying from 8 to 16kJ mol-1. 

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