Reaction center model

Until a recent x-ray diffraction study (17) provided direct evidence of the arrangement of the pigment species in the reaction center of the photosynthetic bacterium Rhodopseudomonas Viridis, a considerable amount of all evidence pertaining to the internal molecular architecture of plant or bacterial reaction centers was inferred from the results of in vitro spectroscopic experiments and from work on model systems (5, 18, 19). Aside from their use as indirect probes of the structure and function of plant and bacterial reaction centers, model studies have also provided insights into the development of potential biomimetic solar energy conversion systems. In this regard, the work of Netzel and co-workers (20-22) is particularly noteworthy, and in addition, is quite relevant to the material discussed at this conference. 

The fullerenes, in particular Cgg, exhibit a variety of remarkable photophysical properties, making them very attractive building blocks for the construction of photosynthetic antenna and reaction center models (Table 1.6) [292-295],... 

Nowadays, most reaction center models carry suitable antenna pigments and acceptor groups and in effect are photosystem models. A typical example for a state-of-the-art system that incorporates many aspects of a photosystem consisted of a boron dipyrrin covalently linked to a zinc(II) porphyrin, which carried a suitably modified C60 derivative as axial ligand. Selective excitation of the boron dipyrrin as antenna pigment resulted in energy transfer to a zinc(II) porphyrin followed by electron transfer to the acceptor109. 

Fullerene-porphyrin architectures as photosynthetic antenna and reaction center models 02CSR22. 

Fig. 7. Three-dimensional structure of the photosynthetic reaction center of Rps. viridis. The a-helices are drawn as cylinders and the cofactors by black rectangles (in perspective). See text for other details. Adapted from the original color drawing by Prof. Jane Richardson of Duke University, based on atomic coordinates provided by Deisenhofer, Michel and Huber. Cf. the broad view of the Rp. viridis reaction-center model on the book cover and side view in Color Plate 1.

Color Plate 1. Side view of Rp. viridls reaction-center model. See book cover for the broad view of the same model. Both figures are kindly provided by Dr. Johann Deisenhofer. (See Chapter 2, Fig. 7.)... 

Another type of reaction center model system is formed when three pyrochlorophyll a molecules are joined via a symmetric triester linkage. Such a system is very interesting because of the proposed role of chlorophyll a as acceptor in photosystem l and the possible role of... 

The information available provides useful constraints on preliminary models of the tertiary structure. As we have previously pointed out (1,2), based on the reaction center model for which the structure is known (cf. 13), residues which are modified in inhibitor resistance strains are likely to map in the tertiary structure to the same volume as the catalytic site at which the inhibitor binds. Together with the heme ligands, which fix the relative positions of helices B and D, the resistance mutations suggest that helices A,D and E should be close at the cytoplasmic ends, while helices C,E, and F should be close at their periplasmic ends. Amphipathic helix cd (at it s N-terminal end) has several mutations on the same side of the helix, and these need to interface with the same volume. Orientation of the helices is indicated by their mutability moment. A preliminary model including these constraints, with helices A-H vertical has been suggested (2), but will obviously need refinement as new information on spatial relationships becomes available. 

Guldi, D. M. Fullerene-porphyrin architectures photosynthetic antenna and reaction center models. Chem. Soc. Rev. 31, 22-36, 2002. 

IV. Reaction Centers Modelling of Structure/Function-Relationship... 

Cluster models with linear free energy relationship s - Reaction center model 2) Periodic DFT models with linear free energy relationship ... 

Figure 3.5 schematically lists the various approaches taken in the literature to applying DFT methods to electrocatalytic reactions. These methods are first differentiated by cluster and periodic representation of the electrode surface. The reaction center model (Model 1 in Figure 3.5), developed by Anderson and coworkers,is an early attempt to evaluate potential dependent reaction energies and activation barriers. It relies on using a small cluster to represent the reaction center of the electrode and evaluates the electron alfinity of such cluster. We will not detail this method, because its fidelity is questionable given its arbitrarily small representation of the electrode when considering its electronic structure and lack of scalability to a more accurate electrode representation. 

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