Molecular modeling, photosynthesis

In this paper we give an account of our ongoing effort to understand bacterial photosynthesis at the atomic level. First, we describe earlier simulations which investigate the nuclear motion coupled to the primary donor excitation in bacterial reaction centers (RC). Then, we discuss the molecular modeling of the chromophores of the RC of rhodohacter sphaeroides. Finally, we report on our latest molecular dynamics simulation results concerning a RC in a detergent micelle. 

Table IV also lists the structure analyses of three bacterial cytochromes. Logically these should be delayed to the second half of the chapter, but the results have had such an important influence on thinking about oxidation-reduction mechanisms that their discussion here is mandatory. A key structure study from an evolutionary standpoint was that of R. rvhrum by Salemme, Kraut, and colleagues at the University of California, San Diego (30,31). That study established in one stroke that the eukaryotic cytochrome fold also extended to a bacterial cytochrome and included the cytochromes of photosynthesis as well as respiration. The fact that this structural homology had been predicted on the basis of amino acid sequence comparisons (32) did not lessen the excitement of seeing direct confirmation from the molecular model. Only one of three possible conclusions can be drawn ...

Molecular catalyst for water oxidation has been attracting a great deal of attention not only as a model for the photosynthetic catalyst but also as a component in an artificial photosynthesis. Many structural models have been synthesized and investigated as the photosynthetic Mn complex. Tetrakis(2,2 -bipyridine)(di /i -oxo)di-Mn complex lu) and tetrakis(2,2 -bipyridine)( jJ. -oxo)di-Ru complex (2, Meyer s complex)14) are typical examples, but many of them failed to show high activity for water oxidation. 

The second article also deals with PET in arranged media, however, this time by discussing comprehensively the various types of heterogeneous devices which may control supramolecular interactions and consequently chemical reactions. Before turning to such applications, photosynthetic model systems, mainly of the triad type, are dealt with in the third contribution. Here, the natural photosynthetic electron transfer process is briefly discussed as far as it is needed as a basis for the main part, namely the description of artificial multicomponent molecules for mimicking photosynthesis. In addition to the goal to learn more about natural photosynthetic energy conversion, these model systems may also have applications, which, for example, lie in the construction of electronic devices at the molecular level. 

Synthesis of porphyrin-based molecular complexes as models for the study of photosynthesis 93UK1020. 

Porphyrins are an important class of -> electron-transfer ligands. Photosynthesis is primarily driven by chromophores (light-harvesting antenna and reaction centers) which consist of special assemblies of porphyrins. Porphyrins have been intensively studied for their possible applications, including their use as photonic materials, catalysts, photosensitizers for photodynamic therapy, receptor models in molecular recognition, and components of -> electrochemical sensors [v]. 

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). 

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