Photosynthesis reaction center models

Porphyrin-quinone compounds as models of photosynthesis reaction center 89UK1032. 

Most of the interest in mimicing aspects of photosynthesis has centered on a wide variety of model systems for electron transfer. Among the early studies were experiments involving photoinduced electron transfer in solution from chlorophyll a to p-benzoquinone (21, 22) which has been shown to occur via the excited triplet state of chlorophyll a. However, these solution studies are not very good models of the in vivo reaction center because the in vivo reaction occurs from the excited singlet state and the donor and acceptor are held at a fixed relationship to each other in the reaction-center protein. 

As discussed above, the photosynthetic reaction center solves the problem of rapid charge recombination by spatially separating the electron and hole across the lipid bilayer. In order to achieve photoinitiated electron transfer across this large distance, the reaction center uses a multistep sequence of electron transfers through an ensemble of donor and acceptor moieties. The same strategy may be successfully employed in photosynthesis models, and has been since 1983 [42-45]. The basic idea may be illustrated by reference to a triad Dj-D2-A, where D2 represents a pigment whose excited state will act as an electron donor, Di is a secondary donor, and A is an electron acceptor. Excitation of D2 will lead to the following potential electron transfer events. 

A depending on the size of the lanthanide metals. Delocalization of electron density on four equivalent nitrogen atoms causes elongation of the Ln-N bonds at about 0.10-0.15 A compared to silylamides. The close proximity of the macrocyclic 7i-systems in sandwich complexes proved to be useful as structural and spectroscopic models for the bacteriochlorophyll [Mg(Bchl)]2, the special pair in the reaction center of bacterial photosynthesis [211,212]. The distance between the pyrrole rings in [Mg(Bchl)]2 is about 3 A. 

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]. 

The field of light-induced electron transfer remains in a state of rapid development in many of the areas on which this brief history has touched and in others that could not be included here. Exciting progress is being made in the delineation of the first picosecond of photosynthesis, in further characterizing photosynthetic reaction centers, in the area of artificial models. The dependence of intramolecular electron transfer upon distance, solvent, orientation is being delineated. Many of these developments are detailed in the following chapters. 

Fig. 5. Three-dimensional organization of the core-antenna Chl-a moiecuies in the PS-i reaction-center core The 1992 version (A), the 1995 version (B) and the 1997 version, as a stereogram (C). Model in (A) Is viewed toward the membrane normal, while those in (B) and (C) ai e viewed along the membrane normal, from the stroma to the lumen. See text for other details. Figure source (A) Witt, Krauli, Hinrichs, (I) Witt, Fromme and Saenger (1992) Three-dimensional crystals of photosystem I from Synechococcus sp. and X-ray structure analysis at 6 A resolution. In N Murata (ed) Research in Photosynthesis (Proc IX Intern Congr on Photosynthesis II 521,526, Kluwer (B) Schubert, Klukas, Krauli, Saenger, Fromme andW H( Q95) Present state of the crystal structure analysis of photosystem I at 4.5 A resolution. In P Mathis (ed) Photosysnthesis from Light to Biosphere. II 9, Kluwer (C) Schubert, Klukas, Krauli, Saenger, Fromme and Witt (1997) Photosystem I of Synechococcus elongatus at 4 A resolution Comprehensive structure analysis. J Mol Biol 272 756.

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. 

Because of the vast quantity of published ET research, we have had to neglect several important areas of current interest. Many investigators have built systems to try to mimic the first steps of photosynthesis as well as the low-temperature (4.2 K) ET seen in photosynthetic reaction centers. Efficient ET has been observed down to 10 K in certain complexes (19, 184), and studies of ET across membranes using model systems have been made (59, 135, 137, 167). A relatively new area related to photosynthctic charge separation encompasses the study of ET in polymers that have been func-... 

Two general approaches of modeling photosynthesis can be considered, one involving mimicking the functions of the photosynthetic reaction center by means of synthetic analogs [25-27]. To this extent the synthesis of linked multicomponent donor-acceptor assemblies could lead to charge separation by means of sequential ET processes as outlined in Eqs. (1) to (4), where S is the light-active component and A and D represent electron acceptor and donor units, respectively. 

The problem of bacterial photosynthesis has attracted a lot of recent interest since the structures of the photosynthetic reaction center (RC) in the purple bacteria Rhodopseudomonas viridis and Rhodobacterias sphaeroides have been determined [56]. Much research effort is now focused on understanding the relationship between the function of the RC and its structure. One fundamental theoretical question concerns the actual mechanism of the primary ET process in the RC, and two possible mechanisms have emerged out of the recent work [28, 57-59]. The first is an incoherent two-step mechanism where the charge separation involves a sequential transfer from the excited special pair (P ) via an intermediate bacteriochlorophyll monomer (B) to the bacteriopheophytin (H). The other is a coherent one-step superexchange mechanism, with P B acting only as a virtual intermediate. The interplay of these two mechanisms can be studied in the framework of a general dissipative three-state model (AT = 3). 

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... 

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