Photosynthetic reaction center components

Figure 6.2. (a) Photosynthetic reaction center of Rhodopseudomonas viridis Reprinted from the Protein Data Bank, H. M. Berman, J. Westbrook, Z. Feng, G. Gilliland, T. N. Bhat, H. Weissig, I. N. Shindyalov, P. E. Bourne, Nucleic Acids Res. 2000, 1, 235 (http // www.pdb.org/) PDB ID IDXR, C. R. D. Lancaster, M. Bibikova, P. Sabatino, D. Oesterhelt, H. Michel, J. Biol Chem. 2000, 275, 39364. (b) arrangement of the essential components in the purple bacterium Rh. sphaeroides. [Adapted from Ref. 5.]... 

Reconstitution of membranes from a small number of molecular components provides simplified structures to study. Thus, cytochrome oxidase or photosynthetic reaction centers, both electron transfer proteins, may be extracted from their native membranes, purified, and reincorporated at relatively high concentration into a simple well defined lipid bilayer. Diffraction investigation then provides information about the distribution and structure of the protein in the membrane. Understanding the mechanism for electron transport in these proteins will require considerable additional information. One key element of structural informations is the location of the redox centres in the membrane profile. 

Many bridging ligands have been constructed that contain porphyrin subunits. Two examples are given to show the versatility of this structural motif. Porphyrins have been used for many years as models for the reactive site in the photosynthetic reaction center because of the resemblance to the natural components and the ability to vary the spectroscopic and redox properties of these chromophores by the use of bulky substituents and by their coordination to different transition metals.192 Molecular dyads of Znn/Irin and Auni/Irm and triads of Znn/Irni/Auin have been synthesized using the porphyrin bridging ligand (66).193-195... 

Among the systems proposed as models for the photosynthetic reaction center, supramolecular assemblies in which Ru(II)-polypyridine complexes and 4,4 -bipyridinium units are held together noncovalently in threaded and interlocked structures have been extensively studied [43, 82-88]. In such assemblies, connections between the molecular components rely on charge transfer interactions between the electron acceptor bipyridinium units and aromatic electron donor groups (Fig. 3). For instance, in the various pseudorotaxanes formed in acetonitrile solution at 298 K by the threading of cyclophane 4 + by the dioxybenzene-containing tethers of 192+ (Fig. 17) [84], an efficient photoinduced electron... 

FI Michel and J Deisenhofer (1986) X-ray diffraction studies on a crystalline bacterial photosynthetic reaction center A progress report and conclusion on the structure of photosystem II reaction centers. In LA Staehelin and CJ Arntzen (eds) Encyclopedia of Plant Physiology, New Ser, Vol 19 pp 371-381. Springer B Svensson, I Vass, E Cedergren and S Styring (1990) Structure of donor side components in photosystem II predicted by computer modeling. The EMBO J 9 2051-2059... 

There are several approaches to mimic functions of photosynthesis, and each has been reviewed extensively. These include the design of molecular assemblies duplicating functions of the photosynthetic reaction center [13,14], development of photosensitive materials capable of light harvesting and inducing electron transfer processes [15-17] and tailoring multi-component systems where light-induced electron transfer processes drive... 

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. 

A particularly intriguing kind of supramolecular self-assembly by axial coordination to metaUoporphyrins, namely self-complementary coordination, is reviewed in the second chapter by Yoshiaki Kobuke. Self-complementarity affords large stability constants. The fascinating systems described in Kobuke s contribution were prepared and investigated as models for components of photosynthetic natural systems self-complementary dimers of porphyrins mimic the special pair of the photosynthetic reaction centers, while macrocychc and three-dimensional porphyrin supramolecules were prepared as photosynthetic antenna models. 

The simplest supramolecular species capable of performing such type of process are covalently-linked three-component systems ("triads"). Two possible schemes for charge separating triads are shown in Fig. 5. Although the scheme in Fig. 5b is reminiscent of the natural photosynthetic reaction center, that of Fig. 5a seems to be more popular in the field of artificial triad systems. The functioning principles are shown in an orbital-type energy diagram in the lower part of Fig. 5. In both cases, excitation of a chromophoric component (1) is followed by a primary photoinduced electron transfer to a primary acceptor (2). This is followed by a secondary thermal electron transfer process (3) electron transifer from a donor component to the oxidized chromophoric component (case a), or electron transfer from the primary acceptor to a secondary acceptor component (case b). The primary process competes with excited-state... 

PS II catalyzes the light driven electron transport from water to plastoquinone. It is an enzyme complex containing a series of redox components and is thought to be constructed in analogy to the photosynthetic reaction center from purple bacteria. 

A relatively large number of theoretical studies on the complete calculation of redox potentials of transition metal active sites in metalloproteins have been published. Metalloproteins studied include manganese superoxide dismutase, iron superoxide dismutase, copper-zinc superoxide dismutase, iron-sulfur proteins, cytochrome f, components of the photosynthetic reaction center, and peroxidases. ... 

Nonheme Fe is a conservative component of the Q-type photosynthetic reaction centers but its function remains unknown. Using Mossbauer spectroscopy, it was shown [81] that in Rhodospirillum rubrum the nonheme Fe exists mostly in a ferrous low-spin state. The binding of Cd ions in the vicinity of the quinone-Fe complex changed the high-spin state of the nonheme Fe into a low-spin one characterized by hyperfine parameters similar to those obtained for the nonheme Fe low-spin state in untreated reaction centers, as confirmed by Mossbauer measurements. 

The bacterial photosynthetic reaction center protein (RC) catalyzes the conversion of light to electrochemical energy through a sequence of photon-initiated electron transfer reactions between redox cofactors held at fixed distances in the protein [1-4]. The primary processes are elicited by absorption of a photon by the primary donor, a dimer of bacteriochlorophyll ([BChIjg). The first excited singlet state of the dimer, [BChljg, transfers an electron over 10 A in 3 ps to a bacteriopheophytin (BPh). The BPh" in turn reduces a quinone bound at the site to form an anionic semiquinone in 0.2 ns (for a review, see [5]). Numerous experimental efforts have aimed to identify the factors which control the remarkable near unit quantum yield and temperature independence of the RC processes [6-9], often with an eye toward emulation in artificial photosynthetic devices [10]. Here, we examine the role of structural components of the quinone cofactor in determining electron transfer rates at the RC Q/y site. 

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