Photochemical Reaction Center Research Articles

Pigment-protein interactions in Rhodobacter sphaeroides Y photochemical reaction center; comparison with other reaction center structures

Structural characteristics of pigments and cofactors are analyzed in the X-ray structure of the Rhodobacter sphaeroides (Y strain) photochemical reaction center, recently refined at 3 A resolution (Arnoux B, Gaucher JF, Ducruix A and Reiss-Husson F (1995) Acta Cryst D51: 368–379). As several structures are now available for these pigment-protein complexes from various Rhodobacter sphaeroides strains and for Rhodopseudomonas viridis, a detailed comparison was done for highlighting converging structural results as well as for pointing to incidental differences. Comparison of mean plane orientations and distances, and also direct superposition of the pigment arrays, indicated that the best agreement between all the structures concerned the dimer and the bacteriopheophytin of the A branch. In the Y reaction center structure the pentacoordination of the Mg++ atoms of the bacteriochlorophylls, and the H bonding pattern of the porphyrin conjugated carbonyls are consistent with the better resolved Rhodobacter sphaeroides recently published structure (Ermler U, Fritzsch G, Buchanan SK and Michel H (1995) Structure 2:925–936). Discrepancies between the various Rhodobacter sphaeroides structures are larger for the quinones, particularly the secondary one. In the Y reaction center structure the phytyl and isoprenoid chains of the cofactors are defined and their local mobility was evaluated by analyzing the temperature factor and the density of neighbouring atoms. Significant differences were observed between the A and B branches, and, within each branch, from the dimer to the quinone molecules.

S-E1-01 Magnetic resonance of transient intermediates in reaction center photochemistry: Proskur yakov II Inst. of soil Sci. and Photosynth. RAS, Pushchino (Russia)

S-E1-01 Magnetic resonance of transient intermediates in reaction center photochemistry: Proskur yakov II Inst. of soil Sci. and Photosynth. RAS, Pushchino (Russia)

Excitation Wavelength Dependence of Bacterial Reaction Center Photochemistry. 2. Low-Temperature Measurements and Spectroscopy of Charge Separation

Excitation with spectrally narrow (5 nm), temporally short duration (200 fs) laser pulses at a variety of wavelengths between 848 and 903 nm results in substantial excitation wavelength dependent differences in the evolution of the bacterial reaction center absorbance spectrum both before and after charge separation occurs. The transient holes in the initial electron donor band showed a more resolved vibrational band structure at 20 K, when compared to those of earlier room temperature transient hole-burning experiments (Peloquin, J. M.; Lin, S.; Taguchi, A. K. W.; Woodbury, N. W. J. Phys. Chem. 1995, 99, 1349). The dominant vibrational band observed is at 120 cm-1, in agreement with dynamic measurements of coherent oscillations on this time scale (Vos, M. H.; Rappaport, F.; Lambry, J.-C.; Breton, J.; Martin, J.-L. Nature 1993, 363, 320). At both room temperature and low temperature, there is a distribution of P to P* transition energies due to a distribution of the protein conformations in the ground state. At 20 K, one can also see a distribution of P* to P stimulated emission energies. As might be expected, the barriers to conformational interconversion are more easily crossed at room temperature, resulting in a smaller difference between the mean transition energies of the photoselectable subpopulations on the picosecond time scale relative to those at low temperature. At 20 K, much of this conformational interconversion is apparently lost when exciting near the 0−0 transition wavelength of P. More conformational interconversion appears to take place at low temperature when higher energy excitation is used, implying that P* is vibrationally hot for at least hundreds of femtoseconds following excitation on the blue side of its QY band. Of particular interest is the insensitivity of the overall charge separation kinetics to the selection of ground state conformational subpopulations by different excitation wavelengths. There appears to be very little coupling between the nuclear motion that defines the ground state transition distribution and the charge separation reaction itself. Given the apparently slow rate of vibrational relaxation of some of the excited state modes most strongly coupled to the P to P* transition, the lack of excitation wavelength dependence of the charge separation rate also suggests that these modes are not strongly coupled to the charge separation reaction. What the ground state (and possibly excited state) conformational distributions and interconversions do affect is the kinetic complexity of the absorbance changes in the 800 nm region. Specifically, the extent of involvement of fast, multiexponential decay components in this region depends strongly on the wavelength of excitation.

Synthesis and electrochemical investigation of azobenzene-substituted porphyrins

The synthesis of azobenzene-substituted porphyrins (3a-g) is described. With respect to possible applications as model systems for photochemical reaction centers the spectroscopic and electrochemical properties of the synthesized compounds were investigated. For compound 3e the obtained spectroscopic and electrochemical data confirm that a light driven electron transfer from a porphyrin to the azobenzene moiety is possible.

Role of the PufX protein in photosynthetic growth of Rhodobacter sphaeroides. 2. PufX is required for efficient ubiquinone/ubiquinol exchange between the reaction center QB site and the cytochrome bc1 complex.

The PufX membrane protein is essential for photosynthetic growth of Rhodobacter sphaeroides because it is required for multiple-turnover electron transfer under anaerobic conditions [see accompanying article; Barz, W. P., Francia, F., Venturoli, G., Melandri, B. A., Verméglio, A., & Oesterhelt, D. (1995) Biochemistry 34, 15235-15247]. In order to understand the molecular role of PufX, light-induced absorption spectroscopy was performed using a pufX- mutant, a pufX+ strain, and two suppressor mutants. We show that the reaction center (RC) requires PufX for its functionality under different redox conditions than the cytochrome bc1 complex: When the kinetics of flash-induced reduction of cytochrome b561 were monitored in chromatophores, we observed a requirement of PufX for turnover of the cytochrome bc1 complex only at high redox potential (Eh > 140 mV), suggesting a function of PufX in lateral ubiquinol transfer from the RC. In contrast, PufX is required for multiple turnover of the RC only under reducing conditions: When the Q pool was partially oxidized in vivo using oxygen or electron acceptors like dimethyl sulfoxide or trimethylamine N-oxide, the deletion of PufX had no effect on light-driven electron flow through the RC. Flash train experiments under anaerobic in vivo conditions revealed that RC photochemistry does not depend on PufX for the first two flash excitations. Following the third and subsequent flashes, however, efficient charge separation requires PufX, indicating an important role of PufX for fast Q/QH2 exchange at the QB site of the RC. We show that the Q/QH2 exchange rate is reduced approximately 500-fold by the deletion of PufX when the Q pool is nearly completely reduced, demonstrating an essential role of PufX for the access of ubiquinone to the QB site. The fast ubiquinone/ubiquinol exchange is partially restored by suppressor mutations altering the macromolecular antenna structure. These results suggest an indirect role of PufX in structurally organizing a functional photosynthetic apparatus.

In vivo participation of a high potential iron-sulfur protein as electron donor to the photochemical reaction center of Rubrivivax gelatinosus.

We have found that the only high redox potential electron transfer component in the soluble fraction of Rubrivivax gelatinosus TG-9 is a high-potential iron-sulfur protein (HiPIP). We demonstrated the participation of this HiPIP in the photoinduced electron transfer both in vivo and in vitro. First, the addition of HiPIP to purified membranes enhanced the rate of re-reduction of the photooxidized reaction center. Second, the photooxidation of HiPIP was observed in intact cells of Ru. gelatinosus TG-9 under anaerobic conditions by EPR and absorption spectroscopies. Analysis of flash-induced absorption changes showed that the equilibration of positive equivalents between the reaction center and HiPIP occurs in less than 1 ms after flash excitation. The complete re-reduction of the photooxidized reaction center is achieved in tens of milliseconds. The turnover of a cyt bc1 is also involved in this reaction, as shown by a slow electrogenic phase of the membrane potential linked to this process.

Determination of the pigment stoichiometry of the photochemical reaction center of photosystem II

The stoichiometry of chlorophyll a, pheophytin α and β-carotene in the photochemical reaction center of Photosystem II was analyzed by reversed phase high-performance liquid chromatography (HPLC) with methanol as the mobile phase, and by the shape of spectra of extracts in 80% acetone. For the HPLC method the molar extinction coefficient of pheophytin a in methanol was redetermined, while for the spectroscopic method spectra of extracts in 80% acetone were simulated by fitting with spectra of isolated chlorophyll a, pheophytin a and β-carotene in 80% acetone. Both methods give internally consistent results, and suggest that the reaction center of Photosystem II isolated by a short Triton X-100 treatment binds 6 chlorophyll a per 2 pheophytin a molecules. We also present evidence that prolonged exposure of the Photosystem II reaction center complex to Triton X-100 does not result in the loss of chlorophyll from the complex. Based on a comparison with spectra reported in publications from other groups, we conclude that the chlorophyll to pheophytin ratio has previously been underestimated to sometimes very significant extents, and that, as yet, no Photosystem II reaction center particles have been purified that bind less than 5–6 chlorophyll a per 2 pheophytin a.

Photoinactivation of Photosystem II Induces Changes in the Photochemical Reaction Center II Abolishing the Regulatory Role of the Qb Site in the Dl Protein Degradation

The effect of 3‐(3,4‐dichlorophenyl)‐1,1 ‐dimethyl urea (diuron) binding at the secondary quinone (QB) binding site of reaction center II (RCII), on the high‐light‐induced degradation of the RCII proteins D1 and D2, and the core proteins CP43 and CP47 was investigated in vivo in Chlamydomonas reinhardtii. The degradation of the RCII‐D2 and the CP43 proteins shows a short lag relative to that of the RCII‐D1 protein. Diuron retards but does not prevent the degradation of RCII‐D1, D2 and CP43 proteins. The degradation of the CP47 protein is not retarded by diuron. The RCII‐D1 protein present in cells photoinactivated in the presence of diuron is subsequently degraded in cells transferred to low light or to darkness. The protein can be replaced (turnover) at least partially under both conditions. The RCII‐D1 protein is not degraded during photoinactivation of a cytochrome‐bf‐defective mutant. Degradation occurs however when the cells are returned to low light permitting slow reoxidation of plastoquinol [Zer, H., Prasil, O. & Ohad, I. (1994) J. Biol. Chem. 269, 17670–17676]. Addition of diuron does not prevent the degradation of the protein at this stage. Tryptic digestion of the RCII‐D1 protein is partially inhibited by diuron in isolated thylakoids [Trebst, A., Depka, B., Kraft, B. & Johanningmeier, U. (1988) Photosynth. Res. 18, 163–177] but not in thylakoids obtained from photoinactivated cells. We conclude that photoinactivation induces a series of sequential changes in RCII exposing the cleavage site of the RCII‐D1 protein to degradation and abolishing the regulatory role of the QB site occupancy by plastoquinone or analog ligands on the cleavage process. The degradation of the RCII‐D2 and CP43 proteins may be a secondary process following modification and/or loss of the RCII‐D1 protein.

Photoinactivation of photosystem II induces changes in the photochemical reaction center II abolishing the regulatory role of the QB site in the D1 protein degradation.

The effect of 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (diuron) binding at the secondary quinone (QB) binding site of reaction center II (RCII), on the high-light-induced degradation of the RCII proteins D1 and D2, and the core proteins CP43 and CP47 was investigated in vivo in Chlamydomonas reinhardtii. The degradation of the RCII-D2 and the CP43 proteins shows a short lag relative to that of the RCII-D1 protein. Diuron retards but does not prevent the degradation of RCII-D1, D2 and CP43 proteins. The degradation of the CP47 protein is not retarded by diuron. The RCII-D1 protein present in cells photoinactivated in the presence of diuron is subsequently degraded in cells transferred to low light or to darkness. The protein can be replaced (turnover) at least partially under both conditions. The RCII-D1 protein is not degraded during photoinactivation of a cytochrome-bf-defective mutant. Degradation occurs however when the cells are returned to low light permitting slow reoxidation of plastoquinol [Zer, H., Prasil, O. & Ohad, I. (1994) J. Biol. Chem. 269, 17,670-17,676]. Addition of diuron does not prevent the degradation of the protein at this stage. Tryptic digestion of the RCII-D1 protein is partially inhibited by diuron in isolated thylakoids [Trebst, A., Depka, B., Kraft, B. & Johanningmeier, U. (1988) Photosynth. Res. 18, 163-177] but not in thylakoids obtained from photoinactivated cells. We conclude that photoinactivation induces a series of sequential changes in RCII exposing the cleavage site of the RCII-D1 protein to degradation and abolishing the regulatory role of the QB site occupancy by plastoquinone or analog ligands on the cleavage process. The degradation of the RCII-D2 and CP43 proteins may be a secondary process following modification and/or loss of the RCII-D1 protein.

Thirty years of fun with antenna pigment-proteins and photochemical reaction centers: A tribute to the people who have influenced my career

The author summarizes the research contributions to photosynthesis made by him, his graduate and postdoctoral students, visiting scientists and by his collaboration with other photosynthesis workers during 1964-1994. The development of isolation procedures and biochemical/biophysical characterization of antenna pigment-proteins and photochemical reaction centers are described together with the author's education and experiences as a scientific researcher. Some anecdotes hopefully add insight into what it was like to be in this area of science during the period.

Rapid turnover of the RCII-D1 protein in the dark induced by photoinactivation of Photosystem II in Scenedesmus wild type and the PS-II-donor defective LF-1 mutant cells

We have investigated the photoinactivation and turnover of the photochemical reaction centre II (RCII) D1 protein in Scenedesmus obliquus wt and LF-1 mutant containing an unprocessed precursor D1 protein (pD1) and defective in donor side activity of Photosystem II (PS II). Photoinactivation of PS II is considerably faster in the LF-1 mutant as compared to the wt cells. The RCII-D1(pD1) protein in photoinactivated cells is irreversibly modified and can be degraded and synthesised in the dark (turnover). Turnover of the D1 protein is proportional to the initial degree of photoinactivation. Diuron retards significantly the protein degradation in the photoinactivated cells. Wild-type cells photoinactivated in the presence of diuron exhibit a fluorescence induction transient similar to that of the LF-1 mutant. Removal of diuron allows rapid turnover of photoinactivated RCII-D1(pD1) in the dark. The protective effect of diuron is gradually lost in photoinactivated cells even when incubated in the dark. Blocking chloroplast translation activity does not prevent D1(pD1) protein degradation. We conclude that the irreversible changes induced by photoinactivation of RCII-D1(pD1) protein in vivo are sufficient to allow complete D1 protein turnover in the dark. Redox control of the D1 protein degradation in vivo is thus related to availability of oxidised plastoquinone which affects the access of the modified protein to the cleavage system (Gong and Ohad (1991) J. Biol. Chem. 266, 21293–21299.)

Structure of the photochemical reaction centre of a spheroidene-containing purple-bacterium, Rhodobacter sphaeroides Y, at 3 A resolution.

The crystal structure of the photochemical reaction centre from Rhodobacter sphaeroides Y, a carotenoid-containing wild-type purple bacterium, has been determined at 3 A resolution. This membrane complex consists of three subunits (281, 307 and 260 residues, respectively) and ten cofactors. It was crystallized in presence of beta-D-octylglucoside. The crystals are orthorhombic with unit-cell dimensions, a = 143.7, b = 139.8, c = 78.7 A, space group P2(1)2(1)2(1) with four molecules in the unit cell. Refinement of the structure by X-PLOR and manual reconstructions yielded an R value of 22.1% for 19630 reflections between 7 and 3 A. The secondary structure is highly homologous to those determined for Rhodopseudomonas viridis (Protein Data Bank entry 1PRC) and Rhodobacter sphaeroides R26 (Protein Data Bank entry 4RCR) reaction centres. In the latter two structures one Fe(2+) ion located between the two quinones is coordinated by four histidines and one glutamic acid. In the Rhodobacter sphaeroides Y structure, Mn(2+) occupies the same position with identical ligands and geometry. The carotenoid conformation which is a non-planar 15-15'-cis spheroidene molecule in our structure differs from the 13-14-cis 2,4-dihydroneusporene in the Rhodopseudomonas viridis structure.

Cloning and sequencing of the genes encoding the light-harvesting B806-866 polypeptides and initial studies on the transcriptional organization of puf2B, puf2A and puf2C in Chloroflexus aurantiacus.

The genes encoding the alpha- and beta-polypeptide subunits of the B806-866 membrane-bound light-harvesting complex of Chloroflexus aurantiacus have been cloned and the nucleotide sequences determined. The gene puf2A, which encodes the B806-866 alpha-polypeptide, began 28 bases downstream of the stop codon of puf2B, which encodes the B806-866 beta gene. The gene-encoding cytochrome c-554, puf2C, was found about 250 bp downstream of puf2A. puf2A encoded a 13 amino acid extension at the C-terminus of the B806-866 alpha-polypeptide that was not present in the mature protein. These genes, unlike those of purple nonsulfur bacteria, did not form a contiguous operon with puf1L or puf1M, the genes encoding the L and M subunits of the photochemical reaction center. The occurrence of the two latter genes and of puf2B and puf2A in two separate operons has not been observed in purple bacteria. Under photoheterotrophic growth conditions, puf2B and puf2A were encoded on an abundant mRNA that was 0.5 kb long. Two monocistronic transcripts for puf2C were observed that had different 5'-ends. One transcript encoding all three genes was also detected. Nucleotide sequences very similar to the consensus promoter sequence of the Escherichia coli RNA polymerase sigma 70 subunit were found seven and eight bases upstream of the 5'-end of mRNA encoding puf2B and for one of the monocistronic mRNA encoding puf2C, respectively.

Excitation Wavelength Dependence of Bacterial Reaction Center Photochemistry. 1. Ground State and Excited State Evolution

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTExcitation Wavelength Dependence of Bacterial Reaction Center Photochemistry. 1. Ground State and Excited State EvolutionJeffrey M. Peloquin, Su Lin, Aileen K. W. Taguchi, and Neal W. WoodburyCite this: J. Phys. Chem. 1995, 99, 4, 1349–1356Publication Date (Print):January 1, 1995Publication History Published online1 May 2002Published inissue 1 January 1995https://doi.org/10.1021/j100004a040RIGHTS & PERMISSIONSArticle Views81Altmetric-Citations37LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1021 KB) Get e-Alerts

Oscillations of Reaction Center II-D1 Protein Degradation in Vivo Induced by Repetitive Light Flashes: CORRELATION BETWEEN THE LEVEL OF RCII-QB- AND PROTEIN DEGRADATION IN LOW LIGHT

The D1 protein subunit of the photochemical reaction center II (RCII) turns over rapidly in oxygenic photosynthetic organisms exposed to the light. At high photon flux densities (PFD), photoinactivation of RCII precedes the degradation of the D1 protein. We found that the apparent quantum yield for the D1 protein degradation in Chlamydomonas cells is severalfold higher at low PFDs (10-100 mumol m-2 s-1) as compared to that observed at PFDs which induce photoinactivation of RCII (1.5-3 x 10(3) mumol m-2 s-1). Relative high levels of reduced RCII secondary plastoquinone acceptor, QB-, are induced in cells exposed to low PFDs as determined by thermoluminescence measurements. The probability of generating elevated levels of QB- which may recombine with the S2,3 oxidized states of the oxygen evolving complex decreases with increase in the light intensities at which consecutive double reduction of QB and exchange with the plastoquinone pool prevail. We have used light flashes to test if a correlation exists between the degradation of D1 protein and the relative level of QB-. D1 protein degradation could be induced in dark-incubated cells exposed to a series of 1.4 x 10(3) single light flashes given at intervals compatible with generation of elevated levels of QB- and its decay by charge recombination. Oscillations of the QB- level in cells exposed to 960-1440 series of 1 to several flashes correlated with oscillations of the D1 protein degradation in Chlamydomonas cells and in the Scenedesmus wild type but not in the LF-1 mutant lacking photosystem II donor side activity. In this mutant the "S state cycle" and QB- oscillations are abolished. We propose that the process of recombination of long lived RCII-QB- with the S2,3 states may involve damaging events related to the D1 protein degradation induced by light flashes or continuous low light in vivo.

Light-dependent degradation of the Photosystem II D1 protein is retarded by inhibitors of chloroplast transcription and translation: possible involvement of a chloroplast-encoded proteinase

The light-dependent degradation of the photochemical Reaction Centre II D1 protein was retarded in higher plant Lemna gibba when inhibitors of chloroplast transcription or translation were present. D1 degradation may correlate with turnover of a chloroplast-encoded proteinase, but this proteinase seems not to be the only enzyme involved. Based on current knowledge and the results presented here, a proposal is made as to the proteinase involved.

Role of plastoquinol oxidoreduction in regulation of photochemical reaction center IID1 protein turnover in vivo

The light-induced turnover of the D1 protein subunit of reaction center II (RCII) was investigated in Chlamydomonas reinhardtii y-1 (control) and D6, AC208, and B4 mutants lacking cytochrome b6/f, plastocyanin or photosystem I activity, respectively, and, thus, impaired in light-dependent plastoquinol (PQH2) oxidation. Charge recombination assayed by thermoluminescence measurements indicated similar RCII properties in control and mutant cells. The D1 protein is not degraded in the mutants during photoinactivation; however, RCII-D1 is irreversibly altered, and the protein is degraded when the cells are incubated in low light permitting slow reoxidation of the PQH2 pool. Photoinactivation precedes D1 degradation also in the control cells. Thus, in vivo under physiological conditions photoinactivation and "tagging" of RCII-D1 are resolved from the degradation process. RCII activity in photoinactivated cells may be recovered only following D1 degradation and replacement. Recovery may occur either in the light or dark in the absence of de novo chlorophyll synthesis. The degradation of the photoinactivated RCII-D1 protein is a prerequisite for the synthesis and stable integration of new D1 indicating that tagged D1 is still assembled in the inactive reaction centers. The physiological implication of these results is that oxidation of the PQH2 pool in photoinactivated cells affects RCII-D1 protein degradation and replacement, and, thus, D1 turnover in vivo is regulated by the turnover of PQ at the binding site of the secondary stable electron acceptor quinone of RCII.

Biomimetic approaches to the construction of supramolecular catalysts: titanium dioxide—platinum antenna catalysts to reduce water using visible light

Thermally and photochemically stable catalysts in systems that convert the energy of visible light into the chemical energy of dihydrogen gas have yet to be fully developed and all proposed systems suffer, to varying extents, from loss of activity with use. We have attempted to model nature's photochemical reaction center through the use of TiO 2 antenna catalysts and very small amounts of Pt that form active centers on the TiO 2 semiconductor surface. This system is relatively inexpensive, reproducible, extremely stable, efficient in conversion of light to dihydrogen in aqueous solutions and displays higher linearity in dihydrogen evolution with greater efficiency in recycling than do other previously reported catalysts. Here we report the self-hydrogenation properties of our TiO 2Pt catalysts in comparison with well known Pt, Ru and Os catalysts as well as their catalytic efficiency in systems for the sacrificial photoreduction of water. The sensitizers employed in this study were a recently synthesized new class of bis-heteroleptic complexes [Ru(bpy) 2(PP)] 2+, [Ru(bpy) 2(PPB)] 2+, [Ru(bpy) 2(PPB-pCl)] 2+ containing one pyridyl—pyrimidine ligand and [Ru(bpy) 3] 2+ and [Ru(bpz) 3] 2+ as well known standard sensitizers. In order to obtain maximum information about the TiO 2Pt catalysts we varied the components and conditions in our sacrificial systems for water reduction: MV 2+, PVS MPVS were used as electron relays, EDTA and TEOA as sacrificial electron donors. Furthermore, the dependence of the dihydrogen evolution rates on the pH of the photolysis solution and its ionic strength were investigated. The techniques used in this study are cyclic voltammetry, steady state and time resolved luminescence spectroscopy, Stern—Volmer quenching, measurements of the quantum yields for PVS reduction, and volumetric measurements of dihydrogen photoproduction. A linear dependence was found between the dihydrogen production rate of the TiO 2Pt catalysts and the quantum yield for the MV 2+ and PVS photoreduction at low ionic strength, showing that the new catalysts are reduced by the electron relays and convert protons into dihydrogen in a highly efficient manner

Shared thematic elements in photochemical reaction centers.

The structural, functional, and evolutionary relationships between photosystem II and the purple nonsulfur bacterial reaction center have been recognized for several years. These can be classified as "quinone type" (type II) photosystems because the terminal electron acceptor is a mobile quinone molecule. The analogous relationship between photosystem I and the green sulfur bacterial (and helicobacterial) reaction centers has only recently become clear. These can be classified as "iron-sulfur type" (type I) photosystems because the terminal electron acceptor consists of one or more bound iron-sulfur clusters. At a fundamental level, the quinone type and iron-sulfur type reaction centers share a common photochemical motif in the early process of charge separation, leading to the speculation that all photochemical reaction centers have a common evolutionary origin. This review summarizes the current state of knowledge in comparative reaction center biochemistry between prokaryotic bacteria, cyanobacteria, and green plants.

Biomimetische Photosysteme zur sakrifiziellen Wasserreduktion / Novel Biomimetic Systems for Visible Light Induced Water Reduction

Novel microheterogeneous systems for the direct photoreduction of water using visible light in analogy to the photochemical reaction center of Rhodopseudomonas viridis are described in detail. These physical model systems for photosynthesis feature the recently synthesized bisheteroleptic metal complexes [Ru(bpy)2(PP)]Cl2, [Ru(bpy)2(PPB)]Cl2, [Ru(bpy)2(PPB-pCl)]Cl2 and [Ru(bpy)3]Cl2, adsorbed on a negatively charged SiO2—TiO2-colloid, the zwitterionic electron relay PVS and a long-term stable and highly efficient TiO2—Pt-“antenna” catalyst as well as TEOA as sacrificial electron donor. Evidence for the directed absorption of the sensitizers on the SiO2—TiO2-colloid is taken from UV-VIS-measurements, steady-state luminescence-spectroscopy and the quantum yields for PVS-reduction. The hydrogen production in the presence of the SiO2—TiO2-colloid is clearly enhanced and proofs the validity of the underlying concept of physical model systems for photosynthesis.