Singlet-singlet annihilation at low temperatures in the antenna of purple bacteria

Remembering Jan Amesz (1934-2001): a great gentleman, a major discoverer, and an internationally renowned biophysicist of both oxygenic and anoxygenic photosynthesisa

We present here the research contributions of Jan Amesz (1934-2001) on deciphering the details of the early physico-chemical steps in oxygenic photosynthesis in plants, algae and cyanobacteria, as well as in anoxygenic photosynthesis in purple, green, and heliobacteria. His research included light absorption and the mechanism of excitation energy transfer, primary photochemistry, and electron transfer steps until the reduction of pyridine nucleotides. Among his many discoveries, we emphasize his 1961 proof, with L. N. M. Duysens, of the "series scheme" of oxygenic photosynthesis, through antagonistic effects of Light I and II on the redox state of cytochrome f. Further, we highlight the following research on oxygenic photosynthesis: the experimental direct proof that plastoquinone and plastocyanin function at their respective places in the Z-scheme. In addition, Amesz's major contributions were in unraveling the mechanism of excitation energy transfer and electron transport steps in anoxygenic photosynthetic bacteria (purple, green and heliobacteria). Before we present his research, focusing on his key discoveries, we provide a glimpse of his personal life. We end this Tribute with reminiscences from three of his former doctoral students (Sigi Neerken; Hjalmar Pernentier, and Frank Kleinherenbrink) and from several scientists (Suleyman Allakhverdiev; Robert Blankenship; Richard Cogdell) including two of the authors (G. Garab and A. Stirbet) of this Tribute.

Excitation Power Modulates Energy-Transfer Dynamics in a Supramolecular RuII -FeII -RuII Triad

Multichromophoric arrays are key to light harvesting in natural and artificial photosynthesis. A trinuclear, symmetric Ru^II -Fe^II -Ru^II triad may resemble a light-harvesting model system in which excitation energy from donor units (Ru-terpyridine fragments) is efficiently transferred to the acceptor (the Fe-terpyridine fragment). The photoinduced dynamics after simultaneous excitation of more than a single chromophoric unit (donor/acceptor) at varying excitation fluence is investigated in this contribution. Data suggests that energy transfer is decelerated if the acceptor states (on the Fe^II unit) are not depopulated fast enough. As a consequence, the lifetime of a high-lying excited state (centered on either of the Ru^II units) is prolonged. A kinetic model is suggested to account for this effect. Although the proposed model is specifically adopted to account for the experimental data reported here, it might be generalized to other situations in which multiple energy or electron donors are covalently linked to a single acceptor site, a situation of interest in contemporary artificial photosynthesis.

Light Absorption and Energy Transfer in the Antenna Complexes of Photosynthetic Organisms

The process of photosynthesis is initiated by the capture of sunlight by a network of light-absorbing molecules (chromophores), which are also responsible for the subsequent funneling of the excitation energy to the reaction centers. Through evolution, genetic drift, and speciation, photosynthetic organisms have discovered many solutions for light harvesting. In this review, we describe the underlying photophysical principles by which this energy is absorbed, as well as the mechanisms of electronic excitation energy transfer (EET). First, optical properties of the individual pigment chromophores present in light-harvesting antenna complexes are introduced, and then we examine the collective behavior of pigment-pigment and pigment-protein interactions. The description of energy transfer, in particular multichromophoric antenna structures, is shown to vary depending on the spatial and energetic landscape, which dictates the relative coupling strength between constituent pigment molecules. In the latter half of the article, we focus on the light-harvesting complexes of purple bacteria as a model to illustrate the present understanding of the synergetic effects leading to EET optimization of light-harvesting antenna systems while exploring the structure and function of the integral chromophores. We end this review with a brief overview of the energy-transfer dynamics and pathways in the light-harvesting antennas of various photosynthetic organisms.

The accumulation of the light-harvesting 2 complex during remodeling of the Rhodobacter sphaeroides intracytoplasmic membrane results in a slowing of the electron transfer turnover rate of photochemical reaction centers

A functional proteomic analysis of the intracytoplasmic membrane (ICM) development process was performed in Rhodobacter sphaeroides during adaptation from high-intensity illumination to indirect diffuse light. This initiated an accelerated synthesis of the peripheral light-harvesting 2 (LH2) complex relative to that of LH1-reaction center (RC) core particles. After 11 days, ICM vesicles (chromatophores) and membrane invagination sites were isolated by rate-zone sedimentation and subjected to clear native gel electrophoresis. Proteomic analysis of gel bands containing the RC-LH1 and -LH2 complexes from digitonin-solubilized chromatophores revealed high levels of comigrating electron transfer enzymes, transport proteins, and membrane assembly factors relative to their equivalent gel bands from cells undergoing adaptation to direct low-level illumination. The GroEL chaperonin accounted for >65% of the spectral counts in the RC-LH1 band from membrane invagination sites, which together with the appearance of a universal stress protein suggested that the viability of these cells was challenged by light limitation. Functional aspects of the photosynthetic unit assembly process were monitored by near-IR fast repetition rate analysis of variable fluorescence arising from LH-bacteriochlorophyll a components. The quantum yield of the primary charge separation during the early stages of adaptation showed a gradual increase (variable/maximal fluorescence = 0.78-0.83 between 0 and 4 h), while the initial value of ~70 for the functional absorption cross section (σ) gradually increased to 130 over 4 days. These dramatic σ increases showed a direct relation to gradual slowing of the RC electron transport turnover rate (τ(QA)) from ~1.6 to 6.4 ms and an ~3-fold slowing of the rate of reoxidation of the ubiquinone pool. These slowed rates are not due to changes in UQ pool size, suggesting that the relation between increasing σ and τ(QA) reflects the imposition of constraints upon free diffusion of ubiquinone redox species between the RC and cytochrome bc(1) complex as the membrane bilayer becomes densely packed with LH2 rings.

Excitation transfer connectivity in different purple bacteria: a theoretical and experimental study

Photosynthetic membranes accommodate densely packed light-harvesting complexes which absorb light and convey excitation to the reaction center (RC). The relationship between the fluorescence yield (phi) and the fraction (x) of closed RCs is informative about the probability for an excitation reaching a closed RC to be redirected to another RC. In this work, we have examined in this respect membranes from various bacteria and searched for a correlation with the arrangement of the light-harvesting complexes as known from atomic force or electron microscopies. A first part of the paper is devoted to a theoretical study analyzing the phi(x) relationship in various models: monomeric or dimeric RC-LH1 core complexes, with or without the peripheral LH2 complexes. We show that the simple "homogeneous" kinetic treatment used here agrees well with more detailed master equation calculations. We also discuss the agreement between information derived from the present technique and from singlet annihilation experiments. The experimental results show that the enhancement of the cross section of open RCs due to excitation transfer from closed units varies from 1.5 to 3 depending on species. The ratio of the core to core transfer rate (including the indirect pathway via LH2) to the rate of trapping in open units is in the range of 0.5 to 4. It is about 1 in Rhodobacter sphaeroides and does not increase significantly in mutants lacking LH2-despite the more numerous contacts between the dimeric core complexes expected in this case. The connectivity in this bacterium is due in good part to the fast transfer between the two partners of the dimeric (RC-LH1-PufX)(2) complex. The connectivity is however increased in the carotenoidless and LH2-less strain R26, which we ascribe to an anomalous LH1. A relatively high connectivity was found in Rhodospirillum photometricum, although not as high as predicted in the calculations of Fassioli et al. (2010). This illustrates a more general discrepancy between the measured efficiency of core to core excitation transfer and theoretical estimates. We argue that the limited core to core connectivity found in purple bacteria may reflect a trade-off between light-harvesting efficiency and the hindrance to quinone diffusion that would result from too tightly packed LH complexes.

Comparison of the fluorescence kinetics of detergent-solubilized and membrane-reconstituted LH2 complexes from Rps. acidophila and Rb. sphaeroides

Picosecond time-resolved fluorescence spectroscopy has been used in order to compare the fluorescence kinetics of detergent-solubilized and membrane-reconstituted light-harvesting 2 (LH2) complexes from the purple bacteria Rhodopseudomonas (Rps.) acidophila and Rhodobacter (Rb.) sphaeroides. LH2 complexes were reconstituted in phospholipid model membranes at different lipid:protein-ratios and all samples were studied exciting with a wide range of excitation densities. While the detergent-solubilized LH2 complexes from Rps. acidophila showed monoexponential decay kinetics (tau(f )= 980 ps) for excitation densities of up to 3.10(13) photons/(pulse.cm(2)), the membrane-reconstituted LH2 complexes showed multiexponential kinetics even at low excitation densities and high lipid:protein-ratios. The latter finding indicates an efficient clustering of LH2 complexes in the phospholipid membranes. Similar results were obtained for the LH2 complexes from Rb. sphaeroides.

In vitro self-assembly of the light harvesting pigment-protein LH2 revealed by ultrafast spectroscopy and electron microscopy

Controlled ensemble formation of protein-surfactant systems provides a fundamental concept for the realization of nanoscale devices with self-organizing capability. In this context, spectroscopic monitoring of pigment-containing proteins yields detailed structural information. Here we have studied the association behavior of the bacterial light-harvesting protein LH2 from Rhodobacter spheroides in an n,n-dimethyldodecylamine-n-oxide/water environment. Time-resolved studies of the excitation annihilation yielded information about aggregate sizes and packing of the protein complexes therein. The results are compared to transmission electron microscopy images of instantaneously frozen samples. Our data indicate the manifestation of different phases, which are discussed with respect to the thermodynamic equilibrium in ternary protein-surfactant-water systems. Accordingly, by varying the concentration the formation of different types of aggregates can be controlled. Conditions for the appearance of isolated LH2 complexes are defined.

Circular symmetry of the light-harvesting 1 complex from Rhodospirillum rubrum is not perturbed by interaction with the reaction center

The effect of the interaction of the reaction center (RC) upon the geometrical arrangement of the bacteriochlorophyll a (BChla) pigments in the light-harvesting 1 complex (LH1) from Rhodospirillum rubrum has been examined using single molecule spectroscopy. Fluorescence excitation spectra at 1.8 K obtained from single detergent-solubilized as well as single membrane-reconstituted LH1-RC complexes showed predominantly (>70%) a single broad absorption maximum at 880-900 nm corresponding to the Q(y) transition of the LH1 complex. This absorption band was independent of the polarization direction of the excitation light. The remaining complexes showed two mutually orthogonal absorption bands in the same wavelength region with moderate splittings in the range of DeltaE = 30-85 cm(-1). Our observations are in agreement with simulated spectra of an array of 32 strongly coupled BChla dipoles arranged in perfect circular symmetry possessing only a diagonal disorder of <or=150 cm(-1). However, in contrast to LH1 complexes alone, excitation spectra that consist of a single absorption band were observed more frequently in the presence of the reaction center. Our results show that the interaction of the RC with the LH1 complex stabilizes the circular symmetric arrangement of the bacteriochlorophyll pigments and are in contradiction to recent studies by other groups using single molecule spectroscopy as well as cryoelectronmicroscopy and atomic force microscopy indicating that the RC induces an elliptical distortion of the LH1 complex. Possible reasons for this discrepancy are discussed.

Projection structures of three photosynthetic complexes from Rhodobacter sphaeroides: LH2 at 6 A, LH1 and RC-LH1 at 25 A

Three photosynthetic complexes, light-harvesting complex 2 (LH2), light-harvesting complex 1 (LH1), and the reaction centre-light-harvesting complex 1 photounit (RC-LH1), were purified from a single species of a purple bacterium, Rhodobacter sphaeroides, and reconstituted into two-dimensional (2-D) crystals. Vesicular 2-D crystals of LH1 and RC-LH1 were imaged in negative stain and projection maps at 25 A resolution were produced. The rings formed by LH1 have approximately the same mean diameter as the LH1 rings from Rhodospirillum rubrum ( approximately 90 A) and therefore are likely to be composed of 15 to 17 alphabeta subunits. In the projection map calculated from the RC-LH1 2-D crystals, the reaction centre is represented by an additional density in the centre of the ring formed by the LH1 subunits. The marked improvement of shape and fine structure after a rotational pre-alignment of the RC-LH1 unit cells before averaging strongly suggests that the RC is not in a unique orientation within the LH1 rings. Tubular crystals of LH2 showed a high degree of order and allowed calculation of a projection map at 6 A resolution from glucose-embedded specimens. The projection structure shows a ring of nine alphabeta subunits. Variation of the alpha-helical projection densities suggests that the 9-fold symmetry axis is tilted with respect to the membrane normal.

Antenna organization in the purple sulfur bacteria Chromatium tepidum and Chromatium vinosum

Structural aspects of the core antenna in the purple sulfur bacteria Chromatium tepidum and Chromatium vinosum were studied by means of fluorescence emission and singlet-singlet annihilation measurements. In both species the number of bacteriochlorophylls of the core antenna between which energy transfer can occur corresponds to one core-reaction center complex only. From measurements of variable fluorescence we conclude that in C. tepidum excitation energy can be transferred back from the core antenna (B920) to the peripheral B800-850 complex in spite of the relatively large energy gap, and on basis of annihilation measurements a model of separate core-reaction center units accompanied by their own peripheral antenna is suggested. C. vinosum contains besides a core antenna, B890, two peripheral antennae, B800-820 and B800-850. Energy transfer was found to occur from the core to B800-850, but not to B800-820, and it was concluded that in C. vinosum each core-reaction center complex has its own complement of B800-850. The results reported here are compared to those obtained earlier with various strains and species of purple non-sulfur bacteria.

Competition between annihilation and trapping leads to strongly reduced yields of photochemistry under ps-flash excitation

Excitation of photosynthetic systems with short intense flashes is known to lead to exciton-exciton annihilation processes. Here we quantify the effect of competition between annihilation and trapping for Photosystem II, Photosystem I (thylakoids from peas and membranes from the cyanobacterium Synechocystis sp.), as well as for the purple bacterium Rhodospirillum rubrum. In none of the cases it was possible to reach complete product saturation (i.e. closure of reaction centers) even with an excitation energy exceeding 10 hits per photosynthetic unit. The parameter α introduced by Deprez et al. ((1990) Biochim. Biophys. Acta 1015: 295-303) describing the competition between exciton-exciton annihilation and trapping was calculated to range between ≈4.5 (PS II) and ≈6 (Rs. rubrum). The rate constants for bimolecular exciton-exciton annihilation ranged between (42 ps)(-1) and (2.5 ps)(-1) for PS II and PS I-membranes of Synechocystis, respectively. The data are interpreted in terms of hopping times (i.e. mean residence time of the excited state on a chromophore) according to random walk in isoenergetic antenna.

Purification and crystallization of the light harvesting LH1 complex from Rhodobacter sphaeroides

The LH1 light harvesting complex has been purified from a mutant of the photosynthetic bacterium Rhodobacter sphaeroides which synthesizes LH1 as the sole pigment protein. Crystallization trials using polyethylene glycol as the precipitant in the presence of the detergent n-octyl glucoside have resulted in the formation of needle like crystals which diffract beyond 3.5 A and which are relatively resistant to radiation damage. X-ray photographs have established that the crystals belong to the tetragonal system and are probably in space group P4(2)2(1)2. Estimates of the crystal density indicate that the asymmetric unit of the crystals contains two oligomers each with an alpha 6 beta 6 stoichiometry.

Structure, function and organization of antenna polypeptides and antenna complexes from the three families of Rhodospirillaneae

Comparative primary structural analysis of polypeptides from antenna complexes from species of the three families of Rhodospirillaneae indicates the structural principles responsible for the formation of spectrally distinct light-harvesting complexes. In many of the characterized antenna systems the basic structural minimal unit is an alpha/beta polypeptide pair. Specific clusters of amino acid residues, in particular aromatic residues in the C-terminal domain, identify the antenna polypeptides to specific types of antenna systems, such as B880 (strong circular dichroism (CD)), B870 (weak CD), B800-850 (high), B800-850 (low) or B800-820. The core complex B880 (B1020) of species from Ectothiorhodospiraceae and Chromatiaceae apparently consists of four (alpha 1 alpha 2 beta 1 beta 2) or three (2 alpha beta 1 beta 2) chemically dissimilar antenna polypeptides respectively. There is good evidence that the so-called variable antenna complexes, such as the B800-850 (high), B800-850 (low) or B800-820 of Rp. acidophila, Rp. palustris and Cr. vinosum, are comprised of multiple forms of peripheral light-harvesting polypeptides. Structural similarities between prokaryotic and eukaryotic antenna polypeptides are discussed in terms of similar pigment organization. The structural basis for the strict organization of pigment molecules (bacteriochlorophyll (BChl) cluster) in the antenna system of purple bacteria is the hierarchical organization of the alpha- and beta-antenna polypeptides within and between the antenna complexes. On the basis of the three-domain structure of the antenna polypeptides with the central hydrophobic domain, forming a transmembrane alpha helix, possible arrangements of the antenna polypeptides in the three-dimensional structure of core and peripheral antenna complexes are discussed. Important structural and functional features of these polypeptides and therefore of the BChl cluster are the alpha/beta heterodimers, the alpha 2 beta 2 basic units and cyclic arrangements of these basic units. Equally important for the formation of the antenna complexes or the entire antenna are polypeptide-polypeptide, pigment-pigment and pigment-polypeptide interactions.

Mutants of Rhodobacter sphaeroides lacking one or more pigment-protein complexes and complementation with reaction-centre, LH1, and LH2 genes

The photosynthetic apparatus of Rhodobacter sphaeroides is comprised of three types of pigment-protein complex: the photochemical reaction centre and its attendant LH1 and LH2 light-harvesting complexes. To augment existing deletion/insertion mutants in the genes coding for these complexes we have constructed two further mutants, one of which is a novel double mutant which is devoid of all three types of complex. We have also constructed vectors for the expression of either LH1, LH2 or reaction-centre genes. The resulting system allows each pigment-protein complex to be studied either as part of an intact photosystem or as the sole complex in the cell. In this way we have demonstrated that reaction centres can assemble independently of either light-harvesting complex in R. sphaeroides. In addition, the isolation of derivatives of the deletion/insertion mutants exhibiting spontaneous mutations in carotenoid biosynthesis provides an avenue for examining the role of carotenoids in the assembly of the photosynthetic apparatus. We show that the LH1 complex is assembled regardless of the carotenoid background, and that the type of carotenoid present modifies the absorbance of the LH1 bacteriochlorophylls.

Isolation and characterization of a structural subunit from the core light-harvesting complex of Rhodobacter sphaeroides 2.4.1 and puc705-BA

A method for isolating a structural subunit, B825, from the B875 core light-harvesting complex (LHC) of Rhodobacter sphaeroides 2.4.1 (wild-type) and a B800-B850(-) mutant, puc705-BA, is presented. This method, based on one developed to prepare a similar subunit, B820, from the core LHC of Rhodospirillum rubrum [Miller et al., Biochemistry 26, 5055-5062 (1987)], requires the dissociation of treated chromatophores with the detergent, octyl-glucoside. A subsequent gel filtration step separates B800-850 (if present), reaction centers, and free bacteriochlorophyll from the subunit complex. B825 was quantitatively reassociated into an 873 nm absorbing form which resembled the in vivo complex as judged by its absorption properties. The polypeptides in B825 and B800-850 were isolated by HPLC and identified by N-terminal amino acid sequencing. Two polypeptides, alpha and beta, were found in each complex in a 1:1 ratio. The spectral and biochemical properties of the subunits isolated from Rhodospirillum rubrum, Rhodobacter sphaeroides, and Rhodobacter capsulatus are compared.

Localized transposon Tn5 mutagenesis of the photosynthetic gene cluster of Rhodobacter sphaeroides

Four genes essential for bacteriochlorophyll biosynthesis were known to be encoded within a 45 kb region of the Rhodobacter sphaeroides genome, the boundaries of which are defined by puh and puf genes for reaction-centre and light-harvesting LH1 complexes. The cluster is represented by eight overlapping inserts cloned in the mobilizable vector pSUP202. We have used localized transposon Tn5 mutagenesis to characterize this cluster further; a total of 87 independent insertions were generated which identify nine genes for bacteriochlorophyll biosynthesis, six for carotenoid biosynthesis, and puhA encoding the reaction-centre H subunit. This work provides an essential framework for a detailed study of the structure and expression of genes for photosynthesis in this bacterium.

Energy-transfer dynamics in three light-harvesting mutants of Rhodobacter sphaeroides: a picosecond spectroscopy study

Picosecond absorption spectroscopy has been used to investigate energy-transfer dynamics within the LH1 and LH2 light-harvesting complexes of three mutants of Rhodobacter sphaeroides. We demonstrate that both complexes are inhomogeneous; each contains a specialized pigment pool which absorbs maximally at a longer wavelength. Within LH2 (mutant NF57), Bchl850 transfers energy to Bchl870 in 39 +/- 9 ps; within LH1 (mutants M21 and M2192), energy is transferred from Bchl875 to Bchl896 in 22 +/- 4 and 35 +/- 5 ps, respectively. Examination of the decay of induced absorption anisotropy indicates that each of these specialized pools exists in a state which is highly organized with respect to the remainder of the pigments. Such an arrangement may facilitate and direct energy migration toward the reaction center.

Temperature dependence of energy transfer from the long wavelength antenna BChl-896 to the reaction center in Rhodospirillum rubrum, Rhodobacter sphaeroides (w.t. and M21 mutant) from 77 to 177K, studied by picosecond absorption spectroscopy

Decay of the bacteriochlorophyll excited state was measured in membranes of the purple bacteria Rhodospirillum (R.) rubrum, Rhodobacter (Rb.) sphaeroides wild type and Rb. sphaeroides mutant M21 using low intensity picosecond absorption spectroscopy. The excitation and probing pulses were chosen in the far red wing of the long wavelength absorption band, such that predominantly the minor antenna species B896 was excited. The decay of B896 was studied between 77 and 177K under conditions that the traps were active. In all species the B896 excited state decay is almost temperature independent between 100 and 177K, and probably between 100 and 300 K. In this temperature range the decay rates for the various species are very similar and close to 40 ps. Below 100 K this rate remains temperature independent in Rb. sphaeroides w. t. and M21, while in R. rubrum a steep decrease sets in. An analysis of this data with the theory of nuclear tunneling indicates an activation energy for the final transfer step from B896 to the special pair of 70cm(-1) for R. rubrum and 30cm(-1) or less for Rb. sphaeroides.

Photosynthetic antenna proteins: 100 ps before photochemistry starts

All photosynthetic organisms require a light harvesting system to funnel excitation energy towards the photosynthetic reaction centre, a process which can take 100 ps. Laser spectroscopy allows us to measure rates of energy transfer between pigments of the light harvesting system for the first time. These rates are correlated with models of the light harvesting apparatus.