The Lhca antenna complexes of higher plants photosystem I

The Lhca antenna complexes of photosystem I (PSI) have been characterized by comparison of native and recombinant preparations. Eight Lhca polypeptides have been found to be all organized as dimers in the PSI-LHCI complex. The red emission fluorescence is associated not only with Lhca1-4 heterodimer, but also with dimers containing Lhca2 and/or Lhca3 complexes. Reconstitution of Lhca1 and Lhca4 monomers as well as of the Lhca1-4 dimer in vitro was obtained. The biochemical and spectroscopic features of these three complexes are reported. The monomers Lhca1 and Lhca4 bind 10 Chls each, while the Chl a/b ratio is lower in Lhca4 as compared to Lhca1. Three carotenoid binding sites have been found in Lhca1, while only two are present in Lhca4. Both complexes contain lutein and violaxanthin while beta-carotene is selectively bound to the Lhca1-4 dimer in substoichiometric amounts upon dimerization. Spectral analysis revealed the presence of low energy absorption forms in Lhca1 previously thought to be exclusively associated with Lhca4. It is shown that the process of dimerization changes the spectroscopic properties of some chromophores and increases the amplitude of the red absorption tail of the complexes. The origin of these spectroscopic features is discussed.     

The Lhca antenna complexes of higher plants photosystem I

The Lhca antenna complexes of photosystem I (PSI) have been characterized by comparison of native and recombinant preparations. Eight Lhca polypeptides have been found to be all organized as dimers in the PSI-LHCI complex. The red emission fluorescence is associated not only with Lhca1-4 heterodimer, but also with dimers containing Lhca2 and/or Lhca3 complexes. Reconstitution of Lhca1 and Lhca4 monomers as well as of the Lhca1-4 dimer in vitro was obtained. The biochemical and spectroscopic features of these three complexes are reported. The monomers Lhca1 and Lhca4 bind 10 Chls each, while the Chl a/b ratio is lower in Lhca4 as compared to Lhca1. Three carotenoid binding sites have been found in Lhca1, while only two are present in Lhca4. Both complexes contain lutein and violaxanthin while β-carotene is selectively bound to the Lhca1-4 dimer in substoichiometric amounts upon dimerization. Spectral analysis revealed the presence of low energy absorption forms in Lhca1 previously thought to be exclusively associated with Lhca4. It is shown that the process of dimerization changes the spectroscopic properties of some chromophores and increases the amplitude of the red absorption tail of the complexes. The origin of these spectroscopic features is discussed.     

De-epoxidation of violaxanthin in light-harvesting complex I proteins

The conversion of violaxanthin (Vx) to zeaxanthin (Zx) in the de-epoxidation reaction of the xanthophyll cycle plays an important role in the protection of chloroplasts against photooxidative damage. Vx is bound to the antenna proteins of both photosystems. In photosystem II, the formation of Zx is essential for the pH-dependent dissipation of excess light energy as heat. The function of Zx in photosystem I is still unclear. In this work we investigated the de-epoxidation characteristics of light-harvesting complex proteins of photosystem I (LHCI) under in vivo and in vitro conditions. Recombinant LHCI (Lhcal-4) proteins were reconstituted with Vx and lutein, and the convertibility of Vx was studied in an in vitro assay using partially purified Vx de-epoxidase isolated from spinach thylakoids. All four LHCI proteins exhibited unique de-epoxidation characteristics. An almost complete Vx conversion to Zx was observed only in Lhca3, whereas Zx formation in the other LHCI proteins decreased in the order Lhca4 > Lhca1 > Lhca2. Most likely, these differences in Vx de-epoxidation were related to the different accessibility of the respective carotenoid binding sites in the distinct antenna proteins. The results indicate that Vx bound to site V1 and N1 is easily accessible for de-epoxidation, whereas Vx bound to L2 is only partially and/or with the slower kinetics convertible to Zx. The de-epoxidation properties determined for the monomeric recombinant proteins were consistent with those obtained for isolated native LHCI-730 and LHCI-680 in the same in vitro assay and the de-epoxidation state found under in vivo conditions in native LHCIs.     

Pigment binding of photosystem I light-harvesting proteins

Light-harvesting complexes (LHC) of higher plants are composed of at least 10 different proteins. Despite their pronounced amino acid sequence homology, the LHC of photosystem II show differences in pigment binding that are interpreted in terms of partly different functions. By contrast, there is only scarce knowledge about the pigment composition of LHC of photosystem I, and consequently no concept of potentially different functions of the various LHCI exists. For better insight into this issue, we isolated native LHCI-730 and LHCI-680. Pigment analyses revealed that LHCI-730 binds more chlorophyll and violaxanthin than LHCI-680. For the first time all LHCI complexes are now available in their recombinant form; their analysis allowed further dissection of pigment binding by individual LHCI proteins and analysis of pigment requirements for LHCI formation. By these different approaches a correlation between the requirement of a single chlorophyll species for LHC formation and the chlorophyll a/b ratio of LHCs could be detected, and indications regarding occupation of carotenoid-binding sites were obtained. Additionally the reconstitution approach allowed assignment of spectral features observed in native LHCI-680 to its components Lhca2 and Lhca3. It is suggested that excitation energy migrates from chlorophyll(s) fluorescing at 680 (Lhca3) via those fluorescing at 686/702 nm (Lhca2) or 720 nm (Lhca3) to the photosystem I core chlorophylls.     

The role of lutein in the acclimation of higher plant chloroplast membranes to suboptimal conditions

Two mutants of Arabidopsis thaliana deficient in lutein have been investigated with respect to their responses to growth under a range of suboptimal conditions. The first mutant, lut1 , was enriched in violaxanthin, antheraxanthin, zeaxanthin and zeinoxanthin compared with the wild-type (WT). In the second mutant, lut2 , the lack of lutein was compensated for only by an increase in xanthophyll cycle (XC) carotenoids. Upon transfer of plants grown under optimal conditions to high light (HL), drought or HL + drought, both mutants acclimated during several days to the new conditions to the same extent as the WT. In contrast, transfer to chilling conditions (6°C) for 6 days induced responses that were different between WT and mutants and between the mutants themselves. In contrast to the WT, the lut2 mutant in particular exhibited a large increase in the Chl a/b ratio and the XC pool size, extensive de-epoxidation and an enhanced extent of non-photochemical quenching. It is suggested that although the role of lutein in the structure and organisation of the light-harvesting complexes can be fulfilled by other xanthophylls under excess light conditions at optimal temperatures, this is not the case at low temperature.     

Molecular configuration of xanthophyll cycle carotenoids in photosystem II antenna complexes

The molecular configuration of the xanthophyll cycle carotenoids, violaxanthin and zeaxanthin, was studied in various isolated photosystem II antenna components in comparison to intact photosystem II membranes using resonance Raman combined with low-temperature absorption spectroscopy. The molecular configurations of zeaxanthin and violaxanthin in thylakoids and isolated photosystem II membranes were found to be the same within an isolated oligomeric LHCII antenna, confirming our recent conclusion that these molecules are not freely located in photosynthetic membranes (Ruban, A. V., Pascal, A. A., Robert, B., and Horton, P. (2001) J. Biol. Chem. 276, 24862-24870). In contrast, xanthophyll cycle carotenoids bound to LHCII trimers had largely lost their in vivo configuration, suggesting their partial dissociation from the binding locus. Violaxanthin and zeaxanthin associated with the minor antenna complexes, CP26 and CP29, were also found to be in a relaxed configuration, similar to that of free pigment. The origin of the characteristic C-H vibrational bands of violaxanthin and zeaxanthin in vivo is discussed by comparison with those of neoxanthin and lutein in oligomeric and trimeric LHCII respectively.     

Carotenoid binding sites in LHCIIb. Relative affinities towards major xanthophylls of higher plants.

The major light-harvesting complex of photosystem II can be reconstituted in vitro from its bacterially expressed apoprotein with chlorophylls a and b and neoxanthin, violaxanthin, lutein, or zeaxanthin as the only xanthophyll. Reconstitution of these one-carotenoid complexes requires low-stringency conditions during complex formation and isolation. Neoxanthin complexes (containing 30-50% of the all-trans isomer) disintegrate during electrophoresis, exhibit a largely reduced resistance against proteolytic attack; in addition, energy transfer from Chl b to Chl a is easily disrupted at elevated temperature. Complexes reconstituted in the presence of either zeaxanthin or lutein contain nearly two xanthophylls per 12 chlorophylls and are more resistant against trypsin. Lutein-LHCIIb also exhibits an intermediate maintenance of energy transfer at higher temperature. Violaxanthin complexes approach a xanthophyll/12 chlorophyll ratio of 3, similar to the ratio in recombinant LHCIIb containing all xanthophylls. On the other hand, violaxanthin-LHCIIb exhibits a low thermal stability like neoxanthin complexes, but an intermediate accessibility towards trypsin, similar to lutein-LHCIIb and zeaxanthin-LHCIIb. Binary competition experiments were performed with two xanthophylls at varying ratios in the reconstitution. Analysis of the xanthophyll contents in the reconstitution products yielded information about relative carotenoid affinities of three assumed binding sites. In lutein/neoxanthin competition experiments, two binding sites showed a strong preference (> 200-fold) for lutein, whereas the third binding site had a higher affinity (25-fold) to neoxanthin. Competition between lutein and violaxanthin gave a similar result, although the specificities were lower: two binding sites have a 36-fold preference for lutein and one has a fivefold preference for violaxanthin. The lowest selectivity was between lutein and zeaxanthin: two binding sites had a fivefold higher affinity for lutein and one has a threefold higher affinity to zeaxanthin.     

Structure of the higher plant light harvesting complex I: in vivo characterization and structural interdependence of the Lhca proteins

We have investigated the structure of the higher plant light harvesting complex of photosystem I (LHCI) by analyzing PSI-LHCI particles isolated from a set of Arabidopsis plant lines, each lacking a specific Lhca (Lhca1-4) polypeptide. Functional antenna size measurements support the recent finding that there are four Lhca proteins per PSI in the crystal structure [Ben-Shem, A., Frolow, F., and Nelson, N. (2003) Nature 426, 630-635]. According to HPLC analyses the number of pigment molecules bound within the LHCI is higher than expected from reconstitution studies or analyses of isolated native LHCI. Comparison of the spectra of the particles from the different lines reveals chlorophyll absorption bands peaking at 696, 688, 665, and 655 nm that are not present in isolated PSI or LHCI. These bands presumably originate from "gap" or "linker" pigments that are cooperatively coordinated by the Lhca and/or PSI proteins, which we have tentatively localized in the PSI-LHCI complex.     

Structural modeling of the Lhca4 Subunit of LHCI-730 peripheral antenna in photosystem I based on similarity with LHCII

Peripheral chlorophyll a/b binding antenna of photosystem I (LHCI) from green algae and higher plants binds specific low energy absorbing chlorophylls (red pigments) that give rise to a unique red-shifted emission. A three-dimensional structural model of the Lhca4 polypeptide from the LHCI from higher plants was constructed on the basis of comparative sequence analysis, secondary structure prediction, and homology modeling using LHCII as a template. The obtained model of Lhca4 helps to visualize protein ligands to nine chlorophylls (Chls) and three potential His residues to extra Chls. Central domain of the Lhca4 comprising the first (A) and the third (C) transmembrane (TM) helices that binds 6 Chl molecules and two carotenoids is conserved structurally, whereas the interface between the first and the second TM helices and the outer surface of the second TM helix differ significantly among the LHCI and LHCII polypeptides. The model of Lhca4 predicts a histidine residue in the second TM helix, a potential binding site for extra Chl in close proximity to Chls a5 and b5 (labeling by Kühlbrandt). The interpigment interactions in the formed pigment cluster are suggested to cause a red spectral shift in absorption and emission. Modeling of the LHCI-730 heterodimer based on the model structures of Lhca1 and Lhca4 allowed us to suggest potential sites of pigment-pigment interactions that might be formed upon heterodimerization or docking of the LHCI dimers to the surface of PSI.     

The nature of a chlorophyll ligand in Lhca proteins determines the far red fluorescence emission typical of photosystem I

Photosystem I of higher plants is characterized by a typically long wavelength fluorescence emission associated to its light-harvesting complex I moiety. The origin of these low energy chlorophyll spectral forms was investigated by using site-directed mutagenesis of Lhca1-4 genes and in vitro reconstitution into recombinant pigment-protein complexes. We showed that the red-shifted absorption originates from chlorophyll-chlorophyll (Chl) excitonic interactions involving Chl A5 in each of the four Lhca antenna complexes. An essential requirement for the presence of the red-shifted absorption/fluorescence spectral forms was the presence of asparagine as a ligand for the Chl a chromophore in the binding site A5 of Lhca complexes. In Lhca3 and Lhca4, which exhibit the most red-shifted red forms, its substitution by histidine maintains the pigment binding and, yet, the red spectral forms are abolished. Conversely, in Lhca1, having very low amplitude of red forms, the substitution of Asn for His produces a red shift of the fluorescence emission, thus confirming that the nature of the Chl A5 ligand determines the correct organization of chromophores leading to the excitonic interaction responsible for the red-most forms. The red-shifted fluorescence emission at 730 nm is here proposed to originate from an absorption band at approximately 700 nm, which represents the low energy contribution of an excitonic interaction having the high energy band at 683 nm. Because the mutation does not affect Chl A5 orientation, we suggest that coordination by Asn of Chl A5 holds it at the correct distance with Chl B5.     

Determination of the stoichiometry and strength of binding of xanthophylls to the photosystem II light harvesting complexes

Xanthophylls have a crucial role in the structure and function of the light harvesting complexes of photosystem II (LHCII) in plants. The binding of xanthophylls to LHCII has been investigated, particularly with respect to the xanthophyll cycle carotenoids violaxanthin and zeaxanthin. It was found that most of the violaxanthin pool was loosely bound to the major complex and could be removed by mild detergent treatment. Gentle solubilization of photosystem II particles and thylakoids allowed the isolation of complexes, including a newly described oligomeric preparation, enriched in trimers, that retained all of the in vivo violaxanthin pool. It was estimated that each LHCII monomer can bind at least one violaxanthin. The extent to which different pigments can be removed from LHCII indicated that the relative strength of binding was chlorophyll b > neoxanthin > chlorophyll a > lutein > zeaxanthin > violaxanthin. The xanthophyll binding sites are of two types: internal sites binding lutein and peripheral sites binding neoxanthin and violaxanthin. In CP29, a minor LHCII, both a lutein site and the neoxanthin site can be occupied by violaxanthin. Upon activation of the violaxanthin de-epoxidase, the highest de-epoxidation state was found for the main LHCII component and the lowest for CP29, suggesting that only violaxanthin loosely bound to LHCII is available for de-epoxidation.     

Energy transfer pathways in the minor antenna complex CP29 of photosystem II: a femtosecond study of carotenoid to chlorophyll transfer on mutant and WT complexes

The energy transfer processes between carotenoids and Chls have been studied by femtosecond transient absorption in the CP29-WT complex, which contains only two carotenoids per polypeptide located in the L1 and L2 sites, and in the CP29-E166V mutant in which only the L1 site is occupied. The comparison of these two samples allowed us to discriminate between the energy transfer pathways from the two carotenoid binding sites and thus to obtain detailed information on the Chl organization in CP29 and to assign the acceptor chlorophylls. For both samples, the main transfer occurs from the S(2) state of the carotenoid. In the case of the L1 site the energy acceptor is the Chl a 680 nm (A2), whereas the Chl a 675 nm (A4-A5) and the Chl b 652 nm (B6) are the acceptors from the xanthophyll in the L2 site. These transfers occur with lifetimes of 80-130 fs. Two additional transfers are observed with 700-fs and 8- to 20-ps lifetimes. Both these transfers originate from the carotenoid S(1) states. The faster lifetime is due to energy transfer from a vibrationally unrelaxed S(1) state, whereas the 8- to 20-ps component is due to a transfer from the S(1,0) state of violaxanthin and/or neoxanthin located in site L2. A comparison between the carotenoid to Chl energy transfer pathways in CP29 and LHCII is presented and differences in the structural organization in the two complexes are discussed.     

Absence of lutein, violaxanthin and neoxanthin affects the functional chlorophyll antenna size of photosystem-II but not that of photosystem-I in the green alga Chlamydomonas reinhardtii

Chlamydomonas reinhardtii double mutant npq2 lor1 lacks the beta, epsilon-carotenoids lutein and loroxanthin as well as all beta,beta-epoxycarotenoids derived from zeaxanthin (e.g. violaxanthin and neoxanthin). Thus, the only carotenoids present in the thylakoid membranes of the npq2 lor1 cells are beta-carotene and zeaxanthin. The effect of these mutations on the photochemical apparatus assembly and function was investigated. In cells of the mutant strain, the content of photosystem-II (PSII) and photosystem-I (PSI) was similar to that of the wild type, but npq2 lor1 had a significantly smaller PSII light-harvesting Chl antenna size. In contrast, the Chl antenna size of PSI was not truncated in the mutant. SDS-PAGE and Western blot analysis qualitatively revealed the presence of all LHCII and LHCI apoproteins in the thylakoid membrane of the mutant. The results showed that some of the LHCII and most of the LHCI were assembled and functionally connected with PSII and PSI, respectively. Photon conversion efficiency measurements, based on the initial slope of the light-saturation curve of photosynthesis and on the yield of Chl a fluorescence in vivo, showed similar efficiencies. However, a significantly greater light intensity was required for the saturation of photosynthesis in the mutant than in the wild type. It is concluded that zeaxanthin can successfully replace lutein and violaxanthin in most of the functional light-harvesting antenna of the npq2 lor1 mutant.     

The Effects of Illumination on the Xanthophyll Composition of the Photosystem II Light-Harvesting Complexes of Spinach Thylakoid Membranes

The xanthophyll composition of the light-harvesting chlorophyll a/b proteins of photosystem II (LHCII) has been determined for spinach (Spinacia oleracea L.) leaves after dark adaptation and following illumination under conditions optimized for conversion of violaxanthin into zeaxanthin. Each of the four LHCII components was found to have a unique xanthophyll composition. The major carotenoid was lutein, comprising 60% of carotenoid in the bulk LHCIIb and 35 to 50% in the minor LHCII components LHCIIa, LHCIIc, and LHCIId. The percent of carotenoid found in the xanthophyll cycle pigments was approximately 10 to 15% in LHCIIb and 30 to 40% in LHCIIa, LHCIIc, and LHCIId. The xanthophyll cycle was active for the pigments bound to all of the LHCII components. The extent of deepoxidation for complexes prepared from light-treated leaves was 27, 65, 69, and 43% for LHCIIa, -b, -c, and -d, respectively. These levels of conversion of violaxanthin to zeaxanthin were found in LHCII prepared by three different isolation procedures. It was estimated that approximately 50% of the zeaxanthin associated with photosystem II is in LHCIIb and 30% is associated with the minor LHCII components.     

The binding of Xanthophylls to the bulk light-harvesting complex of photosystem II of higher plants. A specific requirement for carotenoids with a 3-hydroxy-beta-end group

The pigment composition of the light-harvesting complexes (LHCs) of higher plants is highly conserved. The bulk complex (LHCIIb) binds three xanthophyll molecules in combination with chlorophyll (Chl) a and b. The structural requirements for binding xanthophylls to LHCIIb have been examined using an in vitro reconstitution procedure. Reassembly of the monomeric recombinant LHCIIb was performed using a wide range of native and nonnative xanthophylls, and a specific requirement for the presence of a hydroxy group at C-3 on a single beta-end group was identified. The presence of additional substituents (e.g. at C-4) did not interfere with xanthophyll binding, but they could not, on their own, support reassembly. cis isomers of zeaxanthin, violaxanthin, and lutein were not bound, whereas all-trans-neoxanthin and different chiral forms of lutein and zeaxanthin were incorporated into the complex. The C-3 and C-3' diols lactucaxanthin (a carotenoid native to many plant LHCs) and eschscholtzxanthin (a retro-carotenoid) both behaved very differently from lutein and zeaxanthin in that they would not support complex reassembly when used alone. Lactucaxanthin could, however, be bound when lutein was also present, and it showed a high affinity for xanthophyll binding site N1. In the presence of lutein, lactucaxanthin was readily bound to at least one lutein-binding site, suggesting that the ability to bind to the complex and initiate protein folding may be dependent on different structural features of the carotenoid molecule. The importance of carotenoid end group structure and ring-to-chain conformation around the C-6-C-7 torsion angle of the carotenoid molecule in binding and complex reassembly is discussed.     

The low-energy forms of photosystem I light-harvesting complexes: spectroscopic properties and pigment-pigment interaction characteristics

In this work the spectroscopic properties of the special low-energy absorption bands of the outer antenna complexes of higher plant Photosystem I have been investigated by means of low-temperature absorption, fluorescence, and fluorescence line-narrowing experiments. It was found that the red-most absorption bands of Lhca3, Lhca4, and Lhca1-4 peak, respectively, at 704, 708, and 709 nm and are responsible for 725-, 733-, and 732-nm fluorescence emission bands. These bands are more red shifted compared to "normal" chlorophyll a (Chl a) bands present in light-harvesting complexes. The low-energy forms are characterized by a very large bandwidth (400-450 cm(-1)), which is the result of both large homogeneous and inhomogeneous broadening. The observed optical reorganization energy is untypical for Chl a and resembles more that of BChl a antenna systems. The large broadening and the changes in optical reorganization energy are explained by a mixing of an Lhca excitonic state with a charge transfer state. Such a charge transfer state can be stabilized by the polar residues around Chl 1025. It is shown that the optical reorganization energy is changing through the inhomogeneous distribution of the red-most absorption band, with the pigments contributing to the red part of the distribution showing higher values. A second red emission form in Lhca4 was detected at 705 nm and originates from a broad absorption band peaking at 690 nm. This fluorescence emission is present also in the Lhca4-N-47H mutant, which lacks the 733-nm emission band.     

The current concepts of functional role of carotenoids in the eukaryotic chloroplasts

Current state of knowledge of functional role of carotenoids in algal and higher plant chloroplasts is reviewed. Basic functions of carotenoids are shown to be light-protective, light-absorbing, and structural, as well as participating in photochemical processes of photosystems I and II. Such xanthophylls as neoxanthin, fucoxanthin, peridinin and alloxanthin, which have allenic or acetylenic bond, mostly function as light-absorbers. They transmit absorbed energy to chlorophyll b. Other xanthophylls occupying certain strictly specified loci in light-absorbing chlorophyll-a/b-protein complexes of photosystems have either structural function (lutein) or light-protective function (zeaxanthin, antheraxanthin, violaxanthin). Carotenoids of xanthophyll cycles preserve chlorophylls and lipids of photosynthetic membranes from photodestruction at overlighting in the presence of oxygen. In eukaryotic chloroplasts, three types of xanthophyll cycles were found: violaxanthin, lutein-5,6-epoxide, and diadinoxanthin. The similarities and dissimilarities between epoxidation and de-epoxidation reactions of these cycles are discussed in detail in the present work. The pattern of occurrence of xanthophyll cycles among higher plants and freshwater and marine algae is outlined.     

Analysis of xanthophyll cycle carotenoids and chlorophyll fluorescence in light intensity-dependent chlorophyll-deficient mutants of wheat and barley

Three light intensity-dependent Chl b-deficient mutants, two in wheat and one in barley, were analyzed for their xanthophyll cycle carotenoids and Chl fluorescence characteristics under two different growth PFDs (30 versus 600 mol photons·m^–2 s^–1 incident light). Mutants grown under low light possessed lower levels of total Chls and carotenoids per unit leaf area compared to wild type plants, but the relative proportions of the two did not vary markedly between strains. In contrast, mutants grown under high light had much lower levels of Chl, leading to markedly greater carotenoid to Chl ratios in the mutants when compared to wild type. Under low light conditions the carotenoids of the xanthophyll cycle comprised approximately 15% of the total carotenoids in all strains; under high light the xanthophyll cycle pool increased to over 30% of the total carotenoids in wild type plants and to over 50% of the total carotenoids in the three mutant strains. Whereas the xanthophyll cycle remained fairly epoxidized in all plants grown under low light, plants grown under high light exhibited a considerable degree of conversion of the xanthophyll cycle into antheraxanthin and zeaxanthin during the diurnal cycle, with almost complete conversion (over 90%) occurring only in the mutants. 50 to 95% of the xanthophyll cycle was retained as antheraxanthin and zeaxanthin overnight in these mutants which also exhibited sustained depressions in PS II photochemical efficiency (F_v/F_m), which may have resulted from a sustained high level of photoprotective energy dissipation activity. The relatively larger xanthophyll cycle pool in the Chl b-deficient mutant could result in part from the reported concentration of the xanthophyll cycle in the inner antenna complexes, given that the Chl b-deficient mutants are deficient in the peripheral LHC-II complexes.Abbreviations A antheraxanthin - Chl chlorophyll - F_o and F_m minimal yield (at open PS II reaction centers) and maximal yield (at closed centers) of chlorophyll fluorescence in darkness - F level of fluorescence during illumination with photosynthetically active radiation - F_m maximal yield (at closed centers) of chlorophyll fluorescence during illumination with photosynthetically active radiation - (F_m–F)/F_m actual efficiency of PS II during illumination with photosynthetically active radiation - F_v/F_m+(F_m–F_o)/F_m intrinsic efficiency of PS II in darkness - LHC_II light-harvesting chlorophyll-protein complex of Photosystem II - PFD photon flux density (between 400 and 700 nm) - PS I Photosystem I - PS II Photosystem II - V violaxanthin - Z zeaxanthin     

Evidence for two spectroscopically different dimers of light-harvesting complex I from green plants

A preparation consisting of isolated dimeric peripheral antenna complexes from green plant photosystem I (light-harvesting complex I or LHCI) has been characterized by means of (polarized) steady-state absorption and fluorescence spectroscopy at low temperatures. We show that this preparation can be described reasonably well by a mixture of two types of dimers. In the first dimer about 10% of all Q(y)() absorption of the chlorophylls arises from two chlorophylls with absorption and emission maxima at about 711 and 733 nm, respectively, whereas in the second about 10% of the absorption arises from two chlorophylls with absorption and emission maxima at about 693 and 702 nm, respectively. The remaining chlorophylls show spectroscopic properties comparable to those of the related peripheral antenna complexes of photosystem II. We attribute the first dimer to a heterodimer of the Lhca1 and Lhca4 proteins and the second to a hetero- or homodimer of the Lhca2 and/or Lhca3 proteins. We suggest that the chlorophylls responsible for the 733 nm emission (F-730) and 702 nm emission (F-702) are excitonically coupled dimers and that F-730 originates from one of the strongest coupled pair of chlorophylls observed in nature.