Spectroscopic properties of LHC-II, the main light-harvesting chlorophyll a/b protein complex from chloroplast membranes

Excitation energy equilibration in a trimeric LHCII complex involves unusual pathways

We explore the energy equilibration within the LHCII trimer using various approaches, including the Redfield-Förster method (with different compartmentalization schemes) and the exact hierarchical equation of motion (HEOM). We demonstrate that the inter-monomeric migration in the trimeric LHCII complex is not limited to direct transfers between quasi-equilibrated chlorophylls (Chls) a, but also involves additional pathways with uphill transfers from Chls a to the stromal-side Chls b (connecting the Chls a clusters from different monomeric subunits). Although these uphill transfers are slow they still can increase the total rate of inter-monomeric transfers by a factor of 1.5. The same stromal-side Chls b also promote a depopulation of the Chl a604 long-lived state (blue-shifted and mixed with the lumenal-side Chls b). Due to the connection between the stromal- and lumenal-side Chls b clusters the intra- and inter-monomeric transfers from a604 to the main Chls a become faster by a factor of 1.6 and 1.75, respectively.

Photodynamic Therapy: From the Basics to the Current Progress of N-Heterocyclic-Bearing Dyes as Effective Photosensitizers

Photodynamic therapy, an alternative that has gained weight and popularity compared to current conventional therapies in the treatment of cancer, is a minimally invasive therapeutic strategy that generally results from the simultaneous action of three factors: a molecule with high sensitivity to light, the photosensitizer, molecular oxygen in the triplet state, and light energy. There is much to be said about each of these three elements; however, the efficacy of the photosensitizer is the most determining factor for the success of this therapeutic modality. Porphyrins, chlorins, phthalocyanines, boron-dipyrromethenes, and cyanines are some of the N-heterocycle-bearing dyes' classes with high biological promise. In this review, a concise approach is taken to these and other families of potential photosensitizers and the molecular modifications that have recently appeared in the literature within the scope of their photodynamic application, as well as how these compounds and their formulations may eventually overcome the deficiencies of the molecules currently clinically used and revolutionize the therapies to eradicate or delay the growth of tumor cells.

Pigment structure in the light-harvesting protein of the siphonous green alga Codium fragile

The siphonaxanthin-siphonein-chlorophyll-a/b-binding protein (SCP), a trimeric light-harvesting complex isolated from photosystem II of the siphonous green alga Codium fragile, binds the carotenoid siphonaxanthin (Sx) and/or its ester siphonein in place of lutein, in addition to chlorophylls a/b and neoxanthin. SCP exhibits a higher content of chlorophyll b (Chl-b) than its counterpart in green plants, light-harvesting complex II (LHCII), increasing the relative absorption of blue-green light for photosynthesis. Using low temperature absorption and resonance Raman spectroscopies, we reveal the presence of two non-equivalent Sx molecules in SCP, and assign their absorption peaks at 501 and 535 nm. The red-absorbing Sx population exhibits a significant distortion that is reminiscent of lutein 2 in trimeric LHCII. Unexpected enhancement of the Raman modes of Chls-b in SCP allows an unequivocal description of seven to nine non-equivalent Chls-b, and six distinct Chl-a populations in this protein.

Exciton properties and optical spectra of light harvesting complex II from a fully atomistic description

We present a fully atomistic simulation of linear optical spectra (absorption, fluorescence and circular dichroism) of the Light Harvesting Complex II (LHCII) trimer using a hybrid approach, which couples a quantum chemical description of the chlorophylls with a classical model for the protein and the external environment (membrane and water). The classical model uses a polarizable Molecular Mechanics force field, thus allowing mutual polarization effects in the calculations of the excitonic properties. The investigation is performed both on the crystal structure and on structures generated by a μs long classical molecular dynamics simulation of the complex within a solvated membrane. The results show that this integrated approach not only provides a good description of the excitonic properties and optical spectra without the need for additional refinements of the excitonic parameters, but it also allows an atomistic investigation of the relative importance of electronic, structural and environment effects in determining the optical spectra.

Macroorganisation and flexibility of thylakoid membranes

The light reactions of photosynthesis are hosted and regulated by the chloroplast thylakoid membrane (TM) — the central structural component of the photosynthetic apparatus of plants and algae. The two-dimensional and three-dimensional arrangement of the lipid–protein assemblies, aka macroorganisation, and its dynamic responses to the fluctuating physiological environment, aka flexibility, are the subject of this review. An emphasis is given on the information obtainable by spectroscopic approaches, especially circular dichroism (CD). We briefly summarise the current knowledge of the composition and three-dimensional architecture of the granal TMs in plants and the supramolecular organisation of Photosystem II and light-harvesting complex II therein. We next acquaint the non-specialist reader with the fundamentals of CD spectroscopy, recent advances such as anisotropic CD, and applications for studying the structure and macroorganisation of photosynthetic complexes and membranes. Special attention is given to the structural and functional flexibility of light-harvesting complex II in vitro as revealed by CD and fluorescence spectroscopy. We give an account of the dynamic changes in membrane macroorganisation associated with the light-adaptation of the photosynthetic apparatus and the regulation of the excitation energy flow by state transitions and non-photochemical quenching.

Temperature Dependence of the Energy Transfer in LHCII Studied by Two-Dimensional Electronic Spectroscopy

We measured two-dimensional electronic spectra of light-harvesting complex II (LHCII) at various temperatures (77, 110, 150, 230, and 295 K) under conditions free from singlet-singlet annihilation. We elucidated the temperature-dependent excitation energy transfer dynamics in the Chl a manifold of LHCII. Global analysis revealed that the dynamics can be summarized in distinct time scales from 200 fs up to 15 ps. While the fastest dynamics with a decay time of ∼0.2-0.3 ps are relatively temperature-independent, the lifetimes and relative contributions of slower components showed considerable temperature dependence. The slowest time scale of equilibration with the lowest-energy Chl a increased from ∼5 ps at 295 K to ∼15 ps at 77 K. The final excited state is independent of initial excitation at 230 K and above, whereas static energy disorder is apparent at lower temperatures. A clear temperature dependence of uphill energy transfer processes was also discerned, which is consistent with the detailed-balance condition.

Growth Light Environment Changes the Sensitivity of Photosystem I Photoinhibition Depending on Common Wheat Cultivars

Light is an important factor for determining photosynthetic performance in land plants. At high light intensity, land plants develop photosynthetic activity by increasing electron sinks, such as the Calvin cycle and photorespiration and photoprotective mechanisms in photosystem II (PSII), to effectively utilize light and protect them from photoinhibition. In addition to PSII, photosystem I (PSI) has a risk of undergoing photoinhibition under high light intensity because of the reactive oxygen species (ROS) produced within PSI. However, the acclimation response has hardly been evaluated in the relationship of PSI photoprotection to growth light. In this study, we studied the effect of growth light intensity on the photoprotective mechanisms in PSI using six wheat cultivars. To evaluate the susceptibility of PSI to its photoinhibition, we used the repetitive short-pulse (rSP) illumination method to cause O₂-dependent PSI photoinhibition. We found that PSI photoinhibition induced by rSP illumination was much more alleviated in wheat cultivars grown under high-light conditions compared to those grown under low-light conditions. Here, we observed that wheat plant grown under high-light conditions lowered the susceptibility of PSI to its photoinhibition compared to those grown under low-light conditions. Furthermore, the acclimation response toward PSI photoinhibition was significantly different among the studied wheat cultivars, although the quantum yields both of PSII and PSI were increased by high-light acclimation in all wheat cultivars as reported previously. Interestingly, we observed that total chlorophyll content in leaves correlated with the susceptibility of PSI to its photoinhibition. On the basis of these results, we suggest that high-light acclimation induces protection mechanisms against PSI photoinhibition in land plants, and the increase in the leaf chlorophyll content relates to the susceptibility of PSI photoinhibition in wheat plants.

Temperature Dependence of Chlorophyll Triplet Quenching in Two Photosynthetic Light-Harvesting Complexes from Higher Plants and Dinoflagellates

Chlorophyll (Chl) triplet states generated in photosynthetic light-harvesting complexes (LHCs) can be quenched by carotenoids to prevent the formation of reactive singlet oxygen. Although this quenching occurs with an efficiency close to 100% at physiological temperatures, the Chl triplets are often observed at low temperatures. This might be due to the intrinsic temperature dependence of the Dexter mechanism of excitation energy transfer, which governs triplet quenching, or by temperature-induced conformational changes. Here, we report about the temperature dependence of Chl triplet quenching in two LHCs. We show that both the effects contribute significantly. In LHC II of higher plants, the core Chls are quenched with a high efficiency independent of temperature. A different subpopulation of Chls, which increases with lowering temperature, is not quenched at all. This is probably caused by the conformational changes which detach these Chls from the energy-transfer chain. In a membrane-intrinsic LHC of dinoflagellates, similarly two subpopulations of Chls were observed. In addition, another part of Chl triplets is quenched by carotenoids with a rate which decreases with temperature. This allowed us to study the temperature dependence of Dexter energy transfer. Finally, a part of Chls was quenched by triplet-triplet annihilation, a phenomenon which was not observed for LHCs before.

Light-harvesting complexes of Botryococcus braunii

The colonial green alga Botryococcus braunii (BB) is a potential source of biofuel due to its natural high hydrocarbon content. Unfortunately, its slow growth limits its biotechnological potential. Understanding its photosynthetic machinery could help to identify possible growth limitations. Here, we present the first study on BB light-harvesting complexes (LHCs). We purified two LHC fractions containing the complexes in monomeric and trimeric form. Both fractions contained at least two proteins with molecular weight (MW) around 25 kDa. The chlorophyll composition is similar to that of the LHCII of plants; in contrast, the main xanthophyll is loroxanthin, which substitutes lutein in most binding sites. Circular dichroism and 77 K absorption spectra lack typical differences between monomeric and trimeric complexes, suggesting that intermonomer interactions do not play a role in BB LHCs. This is in agreement with the low stability of the BB LHCII trimers as compared to the complexes of plants, which could be related to loroxanthin binding in the central (L1 and L2) binding sites. The properties of BB LHCII are similar to those of plant LHCII, indicating a similar pigment organization. Differences are a higher content of red chlorophyll a, similar to plant Lhcb3. These differences and the different Xan composition had no effect on excitation energy transfer or fluorescence lifetimes, which were similar to plant LHCII.

Isolating the chiral contribution in optical two-dimensional chiral spectroscopy using linearly polarized light

The full development of mono- or multi-dimensional time-resolved spectroscopy techniques incorporating optical activity signals has been strongly hampered by the challenge of identifying the small chiral signals over the large achiral background. Here we propose a new methodology to isolate chiral signals removing the achiral background from two commonly used configurations for performing two-dimensional optical spectroscopy, known as BOXCARS and gradient assisted photon echo spectroscopy (GRAPES). It is found that in both cases an achiral signal from an isotropic system can be completely eliminated by small manipulations of the relative angles between the linear polarizations of the four input laser pulses. Starting from the formulation of a perturbative expansion of the signal in the angle between the beams and the propagation axis, we derive analytic expressions that can be used to estimate how to change the polarization angles of the four pulses to minimize achiral contributions in the studied configurations. The generalization to any other possible experimental configurations has also been discussed.

Isolation of novel PSII-LHCII megacomplexes from pea plants characterized by a combination of proteomics and electron microscopy

In higher plants, photosystem II (PSII) is a multi-subunit pigment-protein complex embedded in the thylakoid membranes of chloroplasts, where it is present mostly in dimeric form within the grana. Its light-harvesting antenna system, LHCII, is composed of trimeric and monomeric complexes, which can associate in variable number with the dimeric PSII core complex in order to form different types of PSII-LHCII supercomplexes. Moreover, PSII-LHCII supercomplexes can laterally associate within the thylakoid membrane plane, thus forming higher molecular mass complexes, termed PSII-LHCII megacomplexes (Boekema et al. 1999a, in Biochemistry 38:2233-2239; Boekema et al. 1999b, in Eur J Biochem 266:444-452). In this study, pure PSII-LHCII megacomplexes were directly isolated from stacked pea thylakoid membranes by a rapid single-step solubilization, using the detergent n-dodecyl-α-D-maltoside, followed by sucrose gradient ultracentrifugation. The megacomplexes were subjected to biochemical and structural analyses. Transmission electron microscopy on negatively stained samples, followed by single-particle analyses, revealed a novel form of PSII-LHCII megacomplexes, as compared to previous studies (Boekema et al.1999a, in Biochemistry 38:2233-2239; Boekema et al. 1999b, in Eur J Biochem 266:444-452), consisting of two PSII-LHCII supercomplexes sitting side-by-side in the membrane plane, sandwiched together with a second copy. This second copy of the megacomplex is most likely derived from the opposite membrane of a granal stack. Two predominant forms of intact sandwiched megacomplexes were observed and termed, according to (Dekker and Boekema 2005 Biochim Biophys Acta 1706:12-39), as (C2S2)4 and (C2S2 + C2S2M2)2 megacomplexes. By applying a gel-based proteomic approach, the protein composition of the isolated megacomplexes was fully characterized. In summary, the new structural forms of isolated megacomplexes and the related modeling performed provide novel insights into how PSII-LHCII supercomplexes may bind to each other, not only in the membrane plane, but also between granal stacks within the chloroplast.

Observation of Electronic Excitation Transfer Through Light Harvesting Complex II Using Two-Dimensional Electronic-Vibrational Spectroscopy

Light-harvesting complex II (LHCII) serves a central role in light harvesting for oxygenic photosynthesis and is arguably the most important photosynthetic antenna complex. In this work, we present two-dimensional electronic-vibrational (2DEV) spectra of LHCII isolated from spinach, demonstrating the possibility of using this technique to track the transfer of electronic excitation energy between specific pigments within the complex. We assign the spectral bands via comparison with the 2DEV spectra of the isolated chromophores, chlorophyll a and b, and present evidence that excitation energy between the pigments of the complex are observed in these spectra. Finally, we analyze the essential components of the 2DEV spectra using singular value decomposition, which makes it possible to reveal the relaxation pathways within this complex.

Probing molecular events associated with early development of thylakoid membranes by comparative proteomics and low temperature fluorescence

A comparison of protein profiles between prolamellar bodies from dark-grown etioplasts and thylakoid membranes from de-etioplasts illuminated respectively for 1, 5 and 9h revealed 155 differentially expressed CBB-stained spots. Clear results showed that the nonphototransformable Pchlide627-632 was the dominant pigment form in the PLBs of rice etioplasts during plant development in dark and transformed slowly to chlorophyllide in rice etioplasts when exposed to light. The light-induced accumulation of ACC oxidase, which catalyzes the final step of ethylene synthesis using ACC as substrate, would facilitate chlorophyll synthesis by inducing PORa/b expression via ethylene signaling. It could be also suggested that cyclic electron transport might play an important role in generation of ATP for carbon fixation and photoprotection of photosystems from excessive light in prothylakoid. Furthermore, the overproduction of ClpC1, which targets proteins to the ClpPR core complex for degradation, was observed only in Stage 1, during which period PLBs disrupted and converted into prothylakoids, suggesting that ClpC1 was of particular importance for disassembly of PLBs of etioplasts when exposed to light. This study revealed the possible biochemical and physiological processes lead to the formation of functional thylakoid membranes.

BIOLOGICAL SIGNIFICANCE

In this study, we monitored the light-induced transformation of prolamellar bodies into thylakoid membranes, which is correlated to the biogenesis of photosynthetic apparatus involving a complex cascade of biochemical and structural events. Three stages of thylakoid development classified according to the thylakoid development status (Adam et al., 2011) were studied for biogenesis of photosynthetic apparatus: Stage 1, prothylakoids emerge from the disrupted PLBs; Stage 2, prothylakoids converted into primary thylakoids which were dispersed in the stroma; Stage 3, the continuous grana and stroma thylakoids are formed. The development stage-dependent changes in the proteomic profile of the thylakoids were analyzed by two-dimensional electrophoresis (2-DE). This information was complemented with the steady-state 77K chlorophyll fluorescence of thylakoids at the corresponding development stage. Together, these analyses allowed us to further understand the molecular processes connected to the formation of functional thylakoid membranes.

Two-Dimensional Electronic Spectroscopy of Light-Harvesting Complex II at Ambient Temperature: A Joint Experimental and Theoretical Study

We have performed broad-band two-dimensional (2D) electronic spectroscopy of light-harvesting complex II (LHCII) at ambient temperature. We found that electronic dephasing occurs within ∼60 fs and inhomogeneous broadening is approximately 120 cm(-1). A three-dimensional global fit analysis allows us to identify several time scales in the dynamics of the 2D spectra ranging from 100 fs to ∼10 ps and to uncover the energy-transfer pathways in LHCII. In particular, the energy transfer between the chlorophyll b and chlorophyll a pools occurs within ∼1.1 ps. Retrieved 2D decay-associated spectra also uncover the spectral positions of corresponding diagonal peaks in the 2D spectra. Residuals in the decay traces exhibit periodic modulations with different oscillation periods. However, only one of them can be associated with the excitonic cross-peaks in the 2D spectrum, while the remaining ones are presumably of vibrational origin. For the interpretation of the spectroscopic data, we have applied a refined exciton model for LHCII. It reproduces the linear absorption, circular dichroism, and 2D spectra at different waiting times. Several components of the energy transport are revealed from theoretical simulations that agree well with the experimental observations.

Low pH modulates the macroorganization and thermal stability of PSII supercomplexes in grana membranes

Protonation of the lumen-exposed residues of some photosynthetic complexes in the grana membranes occurs under conditions of high light intensity and triggers a major photoprotection mechanism known as energy dependent nonphotochemical quenching. We have studied the role of protonation in the structural reorganization and thermal stability of isolated grana membranes. The macroorganization of granal membrane fragments in protonated and partly deprotonated state has been mapped by means of atomic force microscopy. The protonation of the photosynthetic complexes has been found to induce large-scale structural remodeling of grana membranes-formation of extensive domains of the major light-harvesting complex of photosystem II and clustering of trimmed photosystem II supercomplexes, thinning of the membrane, and reduction of its size. These events are accompanied by pronounced thermal destabilization of the photosynthetic complexes, as evidenced by circular dichroism spectroscopy and differential scanning calorimetry. Our data reveal a detailed nanoscopic picture of the initial steps of nonphotochemical quenching.

Functional rearrangement of the light-harvesting antenna upon state transitions in a green alga

State transitions in the green alga Chlamydomonas reinhardtii serve to balance excitation energy transfer to photosystem I (PSI) and to photosystem II (PSII) and possibly play a role as a photoprotective mechanism. Thus, light-harvesting complex II (LHCII) can switch between the photosystems consequently transferring more excitation energy to PSII (state 1) or to PSI (state 2) or can end up in LHCII-only domains. In this study, low-temperature (77 K) steady-state and time-resolved fluorescence measured on intact cells of Chlamydomonas reinhardtii shows that independently of the state excitation energy transfer from LHCII to PSI or to PSII occurs on two main timescales of <15 ps and ∼ 100 ps. Moreover, in state 1 almost all LHCIIs are functionally connected to PSII, whereas the transition from state 1 to a state 2 chemically locked by 0.1 M sodium fluoride leads to an almost complete functional release of LHCIIs from PSII. About 2/3 of the released LHCIIs transfer energy to PSI and ∼ 1/3 of the released LHCIIs form a component designated X-685 peaking at 685 nm that decays with time constants of 0.28 and 5.8 ns and does not transfer energy to PSI or to PSII. A less complete state 2 was obtained in cells incubated under anaerobic conditions without chemical locking. In this state about half of all LHCIIs remained functionally connected to PSII, whereas the remaining half became functionally connected to PSI or formed X-685 in similar amounts as with chemical locking. We demonstrate that X-685 originates from LHCII domains not connected to a photosystem and that its presence introduces a change in the interpretation of 77 K steady-state fluorescence emission measured upon state transitions in Chalamydomonas reinhardtii.

Direct energy transfer from the major antenna to the photosystem II core complexes in the absence of minor antennae in liposomes

Minor antennae of photosystem (PS) II, located between the PSII core complex and the major antenna (LHCII), are important components for the structural and functional integrity of PSII supercomplexes. In order to study the functional significance of minor antennae in the energetic coupling between LHCII and the PSII core, characteristics of PSII-LHCII proteoliposomes, with or without minor antennae, were investigated. Two types of PSII preparations containing different antenna compositions were isolated from pea: 1) the PSII preparation composed of the PSII core complex, all of the minor antennae, and a small amount of major antennae (MCC); and 2) the purified PSII dimeric core complexes without periphery antenna (CC). They were incorporated, together with LHCII, into liposomes composed of thylakoid membrane lipids. The spectroscopic and functional characteristics were measured. 77K fluorescence emission spectra revealed an increased spectral weight of fluorescence from PSII reaction center in the CC-LHCII proteoliposomes, implying energetic coupling between LHCII and CC in the proteoliposomes lacking minor antennae. This result was further confirmed by chlorophyll a fluorescence induction kinetics. The incorporation of LHCII together with CC markedly increased the antenna cross-section of the PSII core complex. The 2,6-dichlorophenolindophenol photoreduction measurement implied that the lack of minor antennae in PSII supercomplexes did not block the energy transfer from LHCII to the PSII core complex. In conclusion, it is possible, in liposomes, that LHCII transfer energy directly to the PSII core complex, in the absence of minor antennae.

Frequently asked questions about in vivo chlorophyll fluorescence: practical issues

The aim of this educational review is to provide practical information on the hardware, methodology, and the hands on application of chlorophyll (Chl) a fluorescence technology. We present the paper in a question and answer format like frequently asked questions. Although nearly all information on the application of Chl a fluorescence can be found in the literature, it is not always easily accessible. This paper is primarily aimed at scientists who have some experience with the application of Chl a fluorescence but are still in the process of discovering what it all means and how it can be used. Topics discussed are (among other things) the kind of information that can be obtained using different fluorescence techniques, the interpretation of Chl a fluorescence signals, specific applications of these techniques, and practical advice on different subjects, such as on the length of dark adaptation before measurement of the Chl a fluorescence transient. The paper also provides the physiological background for some of the applied procedures. It also serves as a source of reference for experienced scientists.

Photosynthetic light harvesting: excitons and coherence

Photosynthesis begins with light harvesting, where specialized pigment-protein complexes transform sunlight into electronic excitations delivered to reaction centres to initiate charge separation. There is evidence that quantum coherence between electronic excited states plays a role in energy transfer. In this review, we discuss how quantum coherence manifests in photosynthetic light harvesting and its implications. We begin by examining the concept of an exciton, an excited electronic state delocalized over several spatially separated molecules, which is the most widely available signature of quantum coherence in light harvesting. We then discuss recent results concerning the possibility that quantum coherence between electronically excited states of donors and acceptors may give rise to a quantum coherent evolution of excitations, modifying the traditional incoherent picture of energy transfer. Key to this (partially) coherent energy transfer appears to be the structure of the environment, in particular the participation of non-equilibrium vibrational modes. We discuss the open questions and controversies regarding quantum coherent energy transfer and how these can be addressed using new experimental techniques.

Low photosynthetic activity is linked to changes in the organization of photosystem II in the fruit of Arum italicum

The low photosynthetic activity of fleshy green fruits is currently attributed to their special anatomy rather than to a down-regulation of photosystem II (PSII). However, it is unclear whether the organization of PSII, which is highly conserved in leaves, is also shared by non-foliar structures, such as fleshy fruits. To obtain new information on this aspect, the photosynthetic activity and the organization of PSII were investigated in the berry of Arum italicum Miller during maturation (ivory to green) and early ripening (green to yellow). The berry developed an "internal CO(2) recycling" photosynthesis; gross photosynthesis at the green stage was 25% of the leaf lamina. SDS-PAGE, BN-PAGE and 77 K spectrofluorimetry showed that the thylakoid membrane accumulated a very high amount of free LHCII trimers and only few PSII and PSI complexes. The pattern of PSII forms was similar to that of the lamina (monomers, dimers, LHCII-PSII supercomplexes), but increase in CP43-less PSII cores and low F695/F680 fluorescence ratio at room temperature indicated that PSII was less stable than in the leaf lamina. Beside effective PSII photoprotection, we propose that LHCII serves as a temporary storage of chlorophylls to provide a visual signal that fruit is not mature for seed dispersal. We conclude that the low photosynthetic activity of A. italicum berry depends on the scantiness of reaction centres and the reduced functionality of PSII.

Analysis of heat-induced disassembly process of three different monomeric forms of the major light-harvesting chlorophyll a/b complex of photosystem II

The temperature-dependent disassembly process of three monomeric isoforms, namely Lhcb1, Lhcb2, and Lhcb3, of the major light-harvesting chlorophyll (Chl) a/b complexes of photosystem II (LHCIIb) were characterized by observing the changes of absorption spectra, circular dichroism (CD), and dissociation processes of the bound pigments to the in vitro reconstituted complexes subjected to high temperatures. Our results suggest that the three isoforms of LHCIIb undergo conformational rearrangements, structural changes, and dissociations of the bound pigments when the ambient temperature increases from 20 to 90°C. The conformation of the complexes changed sensitively to the changing temperatures because the absorption peaks in the Soret region (436 and 471 nm) and the Qy region (650-660 and 680 nm) decreased immediately upon elevating the ambient temperatures. Analyzing temperature-dependent denaturing and pigment dissociation process, we can divide the disassembly process into three stages: The first stage, appeared from 20°C to around 50-60°C, was characterized by the diminishment of the absorption around 650-660 and 680 nm, accompanied by the blue-shift of the peak at 471 nm and disappearance of the absorbance at 436 nm, which is related to changes in the transition energy of the Chl b cluster, and the red-most Chl a cluster in the LHCIIb. The second stage, beginning at about 50-60°C, was signified by the diminishment of the CD signal between (+)483 nm and (-)490 nm, which implied the disturbance of dipole-dipole interaction of pigments, and the onset of the pigment dissociation. The last stage, beginning at about 70-80°C, indicates the complete dissociation of the pigments from the complex. The physiological aspects of the three stages in the denaturing process are also discussed.

Dynamic properties of photosystem II membranes at physiological temperatures characterized by elastic incoherent neutron scattering. Increased flexibility associated with the inactivation of the oxygen evolving complex

Elastic incoherent neutron scattering (EINS), a non-invasive technique which is capable of measuring the mean square displacement of atoms in the sample, has been widely used in biology for exploring the dynamics of proteins and lipid membranes but studies on photosynthetic systems are scarce. In this study we investigated the dynamic characteristics of Photosystem II (PSII) membrane fragments between 280 and 340 K, i.e., in the physiological temperature range and in the range of thermal denaturation of some of the protein complexes. The mean square displacement values revealed the presence of a hydration-sensitive transition in the sample between 310 and 320 K, suggesting that the oxygen evolving complex (OEC) plays an important role in the transition. Indeed, in samples in which the OEC had been removed by TRIS- or heat-treatments (323 and 333 K) no such transition was found. Further support on the main role of OEC in these reorganizations is provided by data obtained from differential scanning calorimetry experiments, showing marked differences between the untreated and TRIS-treated samples. In contrast, circular dichroism spectra exhibited only minor changes in the excitonic interactions below 323 K, showing that the molecular organization of the pigment-protein complexes remains essentially unaffected. Our data, along with earlier incoherent neutron scattering data on PSII membranes at cryogenic temperatures (Pieper et al., Biochemistry 46:11398-11409, 2007), demonstrate that this technique can be applied to characterize the dynamic features of PSII membranes, and can be used to investigate photosynthetic membranes under physiologically relevant experimental conditions.

Refined structure-based simulation of plant light-harvesting complex II: linear optical spectra of trimers and aggregates

Linear optical spectra of solubilized trimers and small lamellar aggregates of the major light-harvesting complex II (LHCII) of higher plants are simulated employing excitonic couplings and site energies of chlorophylls (Chls) computed on the basis of the two crystal structures by a combined quantum chemical/electrostatic approach. A good agreement between simulation and experiment is achieved (except for the circular dichroism in the Chl b region), if vibronic transitions of Chls are taken into account. Site energies are further optimized by refinement fits of optical spectra. The differences between refined and directly calculated values are not significant enough to decide, whether the crystal structures are closer to trimers or aggregates. Changes in the linear dichroism spectrum upon aggregation are related to site energy shifts of Chls b601, b607, a603, a610, and a613, and are interpreted in terms of conformational changes of violaxanthin and the two luteins involving their ionone rings. Chl a610 is the energy sink at 77K in both conformations. An analysis of absorption spectra of trimers perpendicular and parallel to the C(3)-axis (van Amerongen et al. Biophys. J. 67 (1994) 837-847) shows that only Chl a604 close to neoxanthin is significantly reoriented in trimers compared to the crystal structures. Whether this pigment is orientated in aggregates as in the crystal structures, can presently not be determined faithfully. To finally decide about pigment reorientations that could be of relevance for non-photochemical quenching, further polarized absorption and fluorescence measurements of aggregates or detergent-depleted LHCII would be helpful. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

Intra- and inter-monomeric transfers in the light harvesting LHCII complex: the Redfield-Förster picture

We further develop the model of energy transfer in the LHCII trimer based on a quantitative fit of the linear spectra (including absorption (OD), linear dichroism (LD), circular dichroism (CD), and fluorescence (FL)) and transient absorption (TA) kinetics upon 650 nm and 662 nm excitation. The spectral shapes and relaxation/migration rates have been calculated using the combined Redfield-Förster approach capable of correctly describing fast relaxation within strongly coupled chlorophyll (Chl) a and b clusters and slow migration between them. Within each monomeric subunit of the trimeric complex there is fast (sub-ps) conversion from Chl's b to Chl's a at the stromal side accompanied by slow (>10 ps) equilibration between the stromal- and lumenal-side Chl a clusters in combination with slow (>13 ps) population of Chl's a from the 'bottleneck' Chl a604 site. The connection between monomeric subunits is determined by exciton coupling between the stromal-side Chl's b from the two adjacent subunits (Chl b601'-608-609 cluster) making a simultaneous fast (sub-ps) population of the Chl's a possible from both subunits. Final equilibration occurs via slow (>20 ps) migration between the Chl a clusters located on different monomeric subunits. This migration includes up-hill transfers from the red-most Chl a610-611-612 clusters located at the peripheral side in each subunit to the Chl a602-603 dimers located at the inner side of the trimeric LHCII complex.

How the molecular structure determines the flow of excitation energy in plant light-harvesting complex II

Excitation energy transfer in the light-harvesting complex II of higher plants is modeled using excitonic couplings and local transition energies determined from structure-based calculations recently (Müh et al., 2010). A theory is introduced that implicitly takes into account protein induced dynamic localization effects of the exciton wavefunction between weakly coupled optical and vibronic transitions of different pigments. Linear and non-linear optical spectra are calculated and compared with experimental data reaching qualitative agreement. High-frequency intramolecular vibrational degrees of freedom are found important for ultrafast subpicosecond excitation energy transfer between chlorophyll (Chl) b and Chla, since they allow for fast dissipation of the excess energy. The slower ps component of this transfer is due to the monomeric excited state of Chlb 605. The majority of exciton relaxation in the Chla spectral region is characterized by slow ps exciton equilibration between the Chla domains within one layer and between the lumenal and stromal layers in the 10-20ps time range. Subpicosecond exciton relaxation in the Chla region is only found within the terminal emitter domain (Chls a 610/611/612) and within the Chla 613/614 dimer. Deviations between measured and calculated exciton state life times are obtained for the intermediate spectral region between the main absorbance bands of Chla and Chlb that indicate that besides Chlb 608 another pigment should absorb there. Possible candidates, so far not identified by structure-based calculations, but by fitting of optical spectra and mutagenesis studies, are discussed. Additional mutagenesis studies are suggested to resolve this issue.

Revised assignment of room-temperature chlorophyll fluorescence emission bands in single living cells of Chlamydomonas reinhardtii

Room temperature (RT) microspectrofluorimetry in vivo of single cells has a great potential in photosynthesis studies. In order to get new information on RT chlorophyll fluorescence bands, we analyzed the spectra of Chlamydomonas reinhardtii mutants lacking fundamental proteins of the thylakoid membrane and spectra of photoinhibited WT cells. RT spectra of single living cells were characterized thorough derivative analyses and Gaussian deconvolution. The results obtained suggest that the dynamism in LHCII assembly could be sufficient to explain the variations in amplitudes of F680 (free LHCII), F694 (LHCII-PSII) and F702 (LHCII aggregates); F686 was assigned to the PSII core. Based on the revised assignments and on the variations observed, we discuss the meaning of the two fluorescence emission ratios F680/(F686 + F694) and F702/(F686 + F694), showing that these are sensitive parameters under moderate photoinhibition. In the most photoinhibited samples, the RT spectra tended to degenerate, showing characteristics of mutants that are partly depleted in PSII.

Physical origins and models of energy transfer in photosynthetic light-harvesting

We perform a quantitative comparison of different energy transfer theories, i.e. modified Redfield, standard and generalized Förster theories, as well as combined Redfield-Förster approach. Physical limitations of these approaches are illustrated and critical values of the key parameters indicating their validity are found. We model at a quantitative level the spectra and dynamics in two photosynthetic antenna complexes: in phycoerythrin 545 from cryptophyte algae and in trimeric LHCII complex from higher plants. These two examples show how the structural organization determines a directed energy transfer and how equilibration within antenna subunits and migration between subunits are superimposed.

Site-directed spin-labeling study of the light-harvesting complex CP29

The topology of the long N-terminal domain (approximately 100 amino-acid residues) of the photosynthetic Lhc CP29 was studied using electron spin resonance. Wild-type protein containing a single cysteine at position 108 and nine single-cysteine mutants were produced, allowing to label different parts of the domain with a nitroxide spin label. In all cases, the apoproteins were either solubilized in detergent or they were reconstituted with their native pigments (holoproteins) in vitro. The spin-label electron spin resonance spectra were analyzed in terms of a multicomponent spectral simulation approach, based on hybrid evolutionary optimization and solution condensation. These results permit to trace the structural organization of the long N-terminal domain of CP29. Amino-acid residues 97 and 108 are located in the transmembrane pigment-containing protein body of the protein. Positions 65, 81, and 90 are located in a flexible loop that is proposed to extend out of the protein from the stromal surface. This loop also contains a phosphorylation site at Thr81, suggesting that the flexibility of this loop might play a role in the regulatory mechanisms of the light-harvesting process. Positions 4, 33, 40, and 56 are found to be located in a relatively rigid environment, close to the transmembrane protein body. On the other hand, position 15 is located in a flexible region, relatively far away from the transmembrane domain.

Model for fluorescence quenching in light harvesting complex II in different aggregation states

Low-temperature (77 K) steady-state fluorescence emission spectroscopy and dynamic light scattering were applied to the main chlorophyll a/b protein light harvesting complex of photosystem II (LHC II) in different aggregation states to elucidate the mechanism of fluorescence quenching within LHC II oligomers. Evidences presented that LHC II oligomers are heterogeneous and consist of large and small particles with different fluorescence yield. At intermediate detergent concentrations the mean size of the small particles is similar to that of trimers, while the size of large particles is comparable to that of aggregated trimers without added detergent. It is suggested that in small particles and trimers the emitter is monomeric chlorophyll, whereas in large aggregates there is also another emitter, which is a poorly fluorescing chlorophyll associate. A model, describing populations of antenna chlorophyll molecules in small and large aggregates in their ground and first singlet excited states, is considered. The model enables us to obtain the ratio of the singlet excited-state lifetimes in small and large particles, the relative amount of chlorophyll molecules in large particles, and the amount of quenchers as a function of the degree of aggregation. These dependencies reveal that the quenching of the chl a fluorescence upon aggregation is due to the formation of large aggregates and the increasing of the amount of chlorophyll molecules forming these aggregates. As a consequence, the amount of quenchers, located in large aggregates, is increased, and their singlet excited-state lifetimes steeply decrease.

Importance of trimer-trimer interactions for the native state of the plant light-harvesting complex II

Aggregates and solubilized trimers of LHCII were characterized by circular dichroism (CD), linear dichroism and time-resolved fluorescence spectroscopy and compared with thylakoid membranes in order to evaluate the native state of LHCII in vivo. It was found that the CD spectra of lamellar aggregates closely resemble those of unstacked thylakoid membranes whereas the spectra of trimers solubilized in n-dodecyl-beta,D-maltoside, n-octyl-beta,D-glucopyranoside, or Triton X-100 were drastically different in the Soret region. Thylakoid membranes or LHCII aggregates solubilized with detergent exhibited CD spectra similar to the isolated trimers. Solubilization of LHCII was accompanied by profound changes in the linear dichroism and increase in fluorescence lifetime. These data support the notion that lamellar aggregates of LHCII retain the native organization of LHCII in the thylakoid membranes. The results indicate that the supramolecular organization of LHCII, most likely due to specific trimer-trimer contacts, has significant impact on the pigment interactions in the complexes.

Cadmium-induced inhibition of photosynthesis and long-term acclimation to cadmium stress in the hyperaccumulator Thlaspi caerulescens

Acclimation of hyperaccumulators to heavy metal-induced stress is crucial for phytoremediation and was investigated using the hyperaccumulator Thlaspi caerulescens and the nonaccumulators T. fendleri and T. ochroleucum. Spatially and spectrally resolved kinetics of in vivo absorbance and fluorescence were measured with a novel fluorescence kinetic microscope. At the beginning of growth on cadmium (Cd), all species suffered from toxicity, but T. caerulescens subsequently recovered completely. During stress, a few mesophyll cells in T. caerulescens became more inhibited and accumulated more Cd than the majority; this heterogeneity disappeared during acclimation. Chlorophyll fluorescence parameters related to photochemistry were more strongly affected by Cd stress than nonphotochemical parameters, and only photochemistry showed acclimation. Cd acclimation in T. caerulescens shows that part of its Cd tolerance is inducible and involves transient physiological heterogeneity as an emergency defence mechanism. Differential effects of Cd stress on photochemical vs nonphotochemical parameters indicate that Cd inhibits the photosynthetic light reactions more than the Calvin-Benson cycle. Differential spectral distribution of Cd effects on photochemical vs nonphotochemical quenching shows that Cd inhibits at least two different targets in/around photosystem II (PSII). Spectrally homogeneous maximal PSII efficiency (F(v)/F(m)) suggests that in healthy T. caerulescens all chlorophylls fluorescing at room temperature are PSII-associated.

A comparison of the three isoforms of the light-harvesting complex II using transient absorption and time-resolved fluorescence measurements

In this article we report the characterization of the energy transfer process in the reconstituted isoforms of the plant light-harvesting complex II. Homotrimers of recombinant Lhcb1 and Lhcb2 and monomers of Lhcb3 were compared to native trimeric complexes. We used low-intensity femtosecond transient absorption (TA) and time-resolved fluorescence measurements at 77 K and at room temperature, respectively, to excite the complexes selectively in the chlorophyll b absorption band at 650 nm with 80 fs pulses and on the high-energy side of the chlorophyll a absorption band at 662 nm with 180 fs pulses. The subsequent kinetics was probed at 30-35 different wavelengths in the region from 635 to 700 nm. The rate constants for energy transfer were very similar, indicating that structurally the three isoforms are highly homologous and that probably none of them play a more significant role in light-harvesting and energy transfer. No signature has been found in the transient absorption measurements at 77 K for Lhcb3 which might suggest that this protein acts as a relative energy sink of the excitations in heterotrimers of Lhcb1/Lhcb2/Lhcb3. Minor differences in the amplitudes of some of the rate constants and in the absorption and fluorescence properties of some pigments were observed, which are ascribed to slight variations in the environment surrounding some of the chromophores depending on the isoform. The decay of the fluorescence was also similar for the three isoforms and multi-exponential, characterized by two major components in the ns regime and a minor one in the ps regime. In agreement with previous transient absorption measurements on native LHC II complexes, Chl b --> Chl a energy transfer exhibited very fast channels but at the same time a slow component (ps). The Chls absorbing at around 660 nm exhibited both fast energy transfer which we ascribe to transfer from 'red' Chl b towards 'red' Chl a and slow transfer from 'blue' Chl a towards 'red' Chl a. The results are discussed in the context of the new available atomic models for LHC II.

Temperature-induced isomerization of violaxanthin in organic solvents and in light-harvesting complex II

Three main xanthophyll pigments are bound to the major photosynthetic pigment-protein complex of Photosystem II (LHCII): lutein, neoxanthin and violaxanthin. Chromatographic analysis of the xanthophyll fraction of LHCII reveals that lutein appears mainly in the all-trans conformation, neoxanthin in the 9'-cis conformation and major fraction of violaxanthin in the all-trans conformation. Nevertheless, a small fraction of violaxanthin appears always in a cis conformation: 9-cis and 13-cis (approximately 4% and 2% in the darkness, respectively). Illumination of the isolated complex (5 min, 445 nm, 250 micromolm-2s-1) results in the substantial increase in the concentration of the cis steric conformers of violaxanthin: up to 6% of 9-cis and 4% of 13-cis. Similar effect can be obtained by dark incubation of the same preparation for 30 min at 60 degrees C. Heating-induced isomerization of the all-trans violaxanthin can also be obtained in the organic solvent system but the formation of the 9-cis stereoisomer has not been observed under such conditions. The fact that the appearance of the 9-cis form of violaxanthin is specific for the protein environment suggests that violaxanthin may replace neoxanthin in LHCII in the N1 xanthophyll binding pocket and that the protein stabilizes this particular conformation. The analysis of the electronic absorption spectra of LHCII and the FTIR spectra of the protein in the Amid I band spectral region indicates that violaxanthin isomerization is associated with the disaggregation of the complex. It is postulated that this reorganization of LHCII provides conditions for desorption of violaxanthin from the pigment protein complexes, its diffusion within the thylakoid membrane and therefore, availability to the enzymatic deepoxidation within the xanthophyll cycle. It is also possible that violaxanthin isomerization plays the role of a security valve, by consuming an energy of excessive excitations in the antenna pigment network (in particular, exchanged at the triplet state levels).

Evidence for dissociation of chlorophyll b from the main light-harvesting complex in the oligomerization state isolated from marine alga, Bryopsis corticulans

We investigated the composition and organization of chlorophylls in monomers, trimers and oligomers (small aggregates) of the main light-harvesting complex (LHC II) isolated from marine alga, Bryopsis corticulans, using a combination of measurements with reversed-phase high performance liquid chromatography (RP-HPLC) and steady-state spectroscopy of absorption, circular dichroism (CD) and low temperature fluorescence. The composition and organization of the chlorophylls in monomeric and trimeric LHC II were essentially identical to those of LHC II from higher plants. For LHC II oligomers, a large decrease of chlorophyll (Chl) b absorption and of CD signals corresponding to Chl b was consistent with the quantitative analysis of Chl b by RP-HPLC, indicating that oligomerization of the LHC II proteins significantly influenced spectroscopic properties and led to the dissociation of Chl b molecules from LHC II. Our data strongly suggested that protein oligomerization constitutes a structural basis for the decrease of Chl b molecules in LHC II of B. corticulans. The LHC II of B. corticulans might play a photoprotective role with the reduction of the ability of light absorption via alteration of its own structural conformation.

Stark spectroscopy of the light-harvesting complex II in different oligomerisation states

The electric field-induced absorption changes (Stark effect) of light-harvesting complex II (LHCII) in different oligomerisation states-monomeric, trimeric and aggregated-have been probed at 77 K. All the chlorophyll (Chl) a molecules exhibit electro-optic properties in the Q(y) absorption region characterized by a change in dipole moment /Deltamu-->/ =0.6+/-0.06D/f and polarizability, Tr(Deltaalpha;) approximately 55+/-5 A(3)/f(2) upon electronic excitation, which are similar to those of unbound monomeric Chl a, indicating the absence of strong delocalization of the excitations which would be expected in the presence of strong excitonic interactions. The Stark effect in the Chl b absorption region is significantly bigger with /Deltamu-->/ values of the order of 2.0+/-0.2 D/f and it is attributed to strong interactions with neoxanthin molecules. Clear oligomerisation-dependent differences are observed in the carotenoid region, mainly due to the appearance of a new xanthophyll absorption band at 509 in the spectra of trimers and oligomers. It is ascribed to some lutein molecules, in agreement with previous experimental observations. The electro-optic properties of these lutein molecules are significantly different from those of the other xanthophylls in LHCII, which do not exhibit such a big change in dipole moment upon electronic excitation (/Deltamu-->/ =14.6+/-2.0 D/f). Upon aggregation of LHCII some extra absorption appears on the red side of the main Chl a Q(y) absorption band. In contrast to an earlier suggestion [J. Phys. Chem., A 103 (1999) 2422], no indications are found for the charge-transfer character of the corresponding band. The assignments of the S(2) electronic transitions of neoxanthin and lutein in LHCII and possible origins of the Stark effect are discussed.

Changes in the energy distribution between chlorophyll-protein complexes of thylakoid membranes from pea mutants with modified pigment content. I. Changes due to the modified pigment content

The low-temperature (77 K) emission and excitation chlorophyll fluorescence spectra in thylakoid membranes isolated from pea mutants were investigated. The mutants have modified pigment content, structural organization, different surface electric properties and functions [Dobrikova et al., Photosynth. Res. 65 (2000) 165]. The emission spectra of thylakoid membranes were decomposed into bands belonging to the main pigment protein complexes. By an integration of the areas under them, the changes in the energy distribution between the two photosystems as well as within each one of them were estimated. It was shown that the excitation energy flow to the light harvesting, core antenna and RC complexes of photosystem II increases with the total amount of pigments in the mutants, relative to the that to photosystem I complexes. A reduction of the fluorescence ratio between aggregated trimers of LHC II and its trimeric and monomeric forms with the increase of the pigment content (chlorophyll a, chlorophyll b, and lutein) was observed. This implies that the closer packing in the complexes with a higher extent of aggregation regulates the energy distribution to the PS II core antenna and reaction centers complexes. Based on the reduced energy flow to PS II, i.e., the relative increased energy flow to PS I, we hypothesize that aggregation of LHC II switches the energy flow toward LHC I. These results suggest an additive regulatory mechanism, which redistributes the excitation energy between the two photosystems and operates at non-excess light intensities but at reduced pigment content.

Energy transfer in light-harvesting complexes LHCII and CP29 of spinach studied with three pulse echo peak shift and transient grating

Three pulse echo peak shift and transient grating (TG) measurements on the plant light-harvesting complexes LHCII and CP29 are reported. The LHCII complex is by far the most abundant light-harvesting complex in higher plants and fulfills several important physiological functions such as light-harvesting and photoprotection. Our study is focused on the light-harvesting function of LHCII and the very similar CP29 complex and reveals hitherto unresolved excitation energy transfer processes. All measurements were performed at room temperature using detergent isolated complexes from spinach leaves. Both complexes were excited in their Chl b band at 650 nm and in the blue shoulder of the Chl a band at 670 nm. Exponential fits to the TG and three pulse echo peak shift decay curves were used to estimate the timescales of the observed energy transfer processes. At 650 nm, the TG decay can be described with time constants of 130 fs and 2.2 ps for CP29, and 300 fs and 2.8 ps for LHCII. At 670 nm, the TG shows decay components of 230 fs and 6 ps for LHCII, and 300 fs and 5 ps for CP29. These time constants correspond to well-known energy transfer processes, from Chl b to Chl a for the 650 nm TG and from blue (670 nm) Chl a to red (680 nm) Chl a for the 670 nm TG. The peak shift decay times are entirely different. At 650 nm we find times of 150 fs and 0.5-1 ps for LHCII, and 360 fs and 3 ps for CP29, which we can associate mainly with Chl b <--> Chl b energy transfer. At 670 nm we find times of 140 fs and 3 ps for LHCII, and 3 ps for CP29, which we can associate with fast (only in LHCII) and slow transfer between relatively blue Chls a or Chl a states. From the occurrence of both fast Chl b <--> Chl b and fast Chl b --> Chl a transfer in CP29, we conclude that at least two mixed binding sites are present in this complex. A detailed comparison of our observed rates with exciton calculations on both CP29 and LHCII provides us with more insight in the location of these mixed sites. Most importantly, for CP29, we find that a Chl b pair must be present in some, but not all, complexes, on sites A(3) and B(3). For LHCII, the observed rates can best be understood if the same pair, A(3) and B(3), is involved in both fast Chl b <--> Chl b and fast Chl a <--> Chl a transfer. Hence, it is likely that mixed sites also occur in the native LHCII complex. Such flexibility in chlorophyll binding would agree with the general flexibility in aggregation form and xanthophyll binding of the LHCII complex and could be of use for optimizing the role of LHCII under specific circumstances, for example under high-light conditions. Our study is the first to provide spectroscopic evidence for mixed binding sites, as well as the first to show their existence in native complexes.

Direct evidence for excitonically coupled chlorophylls a and b in LHC II of higher plants by nonlinear polarization spectroscopy in the frequency domain

Occurrence of excitonic interactions in light-harvesting complex II (LHC II) was investigated by nonlinear polarization spectroscopy in the frequency domain (NLPF) at room temperature. NLPF spectra were obtained upon probing in the chlorophyll (Chl) a/b Soret region and pumping in the Q(y) region. The lowest energy Chl a absorbing at 678 nm is strongly excitonically coupled to Chl b.

Excitonic coupling of chlorophylls in the plant light-harvesting complex LHC-II

Manifestation and extent of excitonic interactions in the red Chl-absorption region (Q(y) band) of trimeric LHC-II were investigated using two complementary nonlinear laser-spectroscopic techniques. Nonlinear absorption of 120-fs pulses indicates an increased absorption cross section in the red wing of the Q(y) band as compared to monomeric Chl a in organic solution. Additionally, the dependence of a nonlinear polarization response on the pump-field intensity was investigated. This approach reveals that one emitting spectral form, characterized by a 2.3(+/-0.8)-fold larger dipole strength than monomeric Chl a, dominates the fluorescence spectrum of LHC-II. Considering available structural and spectroscopic data, these results can be consistently explained assuming the existence of an excitonically coupled dimer located at Chl-bindings sites a2 and b2 (referring to the original notation of W. Nühlbrandt, D.N. Wang, and Y. Fujiyoshi, Nature, 1994, 367:614-621), which must not necessarily correspond to Chls a and b). This fluorescent dimer, terminating the excitation energy-transfer chain of the LHC-II monomeric subunit, is discussed with respect to its relevance for intra- and inter-antenna excitation energy transfer.

The calculated in vitro and in vivo chlorophyll a absorption bandshape

The room temperature absorption bandshape for the Q transition region of chlorophyll a is calculated using the vibrational frequency modes and Franck-Condon (FC) factors obtained by line-narrowing spectroscopies of chlorophyll a in a glassy (Rebane and Avarmaa, Chem. Phys. 1982; 68:191-200) and in a native environment (Gillie et al., J. Phys. Chem. 1989; 93:1620-1627) at low temperatures. The calculated bandshapes are compared with the absorption spectra of chlorophyll a measured in two different solvents and with that obtained in vivo by a mutational analysis of a chlorophyll-protein complex. It is demonstrated that the measured distributions of FC factors can account for the absorption bandshape of chlorophyll a in a hexacoordinated state, whereas, when pentacoordinated, reduced FC coupling for vibrational frequencies in the range 540-850 cm(-1) occurs. The FC factor distribution for pentacoordinated chlorophyll also describes the native chlorophyll a spectrum but, in this case, either a low-frequency mode (nu < 200 cm(-1)) must be added or else the 262-cm(-1) mode must increase in coupling by about one order of magnitude to describe the skewness of the main absorption bandshape.