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Experimental study is carried out to model the circular dichroism (CD) spectrum of LHCII, the main light harvesting antenna of photosystem II of higher plants. The findings state that differences in the experimental trimeric CD spectra are caused by changes in the strength of the monomer-monomer interactions due to the differences in detergents used for the purification of the complexes.

The spectroscopic characteristics of carotenoids associated with the antenna complexes of Photosystem I is studied. It is suggested that in the Lhca2-Lhca3 heterodimeric complex energy equilibration is not complete at least on a fast time scale.

Photosystem I (PSI) complexes, possess absorption and emission bands at lower energy than those of the reaction center, is one of its special spectroscopic characteristics. The pigment dimers are responsible for the red states and these red excited states can be interpreted as excimer states.

Research has been conducted on the chlorophyll-xanthophyll-protein CP26 complex purified from leaves or recombinant. The pigment binding site affinity of this complex has been investigated via the use of the biochemical and spectroscopic analysis of the complexes in the presence of different ratios between chromophores and the details are reported.

The pigment-protein interactions in the photosystem II antenna protein, Lhcb4, reveal that removal of a any bound cofactor from any sites of the chlorophyll a binding sites, a2, a4, and a5 leads to a loss of bands of the particular chlorophyll molecule and a structural impact on the protein folding.

Results show that spectral properties and photochemical behavior of xanthophyll pigments are reflections of the individual pigments bound to the light-harvesting complexes. Data on the absorption wavelengths and fluorescence of the bound pigments are discussed.

Existence of two spectroscopically different dimers of light-harvesting complex I from green plants has been supported by recent research. A preparation of isolated dimeric peripheral antenna complexes from green plant photosystem I has been studied using low-temperature polarized steady-state absorption and fluorescence spectroscopy. The chlorophylls responsible for the 733 nm emission and the 702 nm emission are excitonically coupled dimers. F-730 comes from one of the strongest chlorophyll coupled pairs seen in nature.

Photosytem I from maize, having its full antenna complement (PSI-200) has been studied by time-resolved fluorescence methods with about 5 ps resolution at 280 and 170 K in the 690-780 nm wavelength range. Detergent effects of chlorophyll coupling were almost entirely lacking in the preparation from maize. Decay-associated spectra (DAS) data and the time-resolved emission spectra (TRES) data were developed. The data show that trapping in PSI-200 is to a great extent rate-limited by excitation diffusion in the antenna and especially by the gradual 'uphill' transfer from the low-energy forms to the bulk and/or inner core chlorophyll molecules

The thermal broadening of the red absorption tail of the full antenna complement (PSI-200), core and outer antenna (LHCI) and PSI core of the intact photosystem I of maize was investigated in terms of the second-order central moment and compared with that of the bulk antenna pigments. Results of absorption and fluorescence spectroscopic studies showed that at least 80% of the long wavelength absorbing antenna pigments are found in LHCI. The bulk pigments in all three preparations were found to have low values, indicating that Stokes shifts for the absorption forms are in the 1.5-3 nm range.

Thermodynamic analysis of adsorption and fluorescence spectra of the in vitro reconstitution of the photosystem II antenna complex CP29(Lhcb-4) and of modulation of pigment constitution during reconstitution found that all chlorophylls (chl) are energetically connected. Data indicate that most chl sites can bind either chl a or chl b. The invariant position of chl a dn chlb adsorption bands with changing chla/chlb binding stoichiometries suggests that pigment-protein interactions are the main determinants of spectral heterogeneity.

The organization of chl molecules in the membrane plane is affected by the reversible phophorylation of the hydrophilic N-terminal domain of CP29 due to long range allosteric effects. The addition of alkaline phosphatase reverses the variations in the organization of chlorophyll chromatophores of phosphorylated and unphosphorylated forms of CP29. Long-range conformational changes due to phosphorylation are responsible for the variations in the structure of CP29.

The primary donor P700 of photosystem I (PSI) is isoenergetic with its antenna at room temperature. The free energy for excitation transfer from the antenna to P700 is -0.26 kT at room temperature. Fluorescence analysis of PSI between 75 and 280 kelvin with its full antenna complement reveals the presence of a minimum of three chlorophyll spectral forms. The emission maxima of the spectral forms are 720, 730 and 742 nm respectively. At least three or four antenna chlorophylls with absorptions of about 695 nm are linked to P700.

The room-temperature fluorescence decay properties of four monomeric Lhca proteins (Lhca1-4) and a dimeric Lhca1/4 protein are studied using a Streak camera setup with about 5 ps time resolution. This study shows that Lhca proteins have faster decay lifetimes and lower emission yields compared to their counterparts of photosystem II (PSII) antenna proteins, light-harvesting complex II (LHCII) and CP29.

The biochemical and spectroscopic properties of the three groups of genes encoding LHCII components namely Lhcb1, Lhcb2 and Lhcb3 are analyzed. The Lhcb2 pigment-protein shows different spectroscopic properties with respect to Lhcb1 and the hypothesis is suggested that Lhcb3 may be a preferential site of regulation of the antenna function in excess light conditions.

Research demonstrates the biosynthesis of Arabidopsis thaliana recombinant photosystem I antenna complexes, Lhca2 and Lhca3, in Escherichia coli and reconstitution of the pigment-refolded protein complexes. Data reveal that the complexes can modulate the absorption/emission properties and red-shifted spectral characteristics of the native forms.

Research was conducted to examine the orientation of the Q(sub y) transition of the individual chlorophyll (Chl) molecules in the higher-plant antenna protein CP29. Low-temperature linear dichroism and absorption spectroscopy were used to investigate the single-point mutants. Results permit the determination of the orientation of six chromophores within the protein structure. The orientation of the dipole moments of two Chl molecules indicate a role in energy transfer from xanthophyll to Chl.

Increasing the absorption cross section of plants by introducing far-red absorbing chlorophylls (Chls) has been proposed as a strategy to boost crop yields. To make this strategy effective, these Chls should bind to the photosynthetic complexes without altering their functional architecture. To investigate if plant-specific antenna complexes can provide the protein scaffold to accommodate these Chls, we have reconstituted the main light-harvesting complex (LHC) of plants LHCII in vitro and in silico , with Chl d . The results demonstrate that LHCII can bind Chl d in a number of binding sites, shifting the maximum absorption ∼25 nm toward the red with respect to the wild-type complex (LHCII with Chl a and b ) while maintaining the native LHC architecture. Ultrafast spectroscopic measurements show that the complex is functional in light harvesting and excitation energy transfer. Overall, we here demonstrate that it is possible to obtain plant LHCs with enhanced far-red absorption and intact functional properties.

Light-harvesting complex II (LHCII) is a pigment-protein complex present in higher plants and green algae. LHCII represents the main site of light absorption, and its role is to transfer the excitation energy toward the photosynthetic reaction centers, where primary energy conversion reactions take place. The optical properties of LHCII are known to depend on protein conformation. However, the relation between the structural and spectroscopic properties of the pigments is not fully understood yet. In this respect, previous classical molecular dynamics simulations of LHCII in a model membrane [ Sci. Rep. 2015 , 5 , 1-10] have shown that the configuration and excitonic coupling of a chlorophyll (Chl) dimer functioning as the main terminal emitter of the complex are particularly sensitive to conformational changes. Here, we use quantum chemistry calculations to investigate in greater detail the effect of pigment-pigment interactions on the excited-state landscape. While most previous studies have used a local picture in which electrons are localized on single pigments, here we achieve a more accurate description of the Chl dimer by adopting a supramolecular picture where time-dependent density functional theory is applied to the whole system at once. Our results show that specific dimer configurations characterized by shorter inter-pigment distances can result in a sizable intensity decrease (up to 36%) of the Chl absorption bands in the visible spectral region. Such a decrease can be predicted only when accounting for Chl-Chl charge-transfer excitations, which is possible using the above-mentioned supramolecular approach. The charge-transfer character of the excitations is quantified by two types of analyses: one focusing on the composition of the excitations and the other directly on the observable total absorption intensities.

Photosynthetic reactions in plants, algae, and cyanobacteria are driven by photosystem I and photosystem II complexes, which specifically reduce or oxidize partner redox biomolecules. Photosynthetic complexes can also bind synthetic organic molecules, which inhibit their photoactivity and can be used both to study the electron transport chain and as herbicides and algicides. Thus, their development, characterization, and sensing bears fundamental and applied interest. Substantial efforts have been devoted to developing photosensors based on photosystem II to detect compounds that bind to the plastoquinone sites of this complex. In comparison, photosystem I based sensors have received less attention and could be used to identify novel substances displaying phytotoxic effects, including those obtained from natural product extracts. We have developed a robust procedure to functionalize gold electrodes with photo- and redox-active photosystem I complexes based on transparent gold and a thiolate self-assembled monolayer, and we have obtained reproducible electrochemical photoresponses. Chronoamperometric recordings have allowed us to measure photocurrents in the presence of the viologen derivative paraquat at concentrations below 100 nM under lock-in operation and a sensor dynamic range spanning six orders of magnitude up to 100 mM. We have modeled their time course to identify the main electrochemical processes and limiting steps in the electron transport chain. Our results allow us to isolate the contributions from photosystem I and the redox mediator, and evaluate photocurrent features (spectral and power dependence, fast transient kinetics) that could be used as a sensing signal to detect other inhibitors and modulators of photosystem I activity.

Photoprotective nonphotochemical quenching (NPQ) of absorbed solar energy is vital for survival of photosynthetic organisms, and NPQ modifications significantly improve plant productivity. However, the exact NPQ quenching mechanism is obscured by discrepancies between reported mechanisms, involving xanthophyll-chlorophyll (Xan-Chl) and Chl-Chl interactions. We present evidence of an experimental artifact that may explain the discrepancies: strong laser pulses lead to the formation of a novel electronic species in the major plant light-harvesting complex (LHCII). This species evolves from a high excited state of Chl a and is absent with weak laser pulses. It resembles an excitonically coupled heterodimer of Chl a and lutein (or other Xans at site L1) and acts as a de-excitation channel. Laser powers, and consequently amounts of artifact, vary strongly between NPQ studies, thereby explaining contradicting spectral signatures attributed to NPQ. Our results offer pathways toward unveiling NPQ mechanisms and highlight the necessity of careful attention to laser-induced artifacts.

LHCII is the major antenna complex of plants and algae, where it is involved in light harvesting and photoprotection. Its properties have been extensively studied in vitro, after isolation of the pigment-protein complex from the membranes, but are these properties representative for LHCII in the thylakoid membrane? In this work, we have studied LHCII in the cells of the green alga C. reinhardtii acclimated to different light conditions in the absence of the other components of the photosynthetic apparatus. We show that LHCII exists in the membranes in different fluorescence quenching states, all having a shorter excited-state lifetime than isolated LHCII in detergent. The ratio between these populations depends on the light conditions, indicating that the light is able to regulate the properties of the complexes in the membrane.

We have studied thylakoid membranes of Arabidopsis thaliana acclimated to different light conditions and have related protein composition to excitation energy transfer and trapping kinetics in Photosystem II (PSII). In high light: the plants have reduced amounts of the antenna complexes LHCII and CP24, the overall trapping time of PSII is only ∼180 ps, and the quantum efficiency reaches a value of 91%. In low light: LHCII is upregulated, the PSII lifetime becomes ∼310 ps, and the efficiency decreases to 84%. This difference is largely caused by slower excitation energy migration to the reaction centers in low-light plants due to the LHCII trimers that are not part of the C2S2M2 supercomplex. This pool of "extra" LHCII normally transfers energy to both photosystems, whereas it transfers only to PSII upon far-red light treatment (state 1). It is shown that in high light the reduction of LHCII mainly concerns the LHCII-M trimers, while the pool of "extra" LHCII remains intact and state transitions continue to occur. The obtained values for the efficiency of PSII are compared with the values of Fv/Fm, a parameter that is widely used to indicate the PSII quantum efficiency, and the observed differences are discussed.