Structure-Based Modeling of Fluorescence Kinetics of Photosystem II: Relation between Its Dimeric Form and Photoregulation

Ecological diversification of a cyanobacterium through divergence of its novel chlorophyll d-based light-harvesting system

The evolution of novel traits can have important consequences for biological diversification. Novelties such as new structures are associated with changes in both genotype and phenotype that often lead to changes in ecological function.1,2 New ecological opportunities provided by a novel trait can trigger subsequent trait modification or niche partitioning3; however, the underlying mechanisms of novel trait diversification are still poorly understood. Here, we report that the innovation of a new chlorophyll (Chl) pigment, Chl d, by the cyanobacterium Acaryochloris marina was followed by the functional divergence of its light-harvesting complex. We identified three major photosynthetic spectral types based on Chl fluorescence properties for a collection of A. marina laboratory strains for which genome sequence data are available,4,5 with shorter- and longer-wavelength types more recently derived from an ancestral intermediate phenotype. Members of the different spectral types exhibited extensive variation in the Chl-binding proteins as well as the Chl energy levels of their photosynthetic complexes. This spectral-type divergence is associated with differences in the wavelength dependence of both growth rate and photosynthetic oxygen evolution. We conclude that the divergence of the light-harvesting apparatus has consequently impacted A. marina ecological diversification through specialization on different far-red photons for photosynthesis.

Reinvestigation on primary processes of PSII-dimer from Thermosynechococcus vulcanus by femtosecond pump-probe spectroscopy

Cyanobacterial photosynthetic apparatus efficiently capture sunlight, and the energy is subsequently transferred to photosystem I (PSI) and II (PSII), to produce electrochemical potentials. PSII is a unique membrane protein complex that photo-catalyzes oxidation of water and majorly contains photosynthetic pigments of chlorophyll a and carotenoids. In the present study, the ultrafast energy transfer and charge separation dynamics of PSII from a thermophilic cyanobacterium Thermosynechococcus vulcanus were reinvestigated by femtosecond pump-probe spectroscopic measurements under low temperature and weak intensity excitation condition. The results imply the two possible models of the energy transfers and subsequent charge separation in PSII. One is the previously suggested "transfer-to-trapped limit" model. Another model suggests that the energy transfers from core CP43 and CP47 antennas to the primary electron donor ChlD1 with time-constants of 0.71 ps and 3.28 ps at 140 K (0.17 and 1.33 ps at 296 K), respectively and that the pheophytin anion (PheoD1-) is generated with the time-constant of 43.0 ps at 140 K (14.8 ps at 296 K) upon excitation into the Qy band of chlorophyll a at 670 nm. The secondary electron transfer to quinone QA: PheoD1-QA → PheoD1QA- is observed with the time-constant of 650 ps only at 296 K. On the other hand, an inefficient β-carotene → chlorophyll a energy transfer (33%) occurred after excitation to the S2 state of β-carotene at 500 nm. Instead, the carotenoid triplet state appeared in an ultrafast timescale after excitation at 500 nm.

Exciton quenching by oxidized chlorophyll Z across the two adjacent monomers in a photosystem II core dimer

This study aimed to clarify (1) which pigment in a photosystem II (PSII) core complex is responsible for the 695-nm emission at 77 K and (2) the molecular basis for the oxidation-induced fluorescence quenching in PSII. Picosecond time-resolved fluorescence dynamics was compared between the dimeric and monomeric PSII with and without addition of an oxidant. The results indicated that the excitation-energy flow to the 695-nm-emitting chlorophyll (Chl) at 36 K and 77 K was hindered upon monomerization, clearly demonstrating significant exciton migration from the Chls on one monomer to the 695-nm-emitting pigment on the adjacent monomer. Oxidation of the redox-active Chl, which is named Chl_Z caused almost equal quenching of the 684-nm and 695-nm emission bands in the dimer, and lower quenching of the 695-nm band in the monomer. These results suggested two possible scenarios responsible for the 695-nm emission band: (A) Chl11-13 pair and the oxidized Chl_ZD1 work as the 695-nm emitting Chl and the quenching site, respectively, and (B) Chl29 and the oxidized Chl_ZD2 work as the 695-nm emitting Chl and the quenching site, respectively.

Accumulation of quenched LHCII around PSI in Chlamydomonas cells in state2 revealed by cryo-fluorescence lifetime imaging

Fluorescence-spectral microscope observations of photosynthetic organisms at cryogenic temperatures have the ability to spectrally resolve the two photosystems (PSs) and thus provide a powerful tool to elucidate the functional analysis of photosynthesis in vivo. In the present study, a measurement channel of the fluorescence lifetime at 680 nm was added to the cryo-microscope system previously developed by the authors. This provides access to information on the functional state of the light-harvesting system in living cells during regulation by a mechanism called state transitions. The observations of state1-locked and state2-locked Chlamydomonas cells at 80 K enabled us to identify a component showing rapidly decaying fluorescence with a lifetime of ca. 3 ps and emitting at around 676 nm. The component was assigned to the light-harvesting complex II (LHCII) that is isolated from both PSs and in a quenched state, probably due to the formation of aggregates. Simultaneous spectral observations revealed the accumulation of this free LHCII in the photosystem I (PSI)-enriched region within each state2-locked cell. To the best of our knowledge, this is the first in-vivo observation which suggests the localization of the quenched LHCII aggregates.

Chlorophyll excitation energies and structural stability of the CP47 antenna of photosystem II: a case study in the first-principles simulation of light-harvesting complexes

Natural photosynthesis relies on light harvesting and excitation energy transfer by specialized pigment-protein complexes. Their structure and the electronic properties of the embedded chromophores define the mechanisms of energy transfer. An important example of a pigment-protein complex is CP47, one of the integral antennae of the oxygen-evolving photosystem II (PSII) that is responsible for efficient excitation energy transfer to the PSII reaction center. The charge-transfer excitation induced among coupled reaction center chromophores resolves into charge separation that initiates the electron transfer cascade driving oxygenic photosynthesis. Mapping the distribution of site energies among the 16 chlorophyll molecules of CP47 is essential for understanding excitation energy transfer and overall antenna function. In this work, we demonstrate a multiscale quantum mechanics/molecular mechanics (QM/MM) approach utilizing full time-dependent density functional theory with modern range-separated functionals to compute for the first time the excitation energies of all CP47 chlorophylls in a complete membrane-embedded cyanobacterial PSII dimer. The results quantify the electrostatic effect of the protein on the site energies of CP47 chlorophylls, providing a high-level quantum chemical excitation profile of CP47 within a complete computational model of "near-native" cyanobacterial PSII. The ranking of site energies and the identity of the most red-shifted chlorophylls (B3, followed by B1) differ from previous hypotheses in the literature and provide an alternative basis for evaluating past approaches and semiempirically fitted sets. Given that a lot of experimental studies on CP47 and other light-harvesting complexes utilize extracted samples, we employ molecular dynamics simulations of isolated CP47 to identify which parts of the polypeptide are most destabilized and which pigments are most perturbed when the antenna complex is extracted from PSII. We demonstrate that large parts of the isolated complex rapidly refold to non-native conformations and that certain pigments (such as chlorophyll B1 and β-carotene h1) are so destabilized that they are probably lost upon extraction of CP47 from PSII. The results suggest that the properties of isolated CP47 are not representative of the native complexed antenna. The insights obtained from CP47 are generalizable, with important implications for the information content of experimental studies on biological light-harvesting antenna systems.

Identification of assembly precursors to photosystems emitting fluorescence at 683 nm and 687 nm by cryogenic fluorescence microspectroscopy

Photosystem I (PSI) and photosystem II (PSII) play key roles in photoinduced electron-transfer reaction in oxygenic photosynthesis. Assemblies of these PSs can be initiated by illumination of the etiolated seedlings (greening). The study aimed to identify specific fluorescence spectral components relevant to PSI and PSII assembly intermediates emerging in greening seedlings of Zea mays, a typical C4 plant. The different PSII contents between the bundle sheath (BS) and mesophyll (M) cells were utilized to spectrally isolate the precursors to PSI and PSII. The greening Zea mays leaf thin sections were observed with the cryogenic microscope combined with a spectrometer. With the aid of the singular-value decomposition analysis, we could identify four independent fluorescent species, SAS677, SAS685, SAS683, and SAS687, named after their fluorescence peak wavelengths. SAS677 and SAS685 are the dominant components after the 30-minute greening, and the distributions of these components showed no clear differences between M and BS cells, indicating immature cell differentiation in this developing stage. On the other hand, the 1-hour greening resulted in reduced distributions of SAS683 in BS cells leading us to assign this species to PSII precursors. The 2-hour greening induced the enrichment of SAS687 in BS cells suggesting its PSI relevance. Similarity in the peak wavelengths of SAS683 and the reported reaction center of PSII implied their connection. SAS687 showed an intense sub-band at around 740 nm, which can be assigned to the emission from the red chlorophylls specific to the mature PSI.

β-Carotene Probes the Energy Transfer Pathway in the Photosystem II Core Complex

The dynamics of the intact photosystem II core complex (PSII-CC) has been investigated extensively to elucidate its excellent photofunction. However, it is significantly difficult to observe the primary photosynthetic processes in PSII-CC because a vast number of chlorophylls (Chl) in the core complex show similar spectral features. In the present work, the dynamics of the energy transfer (ET) from β-carotene (Bcr) in intact PSII-CC followed by charge separation (CS) at the reaction center (RC) with different excitation wavelengths were compared. Upon excitation at 510 nm, which selectively excites Bcr (Bcr651) inside of the D1-D2 RC, the pheophytin anion absorption band appeared within 9.6 ps. On the other hand, upon excitation at 490 nm, mainly exciting unspecified Bcr in the antenna complex, the anion band appeared after 20 ps. These excitation wavelength dependence experiments revealed a new ET pathway of PSII-CC, which indicates that the initial CS of PSII-CC is limited by ET to the RC.

Adaptation of light-harvesting functions of unicellular green algae to different light qualities

Oxygenic photosynthetic organisms perform photosynthesis efficiently by distributing captured light energy to photosystems (PSs) at an appropriate balance. Maintaining photosynthetic efficiency under changing light conditions requires modification of light-harvesting and energy-transfer processes. In the current study, we examined how green algae regulate their light-harvesting functions in response to different light qualities. We measured low-temperature time-resolved fluorescence spectra of unicellular green algae Chlamydomonas reinhardtii and Chlorella variabilis cells grown under different light qualities. By observing the delayed fluorescence spectra, we demonstrated that both types of green algae primarily modified the associations between light-harvesting chlorophyll protein complexes (LHCs) and PSs (PSII and PSI). Under blue light, Chlamydomonas transferred more energy from LHC to chlorophyll (Chl) located far from the PSII reaction center, while energy was transferred from LHC to PSI via different energy-transfer pathways in Chlorella. Under green light, both green algae exhibited enhanced energy transfer from LHCs to both PSs. Red light induced fluorescence quenching within PSs in Chlamydomonas and LHCs in Chlorella. In Chlorella, energy transfer from PSII to PSI appears to play an important role in balancing excitation between PSII and PSI.

Development of fluorescence quenching in Chlamydomonas reinhardtii upon prolonged illumination at 77 K

Low-temperature fluorescence measurements are frequently used in photosynthesis research to assess photosynthetic processes. Upon illumination of photosystem II (PSII) frozen to 77 K, fluorescence quenching is observed. In this work, we studied the light-induced quenching in intact cells of Chlamydomonas reinhardtii at 77 K using time-resolved fluorescence spectroscopy with a streak camera setup. In agreement with previous studies, global analysis of the data shows that prolonged illumination of the sample affects the nanosecond decay component of the PSII emission. Using target analysis, we resolved the quenching on the PSII-684 compartment which describes bulk chlorophyll molecules of the PSII core antenna. Further, we quantified the quenching rate constant and observed that as the illumination proceeds the accumulation of the quencher leads to a speed up of the fluorescence decay of the PSII-684 compartment as the decay rate constant increases from about 3 to 4 ns^- 1. The quenching on PSII-684 leads to indirect quenching of the compartments PSII-690 and PSII-695 which represent the red chlorophyll of the PSII core. These results explain past and current observations of light-induced quenching in 77 K steady-state and time-resolved fluorescence spectra.

Red shift in the spectrum of a chlorophyll species is essential for the drought-induced dissipation of excess light energy in a poikilohydric moss, Bryum argenteum

Some mosses are extremely tolerant of drought stress. Their high drought tolerance relies on their ability to effectively dissipate absorbed light energy to heat under dry conditions. The energy dissipation mechanism in a drought-tolerant moss, Bryum argenteum, has been investigated using low-temperature picosecond time-resolved fluorescence spectroscopy. The results are compared between moss thalli samples harvested in Antarctica and in Japan. Both samples show almost the same quenching properties, suggesting an identical drought tolerance mechanism for the same species with two completely different habitats. A global target analysis was applied to a large set of data on the fluorescence-quenching dynamics for the 430-nm (chlorophyll-a selective) and 460-nm (chlorophyll-b and carotenoid selective) excitations in the temperature region from 5 to 77 K. This analysis strongly suggested that the quencher is formed in the major peripheral antenna of photosystem II, whose emission spectrum is significantly broadened and red-shifted in its quenched form. Two emission components at around 717 and 725 nm were assigned to photosystem I (PS I). The former component at around 717 nm is mildly quenched and probably bound to the PS I core complex, while the latter at around 725 nm is probably bound to the light-harvesting complex. The dehydration treatment caused a blue shift of the PS I emission peak via reduction of the exciton energy flow to the pigment responsible for the 725 nm band.

Fluorescence property of photosystem II protein complexes bound to a gold nanoparticle

Development of an efficient photo-anode system for water oxidation is key to the success of artificial photosynthesis. We previously assembled photosystem II (PSII) proteins, which are an efficient natural photocatalyst for water oxidation, on a gold nanoparticle (GNP) to prepare a PSII-GNP conjugate as an anode system in a light-driven water-splitting nano-device (Noji et al., J. Phys. Chem. Lett., 2011, 2, 2448-2452). In the current study, we characterized the fluorescence property of the PSII-GNP conjugate by static and time-resolved fluorescence measurements, and compared with that of free PSII proteins. It was shown that in a static fluorescence spectrum measured at 77 K, the amplitude of a major peak at 683 nm was significantly reduced and a red shoulder at 693 nm disappeared in PSII-GNP. Time-resolved fluorescence measurements showed that picosecond components at 683 nm decayed faster by factors of 1.4-2.1 in PSII-GNP than in free PSII, explaining the observed quenching of the major fluorescence peak. In addition, a nanosecond-decay component arising from a 'red chlorophyll' at 693 nm was lost in time-resolved fluorescence of PSII-GNP, probably due to a structural perturbation of this chlorophyll by interaction with GNP. Consistently with these fluorescence properties, degradation of PSII during strong-light illumination was two times slower in PSII-GNP than in free PSII. The enhanced durability of PSII is an advantageous property of the PSII-GNP conjugate in the development of an artificial photosynthesis device.

The lowest-energy chlorophyll of photosystem II is adjacent to the peripheral antenna: Emitting states of CP47 assigned via circularly polarized luminescence

The identification of low-energy chlorophyll pigments in photosystem II (PSII) is critical to our understanding of the kinetics and mechanism of this important enzyme. We report parallel circular dichroism (CD) and circularly polarized luminescence (CPL) measurements at liquid helium temperatures of the proximal antenna protein CP47. This assembly hosts the lowest-energy chlorophylls in PSII, responsible for the well-known "F695" fluorescence band of thylakoids and PSII core complexes. Our new spectra enable a clear identification of the lowest-energy exciton state of CP47. This state exhibits a small but measurable excitonic delocalization, as predicated by its CD and CPL. Using structure-based simulations incorporating the new spectra, we propose a revised set of site energies for the 16 chlorophylls of CP47. The significant difference from previous analyses is that the lowest-energy pigment is assigned as Chl 612 (alternately numbered Chl 11). The new assignment is readily reconciled with the large number of experimental observations in the literature, while the most common previous assignment for the lowest energy pigment, Chl 627(29), is shown to be inconsistent with CD and CPL results. Chl 612(11) is near the peripheral light-harvesting system in higher plants, in a lumen-exposed region of the thylakoid membrane. The low-energy pigment is also near a recently proposed binding site of the PsbS protein. This result consequently has significant implications for our understanding of the kinetics and regulation of energy transfer in PSII.

On the Conflicting Estimations of Pigment Site Energies in Photosynthetic Complexes: A Case Study of the CP47 Complex

We focus on problems with elucidation of site energies (E0n) for photosynthetic complexes (PSCs) in order to raise some genuine concern regarding the conflicting estimations propagating in the literature. As an example, we provide a stern assessment of the site energies extracted from fits to optical spectra of the widely studied CP47 antenna complex of photosystem II from spinach, though many general comments apply to other PSCs as well. Correct values of E0n for chlorophyll (Chl) a in CP47 are essential for understanding its excitonic structure, population dynamics, and excitation energy pathway(s). To demonstrate this, we present a case study where simultaneous fits of multiple spectra (absorption, emission, circular dichroism, and nonresonant hole-burned spectra) show that several sets of parameters can fit the spectra very well. Importantly, we show that variable emission maxima (690–695 nm) and sample-dependent bleaching in nonresonant hole-burning spectra reported in literature could be explained, assuming that many previously studied CP47 samples were a mixture of intact and destabilized proteins. It appears that the destabilized subpopulation of CP47 complexes could feature a weakened hydrogen bond between the 13¹-keto group of Chl29 and the PsbH protein subunit, though other possibilities cannot be entirely excluded, as discussed in this work. Possible implications of our findings are briefly discussed.