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Author(s): Alexander V. Ruban [1, 6]; Rudi Berera [2, 6]; Cristian Ilioaia [3, 4]; Ivo H. M. van Stokkum [2]; John T. M. Kennis [2]; Andrew A. Pascal [3]; Herbert [...]

Analysis on a range of artificial light-harvesting dyads built fro zinc phthalocyanine (Pc) covalently attached to carotenoids with 9,10 or 11 conjugated carbon-carbon double bonds is performed to determine the energy transfer and excited-state deactivation pathway following excitation of the strongly allowed carotenoid [S.sub.2] state as a function of the conjugation length. A new unseen energy relaxation pathways and a unique type of collective carotenoid-phthalocyanine excited state in carotenophthaloyanine dyads are observed.

In the light-harvesting antenna of the Synechocystis PCC 6803 phycobilisome (PB), the core consists of three cylinders, each composed of four disks, whereas each of the six rods consists of up to three hexamers (Arteni et al., Biochim Biophys Acta 1787(4):272-279, 2009). The rods and core contain phycocyanin and allophycocyanin pigments, respectively. Together these pigments absorb light between 400 and 650 nm. Time-resolved difference absorption spectra from wild-type PB and rod mutants have been measured in different quenching and annihilation conditions. Based upon a global analysis of these data and of published time-resolved emission spectra, a functional compartmental model of the phycobilisome is proposed. The model describes all experiments with a common set of parameters. Three annihilation time constants are estimated, 3, 25, and 147 ps, which represent, respectively, intradisk, interdisk/intracylinder, and intercylinder annihilation. The species-associated difference absorption and emission spectra of two phycocyanin and two allophycocyanin pigments are consistently estimated, as well as all the excitation energy transfer rates. Thus, the wild-type PB containing 396 pigments can be described by a functional compartmental model of 22 compartments. When the interhexamer equilibration within a rod is not taken into account, this can be further simplified to ten compartments, which is the minimal model. In this model, the slowest excitation energy transfer rates are between the core cylinders (time constants 115-145 ps), and between the rods and the core (time constants 68-115 ps). [ABSTRACT FROM AUTHOR]

When exposed to intense sunlight, all organisms performing oxygenic photosynthesis implement various photoprotective strategies to prevent potentially lethal photodamage. The rapidly responding photoprotective mechanisms, occurring in the light-harvesting pigment-protein antennae, take effect within tens of seconds, while the dramatic and potentially harmful light intensity fluctuations manifest also on shorter time scales. Here we show that, upon illumination, individual phycobilisomes from Synechocystis PCC 6803, which, in vivo under low-light conditions, harvest solar energy, and have the built-in capacity to switch rapidly and reversibly into light-activated energy-dissipating states. Simultaneously measured fluorescence intensity, lifetime, and spectra, compared with a multicompartmental kinetic model, revealed that essentially any subunit of a phycobilisome can be quenched, and that the core complexes were targeted most frequently. Our results provide the first evidence for fluorescence blinking from a biologically active system at physiological light intensities and suggest that the light-controlled switches to intrinsically available energy-dissipating states are responsible for a novel type of photoprotection in cyanobacteria. We anticipate other photosynthetic organisms to employ similar strategies to respond instantly to rapid solar light intensity fluctuations. A detailed understanding of the photophysics of photosynthetic antenna complexes is of great interest for bioinspired solar energy technologies. [ABSTRACT FROM AUTHOR]

The orange carotenoid protein (OCP)is a crucial player in the process of nonphotochemical quenching ina large number of cyanobacteria. This water-soluble protein bindsone pigment only, the keto carotenoid 3′-hydroxyechinenone,and needs to be photoactivated by strong (blue-green) light in orderto induce energy dissipation within or from the phycobilisome, themain light harvesting system of these organisms. We performed transient-absorptionspectroscopy on OCP samples frozen in the inactive and active formsat 77 K. By making use of target analysis we determined the excitedstate properties of the active form. Our results show that OCP photoactivationmodifies the carotenoid excited state energy landscape. More specificallythe photoactivated OCP is characterized by one state with predominantlyICT character (ICT/S1) and a lifetime of 2.3 ps, and anotherstate with mainly S1character (S1/ICT) witha lifetime of 7.6 ps. We also show that the kinetic model is fullyconsistent with the RT data obtained earlier (Berera et al., J. Phys. Chem.B2012, 116, 2568–2574). We propose that this ICT/S1state acts as the quencher in the OCP mediated nonphotochemicalquenching. [ABSTRACT FROM AUTHOR]

To cope with the deleterious effects of excess illumination,photosyntheticorganisms have developed photoprotective mechanisms that dissipatethe absorbed excess energy as heat from the antenna system. In cyanobacteria,a crucial step in the process is the activation, by blue-green light,of a soluble protein, known as orange carotenoid protein (OCP), whichbinds the carotenoid 3′-hydroxyechinenone as its only pigment.While the spectroscopic properties of the inactive form of OCP havebeen described, the nature of the excited states in the active formstill awaits elucidation. We applied transient absorption spectroscopyto the dark and the light activated forms of OCP to study and comparethe excited state dynamics of both forms. We show that excitationof the photoactivated OCP leads to the population of new carotenoidexcited states. One of these states populated shortly after excitationis characterized by a very pronounced charge transfer character anda lifetime of about 0.6 ps. When the illuminated sample is exposedto a dark relaxation period, it responds to excitation as the originaldark sample, showing that photoactivation and decay of the photoactivatedstate are fully reversible. Thus OCP functions as a light-poweredmolecular switch that modulates its spectroscopic properties as aresponse to specific changes in light environment. We discuss theimportance of this switch in cyanobacteria photoprotection and proposea mechanism wherein the red state of OCP echinenone acts as an energydissipator via its charge transfer state. [ABSTRACT FROM AUTHOR]

Photosynthetic organisms have developed vital strategies which allow them to switch from a light-harvesting to an energy dissipative state at the level of the antenna system in order to survive the detrimental effects of excess light illumination. These mechanisms are particularly relevant in diatoms, which grow in highly fluctuating light environments and thus require fast and strong response to changing light conditions. We performed transient absorption spectroscopy on FCPa, the main light-harvesting antenna from the diatom Cyclotella meneghiniana, in the unquenched and quenched state. Our results show that in quenched FCPa two quenching channels are active and are characterized by differing rate constants and distinct spectroscopic signatures. One channel is associated with a faster quenching rate (16 ns- 1) and virtually no difference in spectral shape compared to the bulk unquenched chlorophylls, while a second channel is associated with a slower quenching rate (2.7 ns- 1) and exhibits an increased population of red-emitting states. We discuss the origin of the two processes in the context of the models proposed for the regulation of photosynthetic light-harvesting. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: Keys to Produce Clean Energy. [ABSTRACT FROM AUTHOR]

When cyanobacteria are grown under iron-limited or other oxidative stress conditions the iron stress inducible pigment–protein IsiA is synthesized in variable amounts. IsiA accumulates in aggregates inside the photosynthetic membrane that strongly dissipate chlorophyll excited state energy. In this paper we applied Stark fluorescence (SF) spectroscopy at 77 K to IsiA aggregates to gain insight into the nature of the emitting and energy dissipating state(s). Our study shows that two emitting states are present in the system, one emitting at 684 nm and the other emitting at about 730 nm. The new 730 nm state exhibits strongly reduced fluorescence (F) together with a large charge transfer character. We discuss these findings in the light of the energy dissipation mechanisms involved in the regulation of photosynthesis in plants, cyanobacteria and diatoms. Our results suggest that photosynthetic organisms have adopted common mechanisms to cope with the deleterious effects of excess light under unfavorable growth conditions. [ABSTRACT FROM AUTHOR]

Abstract: Diatoms are characterized by very efficient photoprotective mechanisms where the excess energy is dissipated as heat in the main antenna system constituted by fucoxanthin–chlorophyll (Chl) protein complexes (FCPs). We performed Stark fluorescence spectroscopy on FCPs in their light-harvesting and energy dissipating states. Our results show that two distinct emitting bands are created upon induction of energy dissipation in FCPa and possibly in FCPb. More specifically one band is characterized by broad red shifted emission above 700nm and bears strong similarity with a red shifted band that we detected in the dissipative state of the major light-harvesting complex II (LHCII) of plants [26]. We discuss the results in the light of different mechanisms proposed to be responsible for photosynthetic photoprotection. [Copyright &y& Elsevier]

In order to cope with the deleterious effects of excess light, photosynthetic organisms have developed remarkable strategies where the excess energy is dissipated as heat by the antenna system. In higher plants one main player in the process is the major light harvesting antenna of Photosystem II (PSII), LHCII. In this paper we applied Stark fluorescence spectroscopy to LHCII in different quenching states to investigate the possible contribution of charge-transfer states to the quenching. We find that in the quenched state the fluorescence displays a remarkable sensitivity to the applied electric field. The resulting field-induced emission spectra reveal the presence of two distinct energy dissipating sites both characterized by a strong but spectrally very different response to the applied electric field. We propose the two states to originate from chlorophyll-chlorophyll and chlorophyll-carotenoid charge transfer interactions coupled to the chlorophyll exciton state in the terminal emitter locus and discuss these findings in the light of the different models proposed to be responsible for energy dissipation in photosynthesis.

The photosynthetic light-harvesting systems of purple bacteria and plants both utilize specific carotenoids as quenchers of the harmful (bacterio)chlorophyll triplet states via triplet-triplet energy transfer. Here, we explore how the binding of carotenoids to the different types of light-harvesting proteins found in plants and purple bacteria provides adaptation in this vital photoprotective function. We show that the creation of the carotenoid triplet states in the light-harvesting complexes may occur without detectable conformational changes, in contrast to that found for carotenoids in solution. However, in plant light-harvesting complexes, the triplet wavefunction is shared between the carotenoids and their adjacent chlorophylls. This is not observed for the antenna proteins of purple bacteria, where the triplet is virtually fully located on the carotenoid molecule. These results explain the faster triplet-triplet transfer times in plant light-harvesting complexes. We show that this molecular mechanism, which spreads the location of the triplet wavefunction through the pigments of plant light-harvesting complexes, results in the absence of any detectable chlorophyll triplet in these complexes upon excitation, and we propose that it emerged as a photoprotective adaptation during the evolution of oxygenic photosynthesis., (Copyright © 2011 Biophysical Society. Published by Elsevier Inc. All rights reserved.)

When grown under a variety of stress conditions, cyanobacteria express the isiA gene, which encodes the IsiA pigment-protein complex. Overexpression of the isiA gene under iron-depletion stress conditions leads to the formation of large IsiA aggregates, which display remarkably short fluorescence lifetimes and thus a strong capacity to dissipate energy. In this work we investigate the underlying molecular mechanism responsible for chlorophyll fluorescence quenching. Femtosecond transient absorption spectroscopy allowed us to follow the process of energy dissipation in real time. The light energy harvested by chlorophyll pigments migrated within the system and eventually reaches a quenching site where the energy is transferred to a carotenoid-excited state, which dissipates it by decaying to the ground state. We compare these findings with those obtained for the main light-harvesting complex in green plants (light-harvesting complex II) and artificial light-harvesting antennas, and conclude that all of these systems show the same mechanism of energy dissipation, i.e., one or more carotenoids act as energy dissipators by accepting energy via low-lying singlet-excited S(1) states and dissipating it as heat.